solectron technology, inc. · 2018. 6. 13. · solectron solectron technology, inc.: solectron...

I

# SOLECTRON

PROJECT SUMMARY:

Solectron Technology Inc. has completed its initial product conversion to VOC-free, no-clean fluxes for assembling printed circuit boards. One product had been converted (now discontinued because it reached end of product life), and a second is starting with the new process.

The conversion offers the advantages of eliminating the Volatile Organic Compounds, solid wastes, liquid hazardous wastes and the handling costs associated with the aqueous washable flux systems previously used. Using a 100,000 board basis (the planned production build of the new card over the next 40 weeks), a reduction of 1176 Ibs of solid waste, 351gallons of hazardous liquid waste and 1033 Ibs of volatile organic emissions will have been eliminated from Solectron’s facility. Yield impacts are negligible in the SMT processes. Yield impact in the wave solder area will be heavily board design dependent. The first card implemented had an increase of 1.5-2 bridgedboard, whereas the second has a simpler design and a yield results will be comparable to an aqueous flux systems’ results.

Significant work and investments were required to convert the process to the new materials. The length of the pay back period financial results will be strongly influenced by the length of time it takes Solectron to reach maximum cleaning capacity within the factory. While excess capacity exists (as it does now), the conversion eliminates the labor associated with the washing processes, but does not eliminate the cost of operating the existing cleaners or eliminate the need to buy new cleaners. Using the 100,000 board basis for this business case, a 3.4 year pay back is expected. Once full cleaning capacity is reached, the conversion to the new process eliminates the need for further investments in cleaners and their operating costs. This situation dramatically reduces the pay back period to .5 years. Although Solectron is not currently at maximum cleaning capacity, given its dramatic growth rate, it will be in the foreseeable future. A pay back period somewhere between the two figures above is expected.

Solectron Technology, Inc. 6900 Mallard Creek Road, P.O. Box 562148, Charlotte, N.C. 28256-2148

Phone (704) 598-3300 Fax (704) 598-3333

# SOLECTRON

SOLECTRON TECHNOLOGY, INC.: Solectron Technology Incorporated (STI) is a contract assembler of printed circuit boards for the electronics industry. Established as a wholly owned subsidiary of Solectron Corporation of Milpitas, California, STI has been in operation at its present facility in Charlotte since 1992 and presently employs about 720 regular and 300 contract employees. Hours of operation are normally three shifts, 5 days a week.

INTRODUCTION TO THE PROBLEM: Although Solectron offers a wide array of pre-manufacturing (such as printed circuit board design), manufacturing (such as printed circuit board assembly and final “box” assembly), and post manufacturing (such as refurbishment and repair) services, the majority of its work is the board assembly process itself. To do this, many sophisticated processes are used through which various electronic components are soldered to printed circuit boards. These processes utilize a fluxing step, by which an acidic compound is used to “clean” chemical oxides from the leads and board, followed by the addition of molten solder to the cleaned joint in such a way that the solder forms a metallurgical bond between the component and board. Although there are several different processes by which soldering is done, volatile materials are inevitably driven from the flux, and a flux residue is left on the finished board. These two results impact the manufacturing process in that they often become environmental and board quality issues.

The flux’s solvent system and the nature of the acidic compounds themselves are the primary determinants on the extent of environmental issues the process will present. Earlier technologies used “rosin” type fluxes, which left a residue on the board that was still “active” (requiring cleaning), but were soluble only in such solvents as chlorinated fluorocarbons (CFC’s). The resulting processes therefore released significant amounts of Volatile Organic Compounds (VOC’s) to the atmosphere, and required costly cleaning step(s) using exotic solvents with all their inherent environmental complications. The Montreal Protocol set the industry into motion to eliminate CFC’s from their processes. A conversion to aqueous fluxes was the result. Aqueous fluxes utilize a water soluble solvent system so that the residues can be removed with a water cleaner, eliminating the CFC’s.

