PRESSURE SENSITIVE ADHESIVE REQUIREMENTS FOR AUTOMOTIVE ELECTRONICS APPLICATIONS

Stanton Rak, PhD, Principal Staff Engr. and Section Manager, Motorola Automotive and Industrial Electronics Group, Northbrook, IL

Jinbao Jiao, PhD, Staff Engr., Motorola Automotive and Industrial Electronics Group, Northbrook, IL

Tom Gall, PhD, Sr. Staff Engr. and Group Leader, Motorola Automotive and Industrial Electronics Group, Northbrook, IL

Abstract:

Pressure sensitive adhesives (PSAs) have been used for the attachment of printed circuit boards to heat sinks in automotive electronics applications. The paper reviews the thermal, adhesive, chemical, and electrical requirements for PSAs in the aggressive automotive environment. Future application requirements and the need for enhanced PSA materials are discussed.

Introduction"

Pressure sensitive adhesives are used for joining flexible circuit substrates to aluminum heat sinks at Motorola's Automotive and Industrial Electronics Group. The final products are electronic engine control modules (ECMs), Figure 1. The ECM manages key engine parameters to reduce emissions and maximize fuel economy. The environment encountered by an ECM is more severe than a typical consumer product environment. Important design considerations include temperature stability, humidity stability, electrical insulation resistance, dielectric breakdown voltage, thermal conductivity, and, in some cases, fluid resistance. Although PSAs are available today that meet typical operating requirements, evolving electronic technologies will surpass the physical limitations of most PSAs. This paper describes existing and emerging environmental requirements for the use of PSAs in automotive electronics.

Overview of the Motorola PSA Application"

In the Motorola ECM manufactt~ng process, a flexible circuit is laminated to an aluminum heat sink with an acrylic PSA to give a "rigidized" circuit assembly. Next, the solder paste is screen-printed, the components are placed, and then the solder is reflowed in an oven giving the "Laminated Circuit" shown in Figure 1. After the components are attached, the rigidized circuit assembly is formed around a housing providing the fully assembled ECM.

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Figure 1. "Rigidized" circuit and electronic engine control module.

Automotive Electronics- Future Trends:

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Application Temperature requirements: Temperature requirements for automotive electronics commonly range between -40°C ' and 125°C. Typical acrylic based PSAs perform adequately in this temperature range. However, industry trendsare pushing the temperature requirements to 150°C and higher. Typical acrylic PSAs begin to degrade at 150°C making it necessary to find alternative materials. Some of the factors driving the operating temperature of automotive electronics are reviewed in this report. There is a general industry trend towards placement of ECMs closer to the engine where the temperature is higher. In some cases, the ECMs are mounted directly on the engine. Another factor is that semiconductor manufacturers are now capable of producing power transistor components that are rated to 175°C peak operating temperature. Previously, most power devices were rated to 150°C maximum by physical packaging limitations. Unpackaged silicon devices like flip-chips can also operate at temperatures exceeding 150°C. Additionally, the automotive industry is preparing for higher power applications operating from a new, 42V power system. Figure 2 shows the increasing trend in anticipated average electrical loads. 1 Some examples of new applications operating from the 42V system are electromechanical valve actuation,

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electronic assist power steering, and electric active suspension (Table 1). These applications will operate at higher power than today's automotive electronics. Thus, the industry trends for automotive controls are indicating the need for new materials, including PSAs, that can withstand higher temperature.

Historical and Ant ic ipated Average Electr ical Load

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Figure 2. Increasing trend in anticipated average electrical loads in the automotive industry. 1

Table 1. Electrical Loads AnticiPated for2010.1 Load Type Electromechanica 1, Engine...Va!ye .Actuator Engine Cooling Fan . . . . . . . . . . . . . . . . . . . . . . . . . .

Engine Cooling P u m p . . . . . . . . . . . . . . . . . . . . . . .

