ANALYSIB OF LOW VOC CONFORMAL COATINGS OVER NO-CLEAN FLUX -

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1 -El35 ANALYSIB OF LOW VOC CONFORMAL COATINGS OVER NO-CLEAN FLUX - RESIDUES L )"/ J / Edward A. Shearls, SAIC, 714 N. Senate Avenue, Indianapolis, Phone: ; Fax: Timothy Crawford, EMPF, 714 N. Senate Avenue, Indianapolis, IN Phone: ; Fax: INTRODUCTION Corrosion and long term reliability are major concerns with electronic assembly operation. Conformal coatings have long been used to protect printed wiring assemblies (PWAs) in harsh operating conditions. The use of conformal coatings presents associated environmental challenges. A conformal coating must be applied to a clean, dry surface to adhere properly to the PWA and provide a good seal. Improper adhesion can jeopardize the reliability of the PWA. Because of the impending phase out of chlorofluorocarbon-based solvents, a new generation of fluxes called "low residueo9 or nno-cleant9 fluxes have been developed. These fluxes are designed to leave little or no residues upon completion of the soldering process and eliminate the need for cleaning. Any residue that is left is benign and should not degrade PWA reliability. The concern with these benign residues is the adhesion of the protective conformal coating to the PWA. Most of the conformal coatings currently in use release significant amounts of volatile organic compounds (VOCs) into the environment during the application process. The ever increasing awareness of the environmental costs of VOC releases makes the identification and implementation of new conformal coatings with low VOC content and less environmental impact an important issue. The Clean Air Act of 1990 drives the eventual implementation of conformal coatings with low VOC content and a negligible environmental impact. Additional concerns are whether a low VOC coating can be applied over a no-clean flux successfully and whether it is practical to do so. The Electronics Manufacturing Productivity Facility (EMPF) is completing a three phase effort to address these concerns. Phases 1 and 2 were performed concurrently. In Phase 1, the adhesion and performance of acrylic, polyurethane, silicone, and parylene conformal coatings applied over test pallets manufactured with low residue fluxes and pastes was evaluated. A nitrogen atmosphere was used for soldering and reflow. In Phase 2 low VOC conformal coatings currently available were applied to simple test pallets made with commonly used RMA and water soluble fluxes and pastes. Environmental screening tests were performed in both phases to down select no-clean materials and low VOC coatings for further testing in Phase 3, the application of low VOC coatings over no-clean fluxes and pastes on functional boards. 1

2 ElETBODOLO~Y Preliminary Compatibility Testing Preliminary compatibility testing was performed to ensure test pallet (Phase 1 and 2) and test board (Phase 3) base fluxes, pastes, and solder mask combinations were compatible. We wanted to ensure that coating problems occurring during environmental screening tests were not due to base material incompatibility. The representative matrix of common fluxes and pastes used included five RMA fluxes, four water-soluble fluxes, seven RMA pastes, and six water-soluble pastes. Two of the more popular solder masks, a liquid photoimageable (LPI) and a dry film mask were used. m and water-soluble fluxes were applied to test pallets using individual spray bottles. RMA and water-soluble pastes were printed onto test pallets using an 80-mesh, 10-mil-thick screen. Pallets were wave soldered or IR reflowed according to manufacturers, technical literature. Those made with RMA materials (fluxes and pastes) were cleaned with 10% Armakleen E detergent in an ECD Model 9300 batch aqueous cleaner. Those made with water-soluble materials were cleaned in the ECD unit using DI water only. Cleaned and baked pallets were visually inspected for icicles, dross, blemishes, discolorations, residues, and associated imperfections. The flux and paste producing the best soldered pallets were used to make test pallets for the Phase 1 and 2 efforts. Test Pallets and Coupons The test pallets used in the Phase 1 and Phase 2 efforts were 4.00 inches by 5.00 inches (10.20 x cm) and had four test coupons to a pallet with break-away tabs between the coupons (see Figure 1). Each test coupon contained a 1.00 inch by 1.50 inch (2.54 x 3.80 cm) copper rectangle with a 0.50 inch by 1.00 inch (1.27 x 2.54 cm) solder mask strip down the center. One half of the test pallets had coupons with LPI film solder mask; the other half had dry film solder mask. The majority of the coupons had bare copper on the areas adjacent to the solder mask. The remainder had hot air solder leveled (HA.%) on the areas adjacent to the solder mask. The laminate was FR4 material. The two masks and the two metal surfaces allowed the following four base metallmask test coupon combinations: 4 CopperlPhotoimageable Liquid 4 CopperjDry Film 4 HASL/Photoimageable Liquid 4 HASL/Dry Film Including the FR4, this arrangement gave us the ability to 2

3 I. evaluate adhesion of the different conformal coatings to five different substrates and interfaces when contaminated with the various no-clean flux residues. A control group was set aside to evaluate conformal coating adhesion to coupons not contaminated with no-clean flux residues. Phase 1. No-Clean Flux Evaluation To evaluate every no-clean flux and every standard Inon-low VOC) conformal Figure I Test coupon coating would be impossible. We decided to randomly select one acrylic, polyurethane, silicone, and parylene to represent their respective coating families. Specialty Coating Services of Indianapolis, Indiana volunteered to apply the coatings to the test pallets. The flux and paste selection was more complicated. Unlike the "good old" rosin flux with a relatively straight-forward chemistry that was rated by its activity, the definition of noclean is broad. The long list of materials available was grouped into resin, rosin, or rosinfresin free categories, and further broken down by solids content, activator and carrier. Twelve pastes and eleven fluxes were chosen from these groups to provide a good representation of the no-clean industry. The liquid fluxes were applied to the test pallets using a high pressure spray system designed for use with no-clean fluxes. Precision Dispensing Equipment of Bay Village, Ohio volunteered to bring the system to the EMPF, set up, and operate the equipment on an as-needed basis. The fluxes were then processed on a nitrogen inerted wave soldering machine using a profile recommended by the material vendor. The no-clean solder pastes were applied to the metallized areas of the test pallets using an 80-mesh, 10-mil-thick screen. The pastes were also reflowed in a nitrogen environment using forced convective air reflow and a profile recommended by the material vendor. In both Phase 1 and 2, after conformal coating application, representative pieces of the coupons were visually examined and tested for adhesion. The rest of the samples were subjected to environmental stress screening (ESS) which included either temperature/humidity, thermal cycle, thermal shock, saltffog, all 3

