Issue link: https://iconnect007.uberflip.com/i/963057
64 PCB007 MAGAZINE I APRIL 2018 As stated previously, water acts as a plasticizer, essentially low- ering the viscosity of the resist and allowing the dry film to flow into the hole faster. As the tech- nology continues to evolve into higher layer counts and greater circuit density, vias continue to get smaller and aspect ratios in- crease. It is the smaller diame- ter vias and thicker boards that present the greatest challenge to complete drying. Resist in a small hole is also harder to remove in the developer. Long hold times al- low the resist to flow further into the holes as w ell. Minimize hold times by moving the panels from lamination to exposure to devel - opment in the shortest time; this is best practice. Plus, invest in a good dryer. After all, fabricating circuit boards requires water for rinsing. One cannot get around that, so improve the drying capability after sur - face prep or even PTH prior to resist lamination. Summary Getting to the root cause or causes of defects is never a simple matter. It takes skill, care- ful thought, and collaboration among those in- volved. The information presented here is a good example of how the defect noted was not related to the PTH process, as some would be quick to conclude. Recognizing how upstream and downstream processes, even as simple as drying the boards prior to resist lamination, can lead to defect formation is critical for the defect resolution team to understand. PCB007 References "Tech Talk," Karl H. Dietz, Circuitree Magazine, January 2002. Michael Carano is VP of technology and business development for RBP Chemical Technology. To reach Carano, or read past columns, click here. area. The resist is removed in the stripper, and the electroless copper is etched leaving a hole void. The resist in the hole is usually difficult to observe after developing and is seldom pres- ent in the cross-section of a board after final processing. Why does the resist flow into the hole? Ac- cording to Dr. Karl Dietz, the root cause is that the air pressure in the via that is covered by the dry film resist is significantly lower than atmo- spheric pressure. With lower pressure the air in the hole is hot during resist lamination and the pressure drops when the air cools to room temperature. This pressure differential causes the resist to slowly flow into the through-hole, making it extremely difficult to develop out [1] . There are at least two additional factors along with entrapped moisture in the via that contribute to resist flow: • Higher aspect ratio vias—narrow diameters and thicker printed circuit boards • Long hold times between lamination and developing Figure 4: Rim void caused by resist flowing into PTH. Note the electroless copper is intact, but there is absence of electrolytic copper at the rim. (Source: IPC-9121)