Electric Assist Powe r S t e e r i n g . . . . . . . . . . . . . .

Heated Glass (front) Electricall), Heated' Cata!yt ic 'c,,, 9 n y e r t e r ,

Electric Active Suspensio n . . . . . . . . . . . . .

Infotronic and Telematics Total

Peak Power (W) 3200 800 500

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2500 • , , , , , , , , , , , , , , , , ,

3000 12000

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Winter Avg. (W) ........ 1000

80 ._. 1 0 0 . . . . . . . . . . .

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100 _ 2010

PSA Requirements- Thermal, Adhesive, Chemical, and Electrical

Temperature Stability of an Acrylic PSA: Figure 3 shows the thermal gravimetric analysis (TGA) results for a typical acrylic PSA. At 150°C isothermal, a slight decrease in weight loss is observed after approximately 30 minutes. However, the PSA degradation accelerates after 2.5 hr. (not shown). At 175°C, the temperatm'e that new power components operate, we observe an inflection point after only 15 minutes of heating showing the onset

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of decomposition. Thus, it is believed that existing acrylic PSA materials will be unsuitable for many of the new automotive electronic applications described above.

Acrylic PSA Weight Loss at 150C and 175C

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Time (min)

Figure 3. Thermal stability of an acrylic PSA as measure by TGA.

Process Temperature Requirements: The automotive environment is severe but the production environment can also be a significant test for a PSA material. For the ECM application described in this paper, the circuit is laminated to the aluminum heat sink prior to the solder reflow process. Common solder reflow processes reach a peak temperature of>200°C, > 17°C above the liquidus temperature of eutectic 63 Sn / 37 Pb solder (melting point - 183°C). The pre-heat cycle and spike in temperature to >200°C will cause volatiles such as water or low molecular weight organics in the PSA to be expelled rapidly. The rapid outgassing of volatiles can lead to the formation of blister and void defects in the laminated assembly (Figure 4). Thus, the outgassing of volatiles is unsuitable for high volume electronic soldering processes.

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In addition to the existing challenges, there is also government legislation being proposed in different parts of the world that could ban the use of Pb in solders and drive a change to SnAg and SnAgCu alloys. These alloys have higher melting points of 221 °C and 219°C respectively, and higher reflow process temperatures. Typical reflow temperatures for the Pb-free SnAg solders can exceed 240°C. The TGA in Figure 5 shows that an acrylic PSA decomposes rapidly above 220°C. Thus, high temperature challenges exist from both design and manufacturing standpoints for automotive ECMs.

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P S A Bl is ter R e s i d u e on AI

Figure 4. Blister 'residue'

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Figure 5. TGA of an acrylic PSA at typical solder reflow temperatures.

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Peel Adhesion at Elevated Temperature: Many PSA suppliers publish the peel data at room temperature exclusively. For automotive electronics, it is important to understand the adhesive strength at the operating temperature of the application. Four acrylic PSA materials of identical thickness were peel tested 90 degrees at 25°C and 125°C using an Instron. The peel samples consisted of the flexible circuit and the aluminum rigidizer. The filled acrylics contained inorganic additives to improve thermal conductivity. The results displayed in Figure 6 show a >75% loss of peel strength at 125°C for all the PSAs evaluated. The filled acrylics #2 & #3 had < 50% of the strength of the unfilled acrylic. Filled acrylic #3 performed well in comparison to the

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unfilled acrylic. The absolute peel strength required for an application will be product dependent. Heat aging tests at 125°C for 1000 hr. and humidity aging tests at 85°C / 85% RH for 1000 hr. were also performed. The results are not presented here. It can be noted that humidity aging did not have a significant effect on any of the PSAs tested but heat aging at 125°C gave varied results that were material dependent.

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90 Degree Peel Strength of P S A Laminated Circuit vs .Temperature

Acrylic Filled Acrylic Filled Acrylic Filled Acrylic 1 2 3

PSA Material (same thickness)

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Figure 6. Peel strength of different PSA materials at 25°C and 125°C.