Since its inception in Charlotte, Solectron Technology has used such an aqueous based flux system for its board production. Although the process is far superior to the rosin systems requiring CFC’s, they still present two additional areas for potential further “environmental improvements”. First, most aqueous based fluxes use a water and alcohol blend for the solvent system. Thus, alcohol is emitted to the environment in the form of VOC’s. Second, its residues are still active and must be removed. Although CFC’s are not needed to remove them, significant capital, energy, and disposal costs are still associated with removing the residue, treating the waste water, and paying for solid waste disposal. For these reasons, Solectron has been working to convert to new flux systems which contain non-VOC solvent systems, and whose residues are inactive (thus being referred to as “no-clean” fluxes). These materials offer these two “final”

Solectron Technology, Inc. 6900 Mallard Creek Road, P.O. Box 562148, Charlotte, N.C. 28256-2148

Phone (704) 598-3300 Fax (704) 598-3333

improvements for the environment. In addition to eliminating the VOC’s, the residues left on the board are harmless, eliminating the entire cleaning process, with all of its inherent costs and waste disposal issues.

These conversions are not trivial, however. A number of issues have to be addressed. For example, a significant amount of work has to be done to first qualify the material systems for the user’s processes and end use products. In addition to picking the material for ease in processing, the selection must reflect the material’s long term impact on board performance. This selection and testing phase alone takes several months, and was completed at Solectron’s Milpitas, California facility. Once that work is done, extensive additional work is still required. The changes in the materials dictate very fundamental process changes (such as flux application method), which dictate new pieces of capital equipment. Once those are in place, process optimization must occur for the board(s) to be run. Next, the process must be documented and the operators trained. After that is done, customers must be converted to the new process. Based on their end product usage/application, the type of board in question, and even their pre-conceived notions about no-clean flux chemistry, they may be quite willing or very reluctant to convert. Therefore, the conversion rate is variable within a factory by customer. Therefore, often the best conversion strategy is to work with a customer familiar and comfortable with no-clean on a high volume board. Once that board is in production, work can begin on the next, and continue until all likely candidates are being run with the no-clean process.

TECHNICAL DESCRIPTION OF EXISTING PROCESS:

Surface Mount Process:

Surface Mount Technology (SMT) components are designed to rest on the top surface of the board with the individual leads placed directly onto a “pad”, where the joint is made. This is done by first screening a solder paste (mixture of very small solder particles and flux) through a stencil so that discrete deposits of solder paste are applied directly onto the board’s pads. The board is then loaded into a “placer” which places the SMT components in their proper position. At this point in the process each discrete deposit of paste is sandwiched between the board’s pad and the component‘s lead. The board is then carefully passed through a reflow oven, whose function is to heat the board, paste, and components in a very controlled manner. During this “reflow” process, the fluxes are activated (the addition of heat increases the rate of the chemical reaction) so that the oxides on board and leads are removed. The volatiles are then driven off, and the temperature increased until the solder particles melt, forming the metallurgical bond between the pad and leads. The board is then cooled permitting the solder to harden.

STI has historically used Alpha 1208@water soluble paste for this process. By weight it is 8590% solids (solder) and IO-15% flux. VOC’s account for 2.5% of the paste. In normal production, VOC emissions are estimated to be .064 Ibs/machine/day. In 1995 STI emitted 208 pounds of VOC from its reflow ovens. Since 1208@ is not a no-clean formulation, its residues must be removed in an in-line aqueous cleaner. In-line cleaners cost anywhere from $60,000-$200,000 new and about $36/hour/machine to run (this number includes maintenance labor, chemicals, and energy). In addition, the disposal of the waste filter cake adds another $1.25/hour/machine manufacturing costs.

Occasionally boards are “misprinted” (the paste is not screened in the proper positions, or not deposited as discrete deposits) and the paste has to be removed, the board cleaned, and the

@ Alpha 1208 is a registered trademark of Alpha Metals Corporation

D.W. Greenfield 2 Solectron Technology, Inc.

process started over. This is done by wiping the excess paste from the board and then passing it through the same in-line aqueous cleaner used to clean residues off.

