LED Lighting Reliability Improvement

- Volume I -

May 2013

Team Members: Ying Lin

Robert Muellerleile James Schultz

Eric Wikan Sicong Wu

Advisor: Jeremy Vearrier

Client:

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Contributions: Robert Muellerleile

Did background patent research

Conducted thermal cycling tests

Contributed to analysis reports in volume II

Helped with prototype construction

Volume I and Volume II Ying Lin

Conducted all water protection tests

Contacted companies in China for samples and adhesives

Conducted ANSYS simulation

Conducted visits at Lumificient to record manufacturing process and receive samples James Schultz

● Conducted tests on the five returned tubes from Lumificient ● Created Block Diagram and Overview Drawings ● Created Mesh of End Cap in ANSYS ● Drew Prototype Designs ● Calculations on LED temperature ● Communicated with MoCap and Protolabs representatives ● Volume I and Volume II

Eric Wikan

Developed CAD Prototype

Communicated with Jeremy Vearrier/Patrick Delaney about design requirements

Spoke with machine shop

Developed 3D printed models

Helped with water pressure test

Created Mold Prototype Sicong Wu

Researched failure causes due to corrosion

Summarized RMA spreadsheet

Performed pressure sensor test to find out baseline requirements

Wrote corrosion background in design report

Drew apparatus sketch in design report for each test

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Executive Summary

Lumificient Corporation in Maple Grove, MN, has been working in the LED signage and display industry since 2000. One of their products, the Lumeon 360 LED light system consists of an LED strip inserted into a polycarbonate tube with both ends sealed. The end cap assembly used to seal the tube is shown below on the left. This product has been experiencing a high rate of failure over the last few years and customers have returned purchases due to the tubes dimming or not lighting up at all. When we first met with Lumificient they knew that moisture in the tube was a leading cause of failure, but they were not sure how the moisture was getting in. They suspected the end cap of being a contributing factor to moisture intrusion and assigned us the task of creating a superior way to seal the end tubes.

We spent a considerable effort conducting experiments in order to determine the exact modes of failures of the LED tubes. Through our experiments, we were able to confirm that water is able to seep through gaps in the end cap. These gaps usually are the result of the product being used in the field for years. Over time, the caps become damaged and gaps or cracks are formed.

The new design, pictured below on the right, is a single end cap made of injection molded polycarbonate that can match the colors currently used in Lumeon tubes. Wires that make the electrical connection to the internal LED strip are passed through holes in the side of the end cap. As seen from the two pictures, our design combines Lumificient’s end cap (the red component), and end seal ( the white circular component) into one piece. Whereas Lumificient’s design uses two adhesives, ours uses only one: Loctite Quick Set Epoxy. We believe that this simplifying of the design will save cost on not only parts but also on assembly time.

The new design on the right improves on the water ingress protection properties of the end cap by creating a much larger contact area between the end cap and the tube. We verified the performance of our design by subjecting it to several different tests that simulate conditions it would be exposed to in the environment. We fully submerged the tube and end cap in water for 30 hours, and afterwards opened the tube to see if any water was present inside the end cap. Samples of Lumificient’s original design that had been exposed to the elements failed this test after 30 hours. Our new design passed this stress in its stock state, but also passed it after being thermal cycled from -10 to 100 degrees Celsius and again after the wires were fatigued by being bent back and forth repeatedly 10,000 times. Additionally we pressurized the inside of the tube to 12 PSI, a level that would replicate environmental conditions, and found that Lumificient’s original design allowed water intrusion under these conditions, while a new sample with our new end cap design did not.

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Table of Contents 1 - Problem Definition .................................................................................................................................. 5

1.1 - Problem Scope .................................................................................................................................. 5

1.2 - Technical Review ............................................................................................................................... 5

1.2.1 - Background ................................................................................................................................ 5

1.2.2 - Lumificient’s Product:................................................................................................................. 5

1.2.3 - Reasons for Failure ..................................................................................................................... 7

1.2.4 - Corrosion Resulting From Moisture ........................................................................................... 9

1.2.5 - Lumificient’s Competitors .......................................................................................................... 9

1.3 - Design Requirements ...................................................................................................................... 10

1.3.1 - Water Protection. ..................................................................................................................... 10

1.3.2 - Thermal Resistance .................................................................................................................. 10

1.3.3 - Pressure Resilience ................................................................................................................... 10

1.3.4 - Length of End Cap .................................................................................................................... 11

2- Design Description ................................................................................................................................. 12

2.1 - Summary of the Design ................................................................................................................... 12

2.2 – Detailed Description ....................................................................................................................... 12