Outgassing'. As discussed above, volatiles are expelled from the PSA during the solder reflow process. The TGA in Figure 7 shows the weight loss of water and volatile organics from an acrylic PSA at 100°C. Approximately 0.9 wt.% total was lost during the drying step. Removal of heat after 6 hours leads to a re-absorption of water. Approximately 0.4 wt.% of water is reabsorbed. Water uptake can be reversed in a another baking step. From the data, we can determine the percentage of water and volatile organic compounds in the acrylic PSA.

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The outgassing volatile organic compounds were identified by GC-MS. They consisted of saturated chain esters, carboxylic acids, and alcohols. The water and volatile organics are believed to be responsible for the blistering observed during the reflow process (Figure 4). The soldering process requires that volatiles be eliminated. Thus, any PSA advancements in reduced water absorption or increased stability above 100°C would be welcomed.

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Drying and Water Re-absorption at ambient of Acrylic PSA

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Outgassing and Blister Formation: A blister test was also designed to evaluate the adhesion of PSA to the circuit board at high temperature. The setup of the test is schematically shown in Figure 8. An aluminum plate was used as the substrate and the PSA with a matching hole was laminated onto the aluminum. The circuit board was laminated on the top of the PSA. The relationship of the adhesion (energy release rate G), pressure, and blister size has beefl described in the literature. 2'3 For simplicity, we monitored the critical, blister-growth pressure (the pressure required for a blister to grow) at various temperatures. This allowed for estimation of relative adhesive strength at temperature.

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Pressure Needed to Form Blisters at Different Temperatures

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Temperature (C)

Figure 9. Critical pressure under circuit and PSA to initiate blistering at different temperatures.

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Shown in Figure 9, the curve is the change in critical pressure at different temperatures. When the temperature increases to 100°C, only about 1/10 of the initial pressure is needed to cause blister growth. Therefore, the blister formation is much easier at high temperature due to the weakening of the PSA adhesion. A separate test was performed (not shown) where 5 micro-liters of water was intentionally added to the interface of a rigid aluminum plate and PSA. Aluminum foil laminated to the exposed surface of the PSA. When the plate was heated to a certain temperature, a blister formed and started to propagate. It was determined that at temperatures above 150°C, blisters easily form and grow to a diameter of over 4 cm. This test also confirms that moisture at the circuit / PSA interface results in the formation of blisters, especially when temperature increases to over 200°C as in the solder reflow process. Thus, there is a balance between adhesive strength and the pressure induced by outgassing volatiles that must be understood and controlled.

Ionic Content: PSAs used in automotive electronics should be low in ionic content to minimize corrosion of electronic circuits and components. Five PSA materials were evaluated by ion chromatography (IC). The PSAs were individually extracted with deionized water at 80°C for 1 hr. The results are presented in Table 2. The identification labels are consistent throughout this report. Four of the PSAs were formulated with inorganic fillers in order to improve thermal conductivity. The acrylic PSA with no filler was 'clean.' The level of ionics was below the detection limit of our equipment. The acrylic PSAs with fillers had higher levels of anions and cations. Typical anions observed were fluoride and chloride. Cations

192

observed were sodium, calcium, and ammonia. It is believed that the inorganic fillers are responsible, in part, for the higher level of ionics in the filled PSAs. Anions such as chloride are known to create corrosion problems for electronic circuits. The acceptable level of ionics will depend on the specific application and environmental requirements. In addition to corrosion, the insulation resistance of the PSAs will also be negatively impacted by ionics.