Wave and Touch-up/Hand Soldering Processes:

Pin Through Hole (PTH) components are designed so that the leads stick through holes in the board. The joints are formed by first inserting the components into the board and then loading it onto the conveyor of a wave solder machine. As the board travels along on the conveyor, flux is first applied to the bottom of the assembled board. The board is then preheated to get it near soldering temperatures, activate the flux, and drive off the volatile solvents in the flux system. Once that is completed, the board is carefully passed over a “wave” of molten solder so that only the bottom of the board is exposed to the wave. During the 1-2 seconds that the wave is in contact with each pin and pad, the solder “wets” their surfaces and flows up through the hole. It then exits the wave, and cools to form the joint.

STI has historically used Lonco 3355-1 I@’ for wave soldering. Its solvent system consists of 42% Isopropyl Alcohol (IPA) and 39% water. Both its composition and the application process result in a significant VOC emissions. The flux is applied by bubbling air up through a reservoir of the flux so that a “foam head” is formed. The assembled board is passed over this head during processing, so that a film of flux is smeared over the bottom of the board. During the preheating and soldering process 100% of the IPA is driven off into the exhaust stream. In addition, when air is bubbled through the flux reservoir, a significant amount of IPA is driven off, even when boards are not being fluxed. This is due both to the volatility of the IPA and the large surface area the flux foam head presents to the atmosphere. As the IPA evaporates, the solids level in the flux increases. In order to maintain the optimum solids level for soldering, additional IPA has to be periodically added to the flux reservoir. In 1995 STl’s actual VOC from wave and hand soldering was 32,510 pounds, or about 8100 pounds per wave solder machine. Of this quantity, the vast majority can be attributed to the wave soldering process itself.

The residues are removed with the same in-line cleaners used for washing the boards after they complete SMT reflow. Currently there are three such cleaners in operation at STI, one to support SMT and two to support wave and hand soldering. Their total operating costs would therefore be the same, at $37.25/hour/machine.

Hand Soldering/Touch-up:

(STI permits hand soldering under the same permit as the wave solder machines; the emissions are included in the numbers mentioned above.)

TECHNICAL DESCRIPTION OF THE NEW VOC-FREE, NO- CLEAN PROCESS:

SMT Process:

STI is using Alpha LR725@’ for its no-clean SMT operation. The new process does not present any major changes for the actual SMT process. The processes of screening the paste on the board and reflowing the paste have only minor changes, if any at all. The placing process is identical. The most significant change occurs in the cleaning of misprinted boards and stencils

‘ Lonco 3355-1 1 is a registered trademark of Alpha Metals Corporation. Alpha LR725 is a registered trademark of Alpha Metals Corporation. 2


after use. The SMT paste is not soluble in water alone, but requires the addition of a saponifier to the water to help cleaning. While cleaning misprinted boards and dirty stencils is an intermittent event, is does require the purchase of a new stencillboard cleaning machine capable of handling the saponifier. STI has purchased an Austin American batch cleaner for this purpose.

Wave Soldering Process and Touch-up/Hand Soldering:

The VOC-free, no-clean process presents significant technical hurdles for wave soldering. First, it is important to apply a more controlled, thinner layer of flux to the bottom of the board. Second, the lack of IPA makes it difficult to bubble (as was done with the Lonco 3355-1 I@). For these two reasons, the flux is now applied through a spray fluxer. STI has purchased two Sensbey Microx spray fluxers for this purpose. Currently one is on-line; the other will be brought on-line once more product is converted. This major change dictates a significant amount of work to be done, including: process optimization, procedure documentation, establishing maintenance spare parts in the tool crib, maintenance procedures, training, etc.

The second major hurdle associated with no-clean wave solder fluxes is that the solids are not nearly as active as those in water soluble fluxes. As such, the “process” window (the range in process parameters over which the variable settings may vary and still produce acceptable product) is very significantly reduced. In order to increase it as much as possible, additional capital is spent on the wave solder machine. First, the soldering process will perform better if oxygen is not present around the wave. Therefore, a nitrogen inerting wave was added to the unit. Second, the flux requires a more uniform preheating than was previously required. Rather than using a radiant type preheater, as was used on earlier machines, a convection preheater is used. Both of these items add cost to the machine. Even with these, however, the process still has a much tighter window, with a higher level of touch-up (the degree dependent upon the board complexity), the result. Extensive work is required to convert the process over.