2.2.1 - The End Cap ............................................................................................................................. 12

2.2.2 - The Adhesives ........................................................................................................................... 14

2.2.3 - Functional Block Diagram: ....................................................................................................... 14

2.2.4 - Functional Description ............................................................................................................. 15

2.2.4 - Overview Drawings .................................................................................................................. 16

2.3 - Additional Uses ............................................................................................................................... 18

3 - Evaluation .............................................................................................................................................. 19

3.1- Evaluation Plan ................................................................................................................................ 19

3.1.1 - Design Requirements and Testing............................................................................................ 19

3.1.2 - Water Protection ...................................................................................................................... 19

3.1.3 - Thermal Protection .................................................................................................................. 19

3.1.4 - Pressure Resilience ................................................................................................................... 19

3.1.5 - Length of End Cap Assembly .................................................................................................... 19

3.2 - Evaluation Results ........................................................................................................................... 20

3.2.1 - The Prototype ........................................................................................................................... 20

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3.2.2 - Water Protection ...................................................................................................................... 20

3.2.3 - Thermal Resistance .................................................................................................................. 20

3.2.4 - Pressure Resilience ................................................................................................................... 20

3.2.5 - Length of End Cap Assembly .................................................................................................... 21

3.3 - Discussion ....................................................................................................................................... 21

3.3.1 - Strengths and Weaknesses ...................................................................................................... 21

3.3.2 - Next Steps ................................................................................................................................ 22

4 - References ............................................................................................................................................. 23

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1 - Problem Definition

1.1 - Problem Scope

The current design of the Lumeon 360 LED light has an unacceptable reliability history and its manufacturer, Lumificient, is in the process of redesigning the product series to address this issue. Many of the failures have been attributed to water seeping into the tubing (which holds the LED strip) and causing electrical failure. The end cap, end seal, and adhesives used to seal the ends of the LED tubing have been suspected by Lumificient of allowing this water ingress. Our team has been given the task of redesigning these three components to create a superior way of sealing the LED tubes.

When they came to us, Lumificient was not entirely sure how these failures were occurring; they just suspected that water was seeping in somehow. Therefore, a large portion of our time was spent investigating the failures behind the LEDs, either through research or experiments. Once these reasons were thoroughly understood, we could move onto the next phase of the project - creating a superior way of sealing the LED tubing.

1.2 - Technical Review

1.2.1 - Background

The term LED stands for light emitting diode. The essential part of an LED is a semiconductor, a material with a varying ability to conduct electricity. The properties of a semiconductor can be altered by adding impurities (atoms of a different element). This process is called “doping.” In an N-Type semiconductor, impurities have been added so that the material has a surplus of electrons. This results in atoms forming with a negative charge. The opposite is true in a P-type semiconductor – it has been doped to be electron deprived, meaning it has atoms of a positive charge. An LED consists of a P-Type and an N-type placed next to each other. Since the different sections have atoms of opposite charge, the atoms begin to move to the interface between the two sections. At the interface, a small insulating area that repels charge is formed between the two semiconductors. This area prevents the movement of electrons and is known as the “depletion zone” [1]. In order to get the electrons moving again, a power source, such as a battery, can be hooked up to the semiconductors. The negative terminal of a battery repels the negative ions in the N-Type semiconductor through the depletion zone (the same occurs with the positive terminal and the P-Type semiconductor). The depletion zone becomes smaller as the electrons begin to move and eventually it disappears. It is this movement of these electrons that produces the light seen in an LED [2].

LED’s are known for their reliability and efficiency. They typically have a useful lifetime of 30,000 – 50,000 hours, compared with the lifetime for Compact Fluorescent bulbs of 10,000 hours and for incandescent of only 1000 hours [1]. While an individual LED bulb can cost more than $30, they run very efficiently, and over time prove to be the more economical choice over many their competitors [3].

1.2.2 - Lumificient’s Product:

The product we are examining, Lumificient’s Lumeon 360 LED light system, has five main components: the polycarbonate extrusion (tubing), the flexible polyvinylchloride (PVC) end cap, the rigid PVC end seal, the LED insert strip, and the interconnection wires (see Figure 1.1). There are also two adhesives used to connect the tubing and end cap: Plastidip, a multipurpose liquid rubber coating, and Loctite 4307. Loctite 4307 is an adhesive based on the chemical

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Cyanoacrylate. Cyanoacrylate is found in most super glues, many of which, do not need further processing to facilitate curing. This particular glue contains a photo initiator, which requires the use of a UV light to cure the glue and provide a permanent bond [4]. The components that we are allowed to modify in our design include the end cap, the end seal, and the adhesives. A complete list of materials and additional part drawings from Lumificient can be found in the Appendix D of Volume II.

