AQUEOUS CI,rlAN I IiG 2ZR F.3RPi'YSC E Janet H. Sterritt Sterritt Associates, Inc. 31 Irene Drive. Hollis, New Hampshire

Transcription

1 ~ contaminants 6th International S'WE Electronics Conference.June 22-25, 1-99? AQUEOUS CI,rlAN I IiG 2ZR F.3RPi'YSC E Janet H. Sterritt Sterritt Associates, Inc. 31 Irene Drive Hollis, New Hampshire ABSTRAcT A new awareness of the long term environmental effects - resulting from cleaning materials has resulted in the pursuit of alternative materials and methods. One alternative is the use of water, and water based chemistries to clean electronic subassemblies and their components. The key issue in the use af water is whether aqueous cleaning can perform as well or better than solvent based methods, and therefore be an acceptable replacement. This paper discusses the physical and chemical properties necessary to provide cleaning performance. Test data shows that aqueous cleaning is a viable alternative to solvent methods. KEYWORDS: Flux; Rosin; Saponifier; Solvent; Water Soluble 1. INTR ODUCTm Printed circuit board (PCB) cleaning consists of removing ionic and flux residues from the entire PCB, especially an all leads, traces, and test points. The most challenging region are the areas under top side surface mount (SMT) conponents and connectors. mp side components are generally

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3 I 2- AOUEOUS CT4EANI.P.c - ti MECHANICS The most important parameter in aqueous cleaninq mechanics is the force with which the cleaninq fluid passes under components and over the surface of the board. This mechanical force erodes contamination from the desired surfaces. The fluid delivery system needs to be optimized. to provide sufficient impact force uniformly over the surface of the board. Several nozzle designs offer various implementation styles. A flood jet releases a large volume of fluid in a low pressure pattern over a wide distance. Flood jets typically show a wide variance from center to edge in pressure. A fan jet disperses fluid under higher pressure over a narrow band, suffering minor losses in performance from center to edge. A new nozzle was created just for aqueous cleaning which would disperse an even flow of fluid at an even pressure across its length. The nozzle is a high pressure and flow device and is commonly referred to as the Hurricane Jet. The nozzles and pressure diagrams showing lateral fluid distribution are shown in Fiyuro 1. Good cleaning coverage is achieved by assembling these nozzles into manifolds, as shown in Figure 2.

4 l---k RINSE FINAL- I WASTE 1 N COMI NG I RINSE DRY Figure 6: Water Soluble Aqueous Cleaning System Fully aqueous systems require the conversion to water soluble.- in the printed circuit board assembly process. Changing Over chemistries is not always capable of being done. Contracts may exist which predate the desire to change preventing complete conversion. In this case, aqueous cleaners can also use a agonifier in the wash tank. The system water flow must be modified so that the saponifier concentration is not diluted. A sagonifled configuration is shown in Figure 7. MAKEUP INCOMING >>> DRY WASTE WASTE Figure 7: saponified Aqueous Cleaning System

5 Lateral Distance Figure 2:!.;anifold Design

6 cleaning Effectiveness Figure 5: Nozzle Desiqn Effect ~ - + f * +* * I I I " * 0.005" 0.003" " Total Cleaning Effectiveness 40- (%) 2 0- X X c? X 6. DRYW One of the most difficult challenges in aqueous cleaning is drying. Even though a printed circuit board may be clean, a drop of water left on the board can cause corrosion or even short out the circuit during functional testing. Once again mechanical force of air is the best method for drying. Merely heating the board will evaporate the cleaning fluid, but this may also evaporate any contaminants from the cleaning fluid back onto the board. High pressure air is used to blow the

7 over inserted device, or device wnich is placed to deep into che paste, can be placed at 0.003t* and 0.001". jm El j3j m Figure 3: Glass Test Board with Coupons Flux is injected under the coupons. The test board is heated. Some gas escapes from the flux as it heats, causing bubbles to Eorm randomly under coupons. The board is evaluated by determining the area containing flux prior to cleaning. The test board is cleaned. The remaining flux area is read. The post cleaning residue area is divided by the pre-cleaning.%rea and the cleaning effectiveness score is calculated by averaging the results for tile three coupons at each stana off height. The total average per board is the average of the four standoff height results. 4. EFFECT OF' PRESSURE.A cleaning system with fan jets is used to demonstrate the effect of increasing pressure in an aqueous system. Glass boards with water so1 uble flux were run at 3 FPM through a wash bath of 140'F as pressure is increased over the ranqe rrom 20 to 60 psi. Cleaning effectiveness parameters were read and the results are shown in Figure 4.