Hand and touch-up soldering require a different technique when soldering with no-clean fluxes. These differences are taught in STI’s hand soldering classes.

ASSESSMENT OF WASTE REDUCED DURING PROJECT PERIOD, AND FINANCIAL ANALYSIS:

REDUCTION IN WASTES:

Elimination of Waste in the SMT Process:

Assumptions/Data:

1 ). 2). 3). 4).

5). 6). Three shifts / day.

Production rate of 2 boardslminute. VOC emissions of .064 Ibs/day/machine with Alpha 1208@ paste. Filter cake production of 1.5 Ibs/hour of cleaner operation. Volume of filter cake production associated with use of stencil cleaner insignificant in comparison to board washing. Cleaning rate = 5 boardslminute.

Hours of Production Required to Process 100,000 boards through SMT Attach Process = (1 00,000 boards) x (1 minute / 2 boards) x (1 hour / 60 minutes) x (1 / .85 utilization) = 980 hours of production.

D. W. Greenfield 4 Solectron Technology, Inc.

Hours of Production Required to Process 100,000 boards through SMT Wash Process = (100,000 boards) x (1 minute / 5 boards) x (1 hour / 60 minutes) x (1 / .85 utilization) = 392 hours of production.

VOC Eliminated = (980 hours of production) x (.064 Ibs VOC / day) x (1 day / 24 hours) = 2.6 Ibs of VOC emissions eliminated.

Filter Cake Reduction = (392 hours of production) x (1.5 Ibs filter cake / hour) = 588 Ibs of filter cake eliminated.

Elimination of Waste in the Wave Solder process:

Assumptions/Data:

1). 2).

3).

4). 5). 6). Two shifts per day.

Production rate of 3 boards / minute. VOC emissions of 6340 Ibs / year / machine (IPA in flux and IPA make up for solids level control). Disposal of waste IPAlLonco 3355-1 I0 (60/40 w/w%) at 2200 gallons/year/machine. Cost of disposal at $375 / 53 gallon drum. Filter cake production of 1.5 Ibs/hour of cleaner operation. Cleaning rate = 5 boardslminute.

Hours of Production Required to Process 100,000 boards through Slide LineWave Soldering Process = (1 00,000 boards) x ( I minute / 3 boards) x (1 hour / 60 minutes) x (1 / .85 utilization) = 654 hours of production.

Hours of Production Required to Process 100,000 boards through Post Wave Wash Process = (100,000 boards) x (1 minute / 5 boards) x (1 hour / 60 minutes) x (1 / .85 utilization) = 392 hours of production.

VOC Reduction = (654 hours of production) x (1 shift / 8 hours) x ( 1 day / 2 shifts) x x (1 year production / 256 days) x (6340 Ibs VOC / 1 year production) = 1012 Ibs VOc emissions eliminated.

Liquid Waste Elimination = (654 hours of production) x (1 shift / 8 hours) x ( I day / 2 shifts) x (1 year production I 256 days) x (2200 gallons I year /machine) = 351 gallons of liquid hazardous waste eliminated.

Filter Cake Reduction = (392 hours of production) x (1.5 Ibs filter cake / hour) = -- 588 Ibs of filter cake eliminated.

Elimination of waste in the Hand Soldering / Touch-Up Process:

Assumptions/Data:

1). 2). 3).

.25 gallons touch-up flux used per week of wave production. 654 hours of wave production time required to build 100,000 boards. VOC content of Kester 450 NFB3 touch-up flux is 92%, SG = 1.154.

Kester 450NF is a registered trademark of Kester Corporation.


VOC reduction = (.25 gallons / week) x (1 week / 5 days) x ( I day / 2 shifts) x (1 shift / 8 hours) x (654 hours production) x (8.34 Ibs / gallon) x (1.154 s.g.) x (.92 Ibs VOC / 1 Ib of liquid) = 18.1 Ibs VOC eliminated.