Figure 1.1: End Cap Assembly Exploded View

Lumificient uses a multilayered process for attaching the tube to the end cap

components. First, the wires are slid through holes in the end seal and soldered to the LED strip (the holes are not visible in Figure 1.1). Next, Loctite 4307 is applied to the end of the polycarbonate tube and the end seal is placed over the adhesive and tube. The glued end is cured using an ultraviolet (UV) curing light. Once dried, the tip of the tube with the end seal attached is dipped in Plastidip and the end cap (seen in red in Figure 1.1) is placed over it. This is allowed to dry overnight. Next, a final coat of Plastidip is applied over the end cap and a portion of the light tube to provide additional water ingress protection. However, this second layer of Plastidip was not witnessed in some of the returned samples we examined. The whole process is run simultaneously on the other end of the light assembly [4].

LED tubes are assembled mostly by hand at Lumificient’s headquarters in Maple Grove. From there, they ship out orders of LED tubing in sections ranging from 12-96 inches. Once the product reaches the shipment destination, it must be installed according to Lumificient specifications [5]. The full installation procedure that Lumificient uses is listed in Appendix D of Volume II.

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1.2.3 - Reasons for Failure

When we first met with Lumificient, they suspected that the failures of the LEDs were primarily due to moisture intrusion; however, they did not have a clear understanding of how this was happening, and what factors in the environment were allowing it to happen. Therefore, we spent a significant amount of effort investigating the reasons behind the LED failures. Through our investigation and experiments, we now have a much clearer understanding of these failures.

We first came up with three theories of how water could get inside the LED tube. Theory one is that the water seeps through cracks on the end cap-tube connection or wire holes. Theory two is that water vapor is present in the tubes at the time of assembly, before the caps are put on. When the tube is fully sealed and used at its final location, fluctuations in temperature and pressure may cause this vapor to condense into water droplets. Theory three is that water permeates through the actual tubing - the “Polycarbonate Extrusion” shown in Figure 1.1.

It was fairly easy to rule out theory 3 by examining the properties of polycarbonate. Polycarbonate is completely waterproof and is often used as a water container for its physical properties [6].

Our next method of investigation was to examine tubes that had failed in the field and been returned on warranty claims. We were able to examine five 8 foot long LED tubes that had been returned to Lumificient - the reasons for failure were unknown. One key observation we made about these tubes was that the failure points occurred near the ends of the tube. On four of the five tubes looked at, the failure point (see Figure 1.2) was located within 1.5 feet of the end cap.

Figure 1.2: Failure points of LED

Next, we opened up the LED tubes to look at the circuit inside. We witnessed corrosion

and burning on all 5 tubes. Again, for four of the five tubes, the corrosion and burning was located primarily near the end caps. Figure 1.3 illustrates severe burning that was found on a section of LED strip that was located next to an end cap. Parts of the LED strip near the middle of the tube had what we believed to be corrosion as well, but to a much lesser extent. Figure 1.4 illustrates a section of LED strip located in the middle of one of the tubes.

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Figure 1.3: Burning located on LED strip. This strip was located near the very end of one of the

tubes.

Figure 1.4: Mild corrosion found on LED strip located near center of tube.

The fact that the corrosion and burning was witnessed primarily near the end caps

strongly supports the theory that water is seeping in through these end caps. If water were to be condensing due to pressure and temperature fluctuations, we suspect that the failure points would be spread uniformly throughout the tubing. A more detailed write-up of the failure modes of these five LED tubes can be found in Appendix D.

The next method of our investigation was to actually test some of the failed tubes. We conducted the “Waterproof Test,” on several more tubes we received from Lumificient that had been returned to them under warranty. The procedure and results of this test are presented in Section 3.1 of Volume II. From these tests, we concluded that moisture was able to enter the tube through the end cap for several returned Lumificient tubes. The most at-risk point for water intrusion appeared to be the holes where the wires entered the end cap. We suspected that water may be able to seep in through gaps in this area.

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1.2.4 - Corrosion Resulting From Moisture

One of the main ways moisture leads to the failure of electronic components is through moisture intrusion. Within the presence of moisture, copper, in its high-energy “metallic” state after being processed or refined, tends to return to its lower-energy and more stable natural “ore” state [7]. Corrosion can be essentially regarded as an electrical circuit in which the presence of electrical potential between anode and cathode immersed in an electrolyte causes electrons to flow from the anode to the cathode and form metallic ions on the surface of the anode. These metallic ions may react with other chemical reagents to form compounds that either adhere to, or detach from, the metal surface. Though atmospheric, soil and galvanic corrosion are generally the three types of corrosion that occur in the electrical industry, galvanic corrosion of the circuit at soldered junctions is believed to be the main cause of failure in our case. Galvanic corrosion occurs when dissimilar metals are in contact with each other. In this case, moisture that has gained access to the circuit board acts as the electrolyte and the copper and solder used on Lumificient’s circuit make up the differing metals.