Table 2. Ion Chromota Data for Filled and Unfilled PSAs (.Units are in u PSAType 'F ~' C;I 13r NO2 NOa PO4 SO4 Total Anions L

i

Acrylic . . . . . . . . . . . . . . . . . 0.0. Filled Acrylic ! 1.1 1.1 _ Filled Ac~11¢2 5.7 2.8 8.5 Fllled Acrylic 3 412 ...... 110 . . . . . . . . . . . . . . . 15.2 . . . . . . . . Filled Acrylic 4 5.8 0.6 6.4 6.0 14.8 20.7

Note- Empty fields indicate the ion leVel Was below detection iimit of equipment (~ (3il-ug.ig.) . . . . . . . . . . . . . . . . . . .

Na K Mg Ca NHa Total Cations 0.0

_ _

9.1 9.1 _

9.3 6.9 16.2 11.3 11.3

Electronic Properties" The PSA ionic content level is important for another reason in addition to corrosion. As show in the drawing in Figure 10, the PSA performs as a dielectric barrier between the circuit board and the aluminum heat sink. Thus, ionics will influence the PSA's ability to act as a dielectric barrier and can cause current to leak from the circuit to the heat sink. Insulation resistance is often expressed as volume resistivity. PSA manufacturers typically report volume resistivity at room temperature. For automotive electronics applications, however, the volume resistivity at operating temperature is a more pertinent measure of a PSA's capabilities. The volume resistivities of the five PSAs listed in Table 2 were measured from 25°C to 100°C. The test applied was an in-house method where a circuit with a 2.0" diameter, exposed Cu disk was laminated with the respective PSA to an aluminum heat sink. The resistance of the PSA between the Cu disk and aluminum heat sink was measured using a Keithley 617 Programmable Electrometer. A hot plate was used to control the temperature. The results are presented in Figure 11. The best performing PSA at 25°C was the unfilled acrylic. The filled acrylics gave lower volume resistances which we believe are attributed to the higher levels of ionics found in the IC work. As the temperature was increased, the volume resistivity of all of the PSAs decreased. Two of the filled materials approached 1,000 Mf~ at 100C. We would be concerned about a volume resistivity < 100 M ~ at the operating temperature of the control module.

Dielectric breakdown voltage is also an important parameter. Applications operating at high voltage can cause a 'punch-through' or shorting of the circuit to the aluminum heat sink. This test can also be utilized to screen for pinholes in PSAs. The requirements will be program specific. As with other physical tests described in this report, it is important to measure the dielectric breakdown voltage at the maximum operating temperature of the control module.

Thermal \

Power device

evi ce case

~-- Circuit Board ~-- Acrylic PSA ~ - Aluminum Heatsink

Figure 10. Cross-section drawing of the laminated circuit assembly with an attached power device.

193

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V o l u m e R e s i s t i v i t y v s . T e m p e r a t u r e

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25C 50C 75C 100C

" , Acrylic

- - B - - Riled Acrylic1

Riled Acrylic2

~ - ~ , ~ Riled Acrylic3

- ,,, Filled Acrylic4

Temperature

Figure 11. Volume resistivity vs. temperature for five PSA materials

Thermal Conductivity: The PSA must serve in more ways than being a fastener and a dielectric. It should also serve as a good thermal path -- thus explaining our interest in filled PSAs. The test vehicle shown in Figure 12 was used to compare the thermal resistance of filled and unfilled acrylic PSAs having identical thickness. Identical sections of circuit boards having five, T0-220 power device packages soldered in place were prepared. The circuit assembly was placed on a water-cooled heat sink with pressure on the plate only. In another set of tests, a 20 lb. force was applied to the back of each power device to determine the significance of pressure on the thermal resistance. One device at a time was powered-on. V, I, and tab- to-plate temperature drop were measured and Rth was then calculated. Surprising to us, the unfilled acrylic PSA thermally outperformed the filled acrylic PSA when no pressure was applied to the power devices (Figure 13). The unfilled material had a specified thermal conductivity of-~ 0.2 W/m-°C and the filled material had a specified thermal conductivity of N 1.0 W/m-°C. When a 20 lb. force was applied to the back of each power device, the filled PSA thermally outperformed the unfilled PSA. For most automotive electronic packaging applications, the use of clamps and fasteners to provide a constant force is an undesirable option.