SAVINGS ASSOClATED WlTH CONVERTING TO NO-CLEAN: While the estimation of waste elimination associated with conversion to no-clean processes is fairly straight forward (based on known usage rates of chemicals, VOC contents, etc.), the calculation of savings and pay back is not. Some items, such as the labor associated with loading and unloading the boards into cleaners is immediately realized as an “out-of-pocket” savings. Other items, such as the cost of running the cleaners (and buying new units), is clearly related to where the factory is at, relative to the maximum machine capacity. For example, if the factory is currently running at 75% capacity through its cleaners and a “no-clean board” is added to production, then certain costs (such as labor for loading and unloading boards from the cleaner) will be realized. Savings such as the $37.25 / hour to run the cleaner cannot be claimed as a savings since the machine has the capacity to accommodate the new boards and will be running regardless of the process used. On the other hand, when the wash process is near or at capacity, the addition of the board with a no-clean process presents a much more favorable business case. In addition to the labor elimination already mentioned, it now permits the company to avoid the purchase of the cleaner (at a cost of $60,000 - $200,000) and the costs associated with running that new cleaner, at $ 37.25 / hour.

Currently STI cleaning processes are not at capacity. Two factors contribute to make this a temporary situation, however. Future plans include moving part of the operations (with one cleaner) to a second building for further expansion. This, coupled with STl’s strong growth, will result in an “at capacity” situation at some point in the future. Therefore, while the full savings associated with no-clean processing will not be immediately realized, it is only a matter of time before they will. For this reasons, the savings and pay back will be analyzed both as an “under capacity” case and as an “at capacity” case.

SMT Process:

Assumptions/Data:

Cost of labor = $ 13.00 / hour. Cost of running cleaners = $37.25 / hour. Cleaning rate = 5 boards / minute. The misprint (board cleaning) and stencil cleaning rates are unchanged from the existing process. Costs of solder pastes are equivalent. Alpha 21 I O @ saponifier at $25 / gal, 1 gal / day consumption. 980 hours of production required to process the 100,000 boards through the SMT attach process. 392 hours of production required to process the 100,000 boards through the SMT wash process.


“UNDER CAPACITY” SAVINGS:

Labor Savings to LoadlUnload Cleaner = (392 hours to wash boards) x ($1 3.00 / hour / operator) x (2 operators) = $10.192.

Additional expenditures for Alpha 21100 Saponifier = (980 hours of production) x (1 shift / 8 hours) x (1 day / 3 shifts) x 1 gal / day x $25 / gal = [$1.021).

Net Savings = $ 10,192 - $ 1,021 = $9.171.

ADDITIONAL “A T CAPACITY” SA VINGS:

Cleaner Operation Savings = (392 hours of cleaner production time) x ($37.25 / hour)

Cleaner Purchase Savings = $ 150.000 (one unit for both SMT and Wave cleanina). = $14.602.

Wave Solder Process:

Assumptions/Data:

Cost of labor = $13.00 / hour. The labor associated with maintaining the spray fluxer is equal to that of maintaining and monitoring specific gravity of the foam fluxers. Cost of running cleaners = $37.25 / hour. Cleaning rate = 5 boards / minute. Elimination of pallet cleaning of once / 4 hours of operation. 654 hours of production required to process the 100,000 boards through the wave soldering process. 392 hours of production required to process the 100,000 boards through the post wave wash process. Cost of Nitrogen gas = $ .21/ 100 cf. Nitrogen gas consumption is 500 cfh. Lonco 3355-1 1 0 cost at $15.14 / gallon. Consumption of 21 gallon / machine /week. Kester 97OSO4 VOC-free, no-clean flux at $17.00 / gallon. Consumption at 10 gallons / week. IPA at $ 3.29 / gallon. Consumption of 23 gallons / machine / week.


Labor Savings to LoadlUnload Cleaner = (392 production hours) x ($13.00 / hour / operator) x (2 operators) = $ 10.192.