Additionally, the presence of moisture in the air causes an oxide layer to form on the surface of copper. Atmospheric sulfur dioxide can form and can be visible as a green patina that inhibits the function of electrical circuits. This type of corrosion has been purposely used on the Statue of Liberty to provide a layer of protection from the elements. Once the oxide layer forms, a stable bond exists at the surface of the copper and provides a non-reactive layer on the surface. In electrical devices this layer can develop between individual components and impede electrical current. This can cause a short circuit if the contact between the copper and another component is thoroughly oxidized. It can also cause an overloaded path for electricity. If a conductor becomes overloaded, it can heat up and melt the connection or cause burning [8]. Corrosion was witnessed on several LED tubes. We believe that this corrosion also led to failed electrical components, causing some of the burning we witnessed (see Figure 1.3).

1.2.5 - Lumificient’s Competitors

There were patents given for products similar to the Lumeon tubes from this project. Most notably there is a patent for a “Waterproof LED lamp tube and casing of same” [9]. The tube discussed in this patent does not create a full cylinder; however, its cross section, which is included in Volume II, has an opaque bracket that holds a translucent light diffuser. Additionally there is a relevant patent given for a “Sealing device and method for producing a waterproof connector,” [10]. The end cap described in this patent includes a wire connector that passes the electrical current through the solid cap. The wire connector is specified in the patent as including “conductive metal pins to form electric connection with the LED lamp strip.”

We contacted a company in China called TianMu who made a product very similar to Lumificient. They shipped us a sample of their product and adhesives, shown in Figure 1.5.

Figure 1.5: TianMu End Cap

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1.3 - Design Requirements

1.3.1 - Water Protection: The end cap provides a complete seal from water.

The first requirement of our design is that it be waterproof. As discussed in Section 1.2.3, moisture has shown to be leading factor in the failure of Lumificient’s LED’s. To measure the level of water resistance, we will use the “Liquid Ingress Protection” rating laid out in the International Electrotechnical Commission Standard 60529 [11]. Our design should have an IP rating of 7, defined as the following: “Ingress of water in harmful quantity shall not be possible when the enclosure is immersed in water (up to 1 m of submersion)” [11]. The test procedure calls for the end cap and tube assembly being submerged in water at a depth of at least 1 meter for a duration of 30 minutes.

1.3.2 - Thermal Resistance: The end cap does not break its seal when exposed to

repeated temperature changes in the range of -10º C and 100º C

Since the LED tubes will be used in a wide variety of climates, they will see rapid temperature changes on a daily basis, as well as gradual temperature changes on a seasonal basis. This will cause repeated thermal expansion and compression, which will in turn lead to fatigue stress points between the LED tube and end cap connection. The design must be strong enough to withstand these fatigue stresses and hold its seal. ANSYS simulations were performed to predict the extent of thermal expansion that occurs and these are presented in Appendix F.

We suspect that 100º C is well over the highest temperatures the LED tubes will be exposed to. While we expect atmospheric temperatures as high as 45º C (113º F), the LED strip itself will also generate heat. The junction temperature of the LED used reaches a maximum of 120º C but will not be able to transfer enough heat to get the LED tube and LED cap to achieve anywhere 120 º [5]. Therefore 100º C is an overestimate. The low temperature of -10º C was chosen for being an achievable representation of the low end of the temperature spectrum expected in the environment the LED’s would be placed in.

1.3.3 - Pressure Resilience: The end cap does not break its seal when a pressure

difference of 12 psi is present between the inside of the tube and outside of the

tube.

The LED tubes are assembled and sealed off in a factory at room temperature. If the temperature is raised after the seal, pressure inside the tube will increase, and forces will push out on the end caps. By using the ideal gas law, we came up with a relationship between the temperature and the air in the tube, shown in the graph on the following page. We also conducted several ANSYS simulations to corroborate our ideal gas law calculations. The results of these tests are shown in Appendix F.

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Figure 1.6: Pressure vs Temperature

As mentioned earlier, the highest possible temperature the air in the LED could reach is predicted to be 120º C. If all the air were heated to this value, it would reach a pressure value of 4.88 psi. For added assurance and to account for other stresses that may be on the end cap due to thermal expansion, we require our design to withstand pressure values up to 12 psi.