4 .~...,,, \\ tests, or a humidity/thermal cycle/thermal shock combination. The stress tests and conditions used are listed in Table 1. 7 TABLE 1. ENVIRONMENTAL SCREENING TEST SUMMARY I Stress Test specification Conditions I TemperaturefHumidity MIL-STD-810D 168 hours, 30 to 60' C I I Method I relative humidity Thermal Cycling IPC-TM hours, -55 to +12S0 C Method min per temp and 15 min dwell at 25' C between each ramp Thermal Shock MIL-STD-202F 24 hours, -55 to +125" C Method 107D 15 min per temp. Salt/Pog MIL-STD-81OD Method hours, 35' C I

5 . ic It was decided after a literature review that LPI mask pallet results would be the primary criteria for flux and paste selection because the LPI mask would be used on the populated boards in the Phase 3 (no-clean materials with low VOC conformal coatings) effort. Studies have shown LPI mask the most compatible with no-clean fluxes (1) and more compatible with conformal coats than other types (2). A solder mask comparison (3) reports LPI masks reduce tombstoning and solder balls, withstand multiple reflow cycles, and are easily cleaned. Several papers describe favorable LPI maskjsolder ball dynamics (4-7). Phase 3. Low VOC Conformal Coating War No-Clean Flux Evaluation Populated boards were manufactured for the Phase 3 evaluation using the liquid film photoimageable solder mask and the laminate from earlier testing. The no-clean fluxes and pastes that graded best in the Phase 1 effort was used for one set of these boards. The RMA and water soluble fluxes and pastes selected in the compatibility pretesting were used to manufacture a control set of populated boards. Replicate groups of 12 manufactured boards each were coated with the one low VOC conformal coating selected from each generic group in the Phase 2 low VOC conformal coating effort. Coated boards were subjected to the inspections, adhesion testing, and environmental stress screening described earlier. In addition, Surface Insulation Resistance (SIR) testing was performed on three replicates from each group. RESULTS In the initial compatibility testing, the three highest scoring RMA fluxes and two highest scoring water-soluble fluxes used on LPI mask pallets also clearly produced the best looking solder applications in their groups. The two highest scoring RMA pastes and the two highest scoring water-soluble pastes used on LPI mask pallets also clearly produced the two best looking solder applications in their groups. For the Phase 1 testing, the adhesion for each substrate was rated by the 0-5 scale then converted to a percent to establish a uniform rating system. This conversion corrected the rare occurrence of a coupon loss from human or machine error. Phase 2 final results are essentially summarized in Table 2. The results shown are the total grades amassed by each coating during Phase 2 testing. The totals include points awarded at the initial visual inspections and at the visual inspections after the test pallets were subjected to the entire sequence of environmental screening tests. The totals include the adhesion grades for the pallets before and after the entire sequence of environmental tests. The totals include the before and aftpr adhesion grades for those pallets that were exposed only to a single screening test. The totals for the coatings with the most points in each 5

6 group are highlighted Table 2. Phase 3 has just been completed. Data analysis is underway but is not sufficiently completed to warrant any more than the generalization that using low VOC conformal coatings over noclean fluxes and pastes is a viable option. CONCLUBIONB A viable test vehicle (pallet) and methodology for assessing interactions between no-clean materials and low VOC conformal coatings have been developed and used. A report is in process detailing the no-clean and low VOC conformal coating selection and their interaction when used together. The preliminary assessment is that it is practical to use low VOC coatings over no-clean fluxes and pastes, but material compatibilities must be addressed. The final report for this project should be available from the EMPF library by the end of this year. 6

7 TABLE 2 lw!al OWE FOR P m S -E WITH LIQUID FILM MASK. CCI INSPECTION I ADH (SEQ) ADH (IND) TOTAL Acrylics t INIT AF SEQ EMPF HASL EMPF GRADE UrethM08 t

8 BIBLIOQRAPHY 1. Hemens-Davis, Chantal and Roy Sunstrum No-Clean: Material Comuatibility Issues. Circuits Assembly. March. pp Vazirani, Govind, David Krevor, Mark Chavez, Satish Muley, Ron Provancher, Danna Ransier and Gregg McNelly Conformal Coats and Their Comuatibilitv with Solder Masks. Kaiser Electronics, San Jose, California. (R) 3. Tennant, Tracy Solder Mask ODtions for the '90s. Electronic Packaging and Production 34(2). pp Crum, Susan Mask Technolocly. Electronic Packaging and Production 33(9). pp , 5. Feryance, Daryl, and Fred Shubert Matte-Surface solder Masks Reduce Solder Ball Defects. Electronic Packaging and Production 33(9). pp Freitag, Brandon Reducins Solder Microballs in Inert Wave Solderinq. Electronic Packaging and Production 34(2). pp Tuck, John Circuits Assembly 4(10) pp