Flux Savings Associated with Conversion = (654 hours of production) x (Ishift / 8 hours) x ( I day / 2 shifts) x (1 week I 5 days) x (22 gal /week) x ($15.14 / gal) - (654 hours of production) x (1 shift / 8 hours) x (1 day / 2 shifts) x (1 week / 5 days) x (IO gal /week) x ($17.00 /gal) = $1382.

Savings Associated with the Elimination of IPA = (654 hours of production) x (1 shift / 8 hours) x (1 day / 2 shifts) x ( I week / 5 days) x (23 gal / week) x ($3.29 I gal) = $61 7.

($21 / 100 c9. = [$687.) Additional expenditures for Nitrogen = (654 hours of production) x (500 cf / hour) x

Net Savings = $10,192 + $1382 + 617 - $687 = $11.504.

Kester 970s is a registered trademark of Kester Corporation.


ADDITIONAL “AT CAPACITY” SAVINGS:

Cleaner Operation Savings = (392 hours of production required) x ($37.25 / hour) = $14.602.

Cleaner Operation Savings Associated with Elimination of Pallet Cleaning = (654 production hours at wave) x (1 pallet wash / 4 hours) x (.25 hours /wash) x ($37.25 / hour)= $1 523.

Hand and Touch-Up Solder Process:

Assumptions/Data:

1). 2).

No net savings for soldering. Savings for wash elimination accounted for in wave soldering analysis.

In Circuit Test (ICT) Repair Touch-Up Solder Process:

Assumptions/Data:

1). 2). 3).

Assumed yield at ICT = .97. No net savings for hand soldering associated with repair work. Washing after repair labor = 10 minutes per board. (Repaired boards at ICT tend to be run singly, or in small groups with the repair operator catching hidher own boards through the cleaner.)


Labor Savings to LoadlUnload Cleaner = (100,000 boards) x (3 boards repaired / 100 boards) x (IO minutes labor / board repaired) x (1 hour / 60 minutes) x ($13.00 / hour) = $6.500.

ADDITIONAL “AT CAPACITY” SAVINGS:

Cleaner Operation Savings = (1 00,000 boards) x (3 boards repaired / 100 boards) x (1 minute / 5 boards) x (1 hour / 60 minutes) x ($37.25 / hour) = $373.

CAPITAL COSTS ASSOCIATED WITH CONVERTING TO THE NO-CLEAN PROCESS:

Receipts are shown in Appendix A.

SMT Process:

Capital: Austin American Stencil / Board Cleaner: Refrigerator for No-clean solder paste:

$53,995 $ 465


Wave Solder Process:

Capital: Sensbey Spray Fluxer: $ 42,290 Contour Wave for Wave Solder Machine: $ 9,750 Convection Preheat: $ 12,000 Modifications to Spray Fluxer: $ 1,386 Spray Fluxer Filter Box: $ 846

TOTAL EXPENDITURES: $ 120,732

TOTAL SAVINGS FOR 100,000 BOARD BUILD: “UNDER CAPACITY” SAVINGS: $ 27,175 “AT CAPACITY” SAVINGS: $208,275

The current planned build rate for the board is 2,500 /week. At this rate of production, pay back times can be calculated for both the “under capacity” and “at capacity” business cases:

WNDER CAPACITY” PAY BACK = (100,000 boards / $27,275) x (1 week / 2,500 boards) x ($120, 732) x (1 year / 52 weeks) = 3.4vears.

“AT CAPACITY” PAY BACK = (100,000 boards / $208,275) x (1 week / 2,500 boards) x ($120,732) x (1 year / 52 weeks) = .5 vears.

Although STI is not currently at capacity with regard to cleaning, given its growth rate, the actual pay back will inevitably end up somewhere between the two cases shown above. It is also quite evident that its ability to avoid the purchase of a new cleaner (and its operating costs) is the strongest lever it has in shortening the pay back period.