1.3.4 - Length of End Cap: The length of section at the end of the LED tube

without light or with significantly modified light does not exceed ½ inches.

It is important that the end cap not protrude too far in or out of the polycarbonate tube. This would reduce the amount of tubing with actual light being emitted once the tubes are installed end to end. During installation, the LED tubes are mounted end to end very close to each other to minimize gaps of darkness (see Figure 1.6) [5]. Therefore, the end cap design may not have a dark section or section with significantly modified light at the end of the tube of more than one half inch (this accounts for differences in the light emitted caused by the protrusion of end cap out of the tube and insertion of end cap into the tube).

Figure 1.6: Current End Cap Design

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2- Design Description

2.1 - Summary of the Design

Our solution is a fully redesigned end cap that will fit onto the end of the current Lumeon 360 tubes. As discussed in Section 1.2.2, Lumificient currently uses an end seal and an end cap; our design combines the two into a single piece. To avoid confusion in the following sections and in Volume II, this single piece will be referred to simply as the “end cap” and Lumificient’s pieces will be referred to as “Lumificient’s end cap,” and “Lumificient’s end seal.”

In addition to a redesigned end cap, we have also researched different adhesives to work with our design. Because our new design encases the end of the tube to a much greater degree than the original Lumificient design, we recommend using only one adhesive: Loctite Quick Set Epoxy, which is a a two-part mixed epoxy resin instead of the Loctite 4307 and Plastidip that Lumificient currently uses.

2.2 – Detailed Description

2.2.1 - The End Cap

Our design has 5 major features that set it apart from Lumificient’s design (Figure 2.1). The “outer lip” in our design serves the same purpose that Lumificient’s end cap does - it slides around the outside of the LED tube and, with the adhesive, provides a tight seal from water. Having the outer lip also negates the need for having two pieces - it essentially combines Lumificient’s end cap and end seal into one component. Having only one piece not only cuts down on manufacturing costs, but also allows for a reduction in the assembly time. Before, the adhesive on the Lumificient’s end cap would have to dry before the end seal could be applied; our design gets rid of this step in the assembly process.

Figure 2.1: Five Main Components of the End Cap

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The “upper extrusion” seen in Figure 2.1 was present in Lumificient’s end seal, but it has been extended and hollowed out in our end cap. The upper extrusion is inserted into the upper tube cavity pictured in Figure 2.2 and its added length is meant to add overall structural support and produce a stronger bond between the end cap and tube.

A “lower extrusion” piece has also been added in our design. This piece is inserted into the “lower tube cavity,” and the wires pass through it in order to connect to the LED strip. The lower extrusion was added in order to provide a better overall seal, and to allow a slot for the LED strip to insert into.

Figure 2.2: Cavities in the LED Tube. Like Lumificieint’s design, our design features holes for the wires to come through at the

bottom of the cap; however the base of these holes has been widened (Figure 2.3). The wires first enter these holes, then go through the “added support” section (seen in Figure 2.1), and come out of the lower extrusion to attach to the LED strip. These holes are surrounded tightly by material and adhesive all the way up until the point at which they connect to the LED strip. More detailed diagrams of this are shown in Section 2.2.4.

Figure 2.3: Wire Hole Base (on right)

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2.2.2 - The Adhesives

Our recommendation for Lumificent with regards to the adhesives is that Loctite Quick Set Epoxy is used to attach the end cap to the tube. Our testing showed that this “quick set” 5 minute epoxy could be applied easily an quickly, and also performed well in terms of water protection. Because the end cap now completely surrounds the tube, using a UV curable resin is no longer feasible, as UV light would not be able to penetrate the polycarbonate and facilitate the curing of the resin. Thus we recommend an adhesive that can cure with time instead of requiring an outside catalyst. We also found the Locktite Quick Set Epoxy had a workable viscosity at room temperature, meaning it wasn’t to thin that it would drip once applied.

The epoxy should be applied to the end cap and tube after an electrical connection is established. The epoxy should be mixed thoroughly, and then applied to all surfaces on the end cap that will come in contact with the tube. Then the end cap can be placed on the end of the tube and the resin allowed to set for 5-10 minutes.

2.2.3 - Functional Block Diagram:

Figure 2.4: Block Diagram (functions are blue, components are red and green)

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2.2.4 - Functional Description

Function 1: Provide Watertight Seal The end cap and the adhesive work together to provide a complete seal from water and water vapor. The first line of defense is the wire hole base and outer lip. The tiny gap between the outer lip and LED tube is protected by adhesive, so no water is able to enter here (Figure 2.5). The most at risk part is the wire hole base. This is where the wires enter and a potential spot for water to enter as well. This base has been expanded in our design to allow more surface area of material to surround the wires. This decreases the chance that water is able to seep through with the wires and also allows more adhesive to attach to the wires.