8 over inserted device, or device wnich is placed to deep into che paste, can be placed at 0.003" and 0.001". i I---- j 5 ] i m Figure 3: Glass Test Board with Coupons Flux is injected under the coupons. The test board is heated. Some gas escapes from the flux as it heats, causing bubbles to Eorm randomly under coupons. The board is evaluated by determining the area containing flux prior to cleaning. The test board is cleaned. The remaining W area is read. The post cleaning JJJ.X residue area is divided by the pre-cleaning area and the cleaning effectiveness score is calculated by averaging the results for tile three coupons at each stand off height. The total average per board is the average of the four standoff height results. 4. EFFECT OF' PRESSURE A cleaning system with fan jets is used to demonstrate the effect of increasing pressure in an aqueous system. Glass boards with water soluble flux were run at 3 FPM through a wash bath of 140-F as pressure is increased over the range trom 20 to 60 psi. Cleaning effectiveness parameters were read and the results are shown in Figure 4.

9 I Figure 5: Nozzle Desiqn Effect Cleaning Effectiveness (%I f +* + * * 00 cn I I I OD " * 0.005" 0.003'' '' Total Cleaning Effectiveness (%I X X X h 6. DRYING One of the most difficult challenges in aqueous cleaning is drying. Even though a printed circuit board may be clean, a drop of water left on the board can cause corrosion or even short out the circuit during functional testing. Once again mechanical force of air is the best method for drying. Merely heating the board will evaporate the cleaning fluid, but this may also evaporate any contaminants from the cleaning fluid back onto the board. High pressure air is used to blow the

10 Lateral Distance Figure 2: :.;anifold Design

11 .> >> INCOMING I >>, WASH RINSE FINAL RINSE DRY L A I WASTE Figure 6: Water Soluble Aqueous Cleaning System Fully aqueous :eluxes in the over chemistr Contracts may systems require the conversion printed circuit board assembly es is not always capable of be exist which predate the desire to water soluble process. Changing ng done. to change preventing complete conversion. In this case, aqueous cleaners.. can also use a aggnifler in the wash tank. The system water flow must be modified so that the saponifier concentration is not diluted. A _sar>onifled configuration is shown in Figure 7. >>> m MAKEUP -wash--l+ INCOMING FINAL-- RINSE I >>> DRY WASTE WASTE Figure 7: Saponified Aqueous Cleaning System

12 I 2. AOLJEOWS C1,EW - G MECHANICS The most important parameter in aqueous cleaninq mechanics is the force with which the cleaninq fluid passes under components and over the surface of the board. This mechanical force erodes contamination from the desired surfaces. The fluid delivery system needs to be optimized to provide sufficient impact force uniformly over the surface of the board. Several nozzle designs offer various implementation styles. A flood jet releases a large volume of fluid in a low pressure pattern over a wide distance. Flood jets typically show a wide variance from center to edge in pressure. A fan jet disperses fluid under higher pressure over a narrow band, suffering minor losses in performance from center to edge. A new nozzle was created just for aqueous cleaning which would disperse an even flow of fluid at an even pressure across its length. The nozzle is a high pressure and flow device and is commonly referred to as the Hurricane Jet. The nozzles and pressure diagrams showing lateral fluid distribution are shown in Fiyura 1. Good cleaning coverage is achieved by assembling these nozzles into manifolds, as shown in Figure 2.

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14 6th International SAMPE Electronics Conference.June 27-25, 1-99? AQUEOUS C1,rlANING?ZRF.3J?KW~CE Janet E(. Scerritt Sterritt Associates, Inc. 31 Irene Drive Hollis, New Hampshire TRACT A new awareness of the long term environmental effects resulting from cleaning materials has resulted in the pursuit of alternative materials and methods. One alternative is the use of water, and water based chemistries to clean electronic subassemblies and their components. The key issue in the use af water is whether aqueous cleaning can perform as well or better than solvent based methods, and therefore be an scceptable replacement. This paper discusses the physical and chemical properties necessary to provide cleaning performance. Test data shows that aqueous cleaning is a viable alternative to solvent methods. KEYWORDS: Flux: Rosin: Saponifier; Solvent; Water Soluble 1. INTRODUCTW Printed circuit board (PCB) cleaning consists of removing ionic - contaminants and flux residues from the entire PCB, especially an all leads, traces, and test points. The most challenging region are the areas under top side surface mount (SMT) corxponents and connectors. mp side components are generally