ADVANTAGES/DISADVANTAGES OF PROJECT IMPLEMENTATION: The obvious advantages and disadvantages of no-clean are reflected in the waste reduction and costing discussions. VOC free no-clean flux systems significantly reduce VOC emissions while eliminating cleaning. This in turn eliminates de-ionized water generation requirements, energy, capital expense, and water treatment expenditures associated with cleaning. The process does drive its own capital requirements, however, in addition to tighter process windows. Two other aspects have not yet been discussed. In some cases, a no-clean process will eliminate hand soldering of non-wettable components onto boards. Also, on some boards, the tighter process window will manifest itself in the form of lower yields. Both are discussed below.

NON-WETTABLE COMPONENTS:

Occasionally a board is designed with a component not designed to withstand the cleaning process. Once moisture penetrates inside the component, it is impossible to dry, with a long term field failure the expected result. In those cases, the boards are processed in the normal manner, except that the non-wettable component is left off. After the board is built, the component is typically hand soldered on (a slow, and less reliable process) with a no-clean hand soldering flux. Obviously, if the entire board is built with a no-clean process, this additional step is eliminated.

D . W. Greenfield 9 Solectron Technology, Inc.

YIELD / QUALITY:

From an overall product reliability standpoint, solder joints made with a properly qualified and run no-clean flux will be equivalent to those made with an aqueous (or other) flux system. The primary differences show up in the processing of the boards themselves, where different yields may result.

SMT:

The SMT no-clean screening process is at least equivalent to the aqueous approach. As mentioned above, the LR725Q is not water soluble (hence, the need for the saponifier to aid in cleaning the stencil and misprinted boards). While this dictates the need for a new piece of equipment, it also makes the paste less sensitive to humidity in the environment. (Water soluble pastes tend to be hygroscopic. As they absorb moisture, their properties change. If enough moisture is absorbed, the paste may “slump” rather than staying in discreet deposits on the pads. If that happens, the yield will drop due to increased bridging between adjacent pads.

Also, such pastes usually tend to hold their “tack or stickiness longer, resulting in a longer process window between the time the paste is screened on and the time it is reflowed.

Under normal processing conditions, neither of these tend to be a big yield factor. Overall, the SMT yields are considered similar.

Wave Soldering:

The yield impact at wave solder is a strong function of the board being run. The lower solids level tends to result in a system more prone to bridging, but that does not necessarily mean that more bridging wi// occur. For instance, the computer board first converted to no-clean had some closely spaced leads. It also was being run with a process which, although advantageous overall, dictates a special wave solder pallet which aggravates the bridging tendencies at the wave. Regardless, with a proper profile in the aqueous system, the board could be run virtually bridge free. This was not the case in the no-clean system, however. The initial designed experiments resulted in besf case results of 8-12 bridges / board. Further experimentation, tooling and process work reduced that down to where it was running less than 2 bridges / board when production was started. Once those were touched up, the ulimate overall rolled yield fell by about I-2%.

Early work on the new board suggests much more favorable performance throughout the process. The SMT yields will again be comparable. The board design is simple enough that bridging should not be a problem at the wave. Consequently, comparable yields are anticipated through the entire process.

RECOMMENDATIONS FOR FURTHER STUDY AND TESTING: Further work will fall into two areas. First, as new boards are selected for the no-clean process, specific work will be done to optimize the process for that particular design. The process of doing this will obviously provide more knowledge which can be applied to other, similar designs. Second, STI plans to do some fundamental work with the no-clean process to evaluate the fundamental tendencies to bridge, and how best to reduce that bridging through board design modifications. This work, though long term, will hopefully provide fundamental design approaches which can be applied to all new boards slated for both the aqueous and no-clean processes.


I t ' * .

SUMMARY The no-clean process should provide the following savings (all numbers reflect a 100,000 board basis and 2500 board /week build rate, where appropriate):

The financial pay back period will be somewhere between .5 years and 3.4 years, depending upon how quickly the cleaning capacity is reached, and whether a sufficient amount of product is converted to no-clean so that the purchase and operational expenses of additional cleaners can be avoided.


solectron technology, inc. · 2018. 6. 13. · solectron solectron technology, inc.: solectron...

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