Separately, we performed thermal conductivity testing using a Holometrix k-tester. We were able to estimate the thermal contact resistance contribution to be >50% for the PSAs from the combined thermal test results. Thus, contact resistance essentially washed out any benefit of using a thermally conductive, filled PSA material.

1 9 4

FI Circuit

Surface Mount TO-220's powered "on" Force

PSA Material

Figure 12. Test vehicle for measuring thermal resistance !

Effects of Pressure on Thermal Resistance

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t~ 5

n,, 4.5 t~

4

3.5

Unfilled acrylic PSA (Tc = ~ 0.2 W/m-C)

Filled acrylic PSA (Tc =--. 1.0 W/m-C)

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No pressure applied to 20lb. Pressure applied to power devices power devices

Pressure Effect

Figure 13. Comparison of filled and unfilled PSAs at different pressures

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Additional Requirements" The thermal, adhesive, chemical, and electrical requirements for PSAs in automotive engine control applications are described above. There are additional requirements that cannot be overlooked for some specific applications. One such property is fluid compatibility. More automotive electronic applications are appearing near or in automotive fluids. PSAs are required to maintain their adhesion, even when submerged in fluids. Another area that can draw attention in the automotive environment is vibration. The PSAs should maintain adhesion in a typical engine mount application at 20G @ 2000 Hz.

Summary: The trend in automotive engine control electronics is towards higher operating temperatures. The movement of the controls closer to the engine and the higher power loads associated with new electromechanical technologies are going to drive the change. Power component capabilities are already being increased from 150°C to 175°C that will enable the change. Thus, all packaging materials

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including PSA will need to perform at 175°C. The TGA data presented in this report show that typical acrylic PSAs cannot meet the higher temperature requirements.

Manufacturing is another area where enhanced PSA properties are needed. Managing the outgassing of water and organic volatiles in elevated temperature processes like soldering is a struggle. It's not going to get any easier assuming manufactures begin to switch to lead-free solder alloys, many of which require higher processing temperatures.

This paper gave an overview of the thermal, adhesive, chemical, and electrical requirements for PSAs in automotive engine control electronics. It is difficult to separate the properties because they tend to blend together---especially at elevated temperatures. The tape must be developed with intentions of being a dielectric insulator and a thermal conductor in addition to being a mechanical fastener. Most of the data presented were collected in tests evolving from industry standard tests. Some tests were more specific like the thermal conductivity measurements. The main point to remember when developing PSAs for automotive electronics applications is to perform the physical characterization tests at the temperature extremes of the applications.

Literature Citations: 1. Nicastri, P. and Huang, H., "42V PowerNet: Providing the Vehicle Electrical Power for the 21 st

Century," Presented at the SAE International - Future of Transportation Technology Conference in Costa Mesa, CA; August 2000, Report # 2000-01-3050.

2. Chu, Y.Z. and Durning, C.J. (1992), "Application of the Blister Test to the Study of Polymer- Polymer Adhesion," J. Applied Polymer Science, Vol. 45, pp. 1151-1164.

3. Lai, Y-H. and Dillard, D.A. (1996), "A Comparison of Energy Release Rates in Different Membrane Blister and Peel Tests," J. Adhesion, Vol. 56, pp. 59-78.

A cknowled gemen ts: We would like to thank the following people and organizations. Jim Elfering of Motorola for collecting the ion chromatography and electrical insulation resistance data. Tony Asghari of Motorola for creating Figure 10. Steve Larson of Motorola made the thermal contact resistance estimate. John Zimmerman of Motorola for supporting the work. We collaborated with a PSA supplier for the test described in Figure 12. Comell University assisted with some of the thermal and high temperature peel testing.

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