The “added support” and “lower extrusion” also work to prevent water from entering. The “added support” tightly surrounds the wires up until the point that they connect to the LED strip (Figure 2.6). This tight fit is designed to decrease the chance of leaks. Even if water were able to get between the outer lip and LED tube, it would still have to get around the lower extrusion in order to reach the LED strip. Adhesive will be placed on the lower extrusion to prevent gaps between it and the “lower tube cavity.” Function 2: Provide Structural Support The upper extrusion part and outer lip of the end cap work together to provide a structurally stable connection to the LED tube. The upper extrusion is inserted into the “upper tube cavity” (Figure 2.2) and prevents any movement of the cap relative to the tube. If any force acts to shear the cap relative to the tube, these two components are designed to withstand it and take stresses off the adhesive (Figure 2.7). Function 3: Allow Safe Passage of Wires The wire hole base, added support section and lower extrusion all allow for the passage of wires. As mentioned earlier, they mean to maximize the amount of material that tightly surrounds the wires in order to suppress any leaks. Figure 2.8 illustrates the path the wires take to connect to the LED strip. Once the wires get through the added support section, they make a 90 degree turn through the lower extrusion. This creates a kink in the wire; the kink creates more friction between the cap and the wires, which works to prevent the wire from being pulled out of the cap. The adhesive coats the inside of the “added support” section to fill in any gaps between end cap material and the wire. Function 4: Connect Components The adhesive works to connect the end cap to the LED tube and fill in any gaps that exist. Some of these gaps are illustrated in Figure 2.9. Adhesive is placed on the inside of the end cap to connect the upper extrusion to the upper tube cavity and the lower extrusion to the lower tube cavity. It is place inside the wire holes to connect the wires to the end cap (and provide a watertight seal). The adhesive is also placed on the outside of the end cap where it connects the “outer lip” to the outside of the LED tubing.

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2.2.4 - Overview Drawings

Figure 2.5: Assembly view of end cap, wires, and LED tube.

Figure 2.6: Wires enter through the wire hole base on the bottom, then through the “added support” section, and lastly come out of the lower extrusion.

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Figure 2.7: Outer lip and upper extrusion act to absorb shear forces and reduce stress on adhesive.

Figure 2.8: Wire Path

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Figure 2.9: The adhesive provides a strong connection between the end cap, LED tube and wires. It also fills in any gaps that exist between the components. These gaps are visible in the figure above.

2.3 - Additional Uses Because of the specific shape of the Lumeon 360 tube, applying our design directly to other products could be difficult - the end cap would not fit on other sized tubes. However, it may be possible for Lumificient Corporation to implement certain features of this design into some of their other sign lighting products. Specifically, they may apply our method of passing the wires through the end cap to future products. The tests we conducted on adhesives are also directly usable by Lumificient - they may use the results of these tests to choose adhesives for other products.

There are several other variations of our design. Many variations of adhesives are possible. In addition to our one adhesive, a layer of Plastidip could be placed on the outside of the end cap, but it would add to assembly time. We also considered variations of the end cap - one was a design with two pieces, similar to Lumificient’s design. Another design we considered involved screws going through the end cap. However, through the design process we ruled out these designs and arrived at ours as a superior solution. Documentation of all other design alternatives is presented in Volume II.

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3 - Evaluation

3.1- Evaluation Plan

3.1.1 - Design Requirements and Testing

Each test is performed on at least two samples. The first sample is a 1 foot long assembly of the LED tube with Lumificient’s current components attached - meaning their current end cap and end seal, with Loctite 4307 and Plastidip used for adhesives. The second sample is a 1 foot long assembly of the LED tube with our end cap and our glue - Loctite Quick-Set Epoxy. In some tests additional samples are used as well.

3.1.2 - Water Protection

The end cap must provide a seal from water. This is tested by using the liquid “Ingress Protection” ratings defined in the International Electrotechnical Commission Standard 60529 [10]. Our design is required to achieve a liquid IP rating of 7. The test to achieve this IP rating consists of completely immersing the sample in water. The end cap must be immersed at least 15 cm and remain there for 30 hours. After 30 hours, the sample is taken out, and the tube is cut open to examine the inside for water intrusion. If any moisture is visibly present, the sample fails.

