Printed Circuit Design & Fab - November 2008 - (Page 40)

SOLDER rESiStS The LeaD-free SoLDerIng challenges for Peelable resists The appeal of peelable resist technology. by IAN MCDONALD Figure 1. Peelable resist technology and its use in the manufacturing of printed circuit boards can be traced back at least three decades. It was used as selective plating resists, as well as solder resists. The development of hot air solder leveling (HASL) in the early 1980s necessitated technical improvements in peelable technology that enabled these systems to withstand the new demands of the HASL process. The wheel now appears to have turned full circle, and manufacturers of peelable resists have had to review the current level of technology in order to meet the increased demands that lead-free soldering has placed upon these peelable resists. In addition to the new technical demands, raw material health and safety issues will also impact manufacturers. Peelable resists belong to a general category of coatings known as plastisols. These coatings are essentially stable dispersions of powdered PVC resin in a continuous or liquid phase. PVC resins have what is called a linear isotactic structure, the chlorine is always on the same side of the carbon atom, causing crystallization within the PVC polymer. Because this limits the solubility in most common organic solvents, PVC resin is incorporated into the plastisol in a solid or disperse phase. The resin is then dispersed uniformly into what is known as a plasticiser (the liquid continuous phase). The peelable coating is essentially solvent-free with the plasticiser basically forming part Figure 1. of the final, fused film after thermal cure. The choice of both PVC resin and plasticiser types, coupled to the optimal ratio between the two, is critical to the overall performance (solder resistance and peelability) of the coating. Upon thermal curing, the plasticiser is essentially absorbed into the PVC resin to produce a high-build, tough, flexible, chemically resistant film that has excellent cohesive strength but minimal adhesion to the substrate. Hence it is peelable! The Lead-Free Challenge All lead-free alloys have higher melting points than those of eutectic solder, resulting in the soldering temperatures being approximately 30ºC higher. At this increased temperature, many peelable resists lose their usual flexibility and become hard and brittle– and very difficult to remove. FiGurE 1 illustrates what can happen to a standard peelable film when subjected to multiple, high-temperature, lead-free Figure 3 soldering processes. Note the changes in Fig F Figure 1. 40 FiGurE 1. the above two films of peelable resist have each been subjected to lead-free soldering at approximately 300ºC for a total of two passes through the soldering operation. printEd CirCuit dESign & fAB Figure 2 FiGurE 2. Rheology curve for peelable Figure 3 resist material. Figure 4 soldering. lead-free FiGurE 3. New peelable resist after Figure 5 NOVEMBER 2008

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - November 2008

Printed Circuit Design & Fab - November 2008 Contents Our Line Market Watch Around the World Happenings ROI Positive Plating Ten Tips to Improve Manufacturability 3D Chip-Package-Board Modeling Improving Circuit Simulation With The Addition Of Real Measurements Ad Index PCB West: Interview with NBS Design Inc. The Influence of Final Finish on Lead-Free Assembly Reliability The Lead-free Soldering Challenges for Peelable Resists Off the Shelf Marketplace BGA Bulletin

Printed Circuit Design & Fab - November 2008

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