3.1.3 - Thermal Protection

The end cap must retain its functionality and watertight seal over a range of temperatures. We will test this by thermal cycling the sample and then putting it through the “Water Protection” test discussed above. The sample is first heated to 100 C for 15-20 minutes in a conventional cooking oven and then taken out and placed in a freezer at temperature of -10 C for 15 to 20 minutes. This cycle is repeated 10 times. This test is designed to simulate the frequent thermal expansion and compression that the LED tube assembly will experience on a seasonal as well as day-to-day basis. The test we have designed is based on the JEDEC Standard JESD22-A104D [13].

3.1.4 - Pressure Resilience

The end cap must retain its functionality when a pressure difference of 12 psi is present between the inside of the tube and outside of the tube. As mentioned in Section 1.3.3, this test is designed to replicate pressure differences the tube will experience in a heated environment. The test will consist of pumping air into one end of an LED tube while the other end with an end cap attached is submerged in water. The pressure in the tube will be gradually incremented up to 12 psi and held there for 5 minutes. If water bubbles appear from the submerged end of the tube, the sample fails.

3.1.5 - Length of End Cap Assembly

The length of section at the end of the LED tube without light or with significantly modified light output (due to end cap covering the tube) must not exceed 1/2 inches. This test ensures that (1) the end cap assembly will not extend too far out or insert too far into the LED tubing and that (2) the coloring of end cap assembly does not change the light output of the tube.

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3.2 - Evaluation Results

3.2.1 - The Prototype

We developed a prototype to test how well our design meets our design requirements. Our prototype was cast from polycarbonate using the following method. First, a 3D model of our design was printed in ABS. This 3D printed model was placed in silicon that was allowed to set into a solid. The silicone was then cut open, and the 3D printed model was removed. Then wax was melted into the silicone mold and allowed to harden. This wax model was then placed in kiln cement and melted out once the cement set. Then very small polycarbonate pellets were placed in the cement mold, and heated under pressure. This created a polycarbonate end cap prototype that approximates an injection molded end cap. This end cap was adhered to an LED tube and wires using Loctite Quick Set Epoxy to create the final testing prototypes.

3.2.2 - Water Protection

The tubes need to be sealed in such a way that water cannot enter them as water causes many problems with the electronic components. To test the end caps’ resistance to water ingress, they were submerged in water at a depth of 15 centimeters for 30 hours. Then the tubes were removed from the water and the end cap was cut off to see if any water had entered the tube where the electronic components are located. The water protection tests were run on the current Lumeon construction for a number of samples. Three samples directly off the production line had no water ingress. Three Lumificient samples that had been used in the field for two years were also tested. Two out three failed. Lastly, the test was performed on a tube with our end cap prototype attached with our adhesive; this sample also passed the test. These results indicate that through use in the field, Lumificient’s end cap assembly is becoming damaged and moisture is able to seep in. The results also show that without any use in the field, our prototype provides an equal amount of water protection to Lumificient’s design. To further compare our design to Lumificient’s, we would need to put our end cap into use for a number of years, and then perform the water protection test; however, time constraints prohibit this.

3.2.3 - Thermal Resistance

Thermal cycling was done to examine the effects of changes in temperature on the water ingress protection properties of the end seal. This was done to simulate the outdoor environment the tubes could realistically be expected to endure. The thermal test the tubes were subjected to was developed based on the JEDEC Thermal Cycling Test JESD22-A104D; specifically the test was developed to approximate the “J” test condition, cycling from -10 degrees Celsius to 100 degrees Celsius for ten cycles with each cycle lasting approximately 30 minutes. After subjecting the tubes to these conditions, the water protection test described above was conducted. One sample of Lumificient’s current design was tested as well as one sample of our design. Both samples passed the test. This likely implies that our test did not do an adequate job of simulating the actual environment the tubes would be placed in. However, it also tells us that under mild thermal cycling, our design retains its water protection abilities to the same extent as Lumificient’s design.

3.2.4 - Pressure Resilience

The LED tubes are assembled and sealed off in a factory at room temperature. If the temperature is raised after the seal, pressure inside the tube will increase, and forces will push out on the end caps. Therefore, we require our design to be able to withstand a pressure of 12

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psi without leaking. Pressure resilience tests were performed on sealed tubes by securely attaching a bike pump to one end of an LED tube while the other end had an end cap. The end cap was placed in the water and the pressure was gradually pumped up to 12 psi and held there for 5 minutes. If bubbles appeared, that would mean the end cap is leaking and therefore it would fail the test. We tested one original Lumificient end cap design and one tube with our end cap prototype attached. The Lumificient sample started to bubble at 12 psi after about 10 seconds. Our design still had no bubbles after 5 minutes at 12 psi. These results indicate that our design is better able to hold a water tight seal under a pressure. The results also indicate that our end cap has a stronger seal and therefore will be able to better perform in the field for long term periods, but more testing would be needed to confirm this.

3.2.5 - Length of End Cap Assembly

In order to satisfy our customers’ needs the end cap must not protrude out a great distance from the end of the tube. Specifically, Lumificient wanted to limit this length of the end cap to less than half an inch. Testing of this was done simply by measuring the distance the end cap extended beyond the LED strip on the inside of the tube. Our designed end cap extends 0.1” from the end of the tube and .25’ into the tube.

3.3 - Discussion

3.3.1 - Strengths and Weaknesses

We believe our design provides significant improvements over Lumificient’s current end cap and end seal. The increased contact area between the end cap and the tube will provide increased water ingress protection as well as structural support for fatigue stresses. Additionally, our design simplifies the manufacturing process for Lumificient by removing the need for a separate end seal and end cap by combining them into one component. Our design also allows for the use of a single adhesive that sets relatively quickly compared to the two different glues that Lumificient currently uses which take more time to dry. These two simplifications will save Lumificient money in part costs as well as assembly costs.

The major cause of failure in the old design was that wires were passed directly through the end cap to the interior of the LED tube, which allowed water to enter into the LED chamber and led to failure. Our design still has this same feature, and while making the wires turn a corner and completely surrounding them with much more area will improve performance in regards to water ingress protection, a design where wires are not passed directly into the LED chamber would have been ideal.

Another weakness in our design is that it hasn’t been proven in the field. It performed better than 2 year old Lumificient samples in the water protection test, but these old samples had been weathered by the environment. We tried to replicate environmental conditions by thermal cycling our end cap and subjecting it to pressure; however, given our time constraints, there is no way to exactly replicate the conditions it would face over years in the field. The only way to truly find out if our design is superior in terms of water ingress protection would be to put it into use in the field for years at a time. .

Lastly, we are not entirely sure if our part is able to be successfully injection molded. While we have created prototypes, we didn’t use an injection molding method that industry uses. However, we did speak with ProtoLabs in Maple Plain, and there preliminary estimate was that the design would be able to be injection molded.

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3.3.2 - Next Steps

Lumificient can take the CAD file we provided for our final end cap design, and make whatever changes they deem necessary. They may want to perform long term tests of the tubes to validate their performance. To create the end caps, they can then send the part out to be produced by injection molding, most likely to a company in China, where their current end cap and end seal are produced. Production can then begin on the Lumeon 360 using the new end cap design, and they can phase out the old end cap design.

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4 - References [1] Alex Mak. “Corrosion of Steel, Aluminum and Copper in Electrical Applications.” [Online Article], [2013 March 6], Available at http://www.stabiloy.com/CablePublic/en-US/Information+Center/Papers+and+Studies/White+Papers/ [2] Georgia State University "P-N Junction." [Website], [2013 March 6], Available at http://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html [3] “Energy Efficient Lighting: LED Light Bulbs.” [Website], [2013 March 8], Available at http://eartheasy.com/energy-efficiency/energy-efficient-led-lighting [4] Jeremy Verier. Production Manager, Lumificient Corporation. Interview. Maple Grove, MN. January 29, 2013. [5] Patrick Delaney. Project Supervisor, Lumificient Corporation. Interview. Maple Grove, MN. January 29, 2013. [6] “What is Polycarbonate (with pictures)” [Website], [2013 February 25], Available at http://www.wisegeek.org/what-is-polycarbonate.htm [7] J. Bushman, Corrosion and Cathodic Protection Theory. Medina, OH. Bushman & Associates, Inc., pp. 3-10.

[8] D. Cullen, “Preventing Corrosion Of PCB Assemblies,” OnBoard Technology, pp. 52-57, Oct. 2008

[9] Qing Ma, Chienchen Tung, Dellang Xiao, Kegang Yue, & Xianhua Zhao, US Patent 20120250301 A1 Waterproof led lamp tube and casing of same, 2011.

[10] Masaaki Tabata, & Shigekazu Wakata, US Patent 5927725 Sealing device for a cavity of a waterproof connector housing, 1997.

[11] Maxim Integrated, “Understanding the IP (Ingress Protection) Ratings of iButton Data Loggers and Capsule” Oct 31 2007. [Online], [2013 March 8], Available at http://www.maximintegrated.com/app-notes/index.mvp/id/4126 [12] K. T. Ulrich, S. D. Eppinger, Product Design and Development. New York, NY: McGraw-Hill Companies, 2004, pp. 173-175.

[13] JEDEC Standard Temperature Cycling: JESD22-A104D. Arlington, VA. JEDEC Solid State Technology Association, pp. 5-14.