Issue link: https://iconnect007.uberflip.com/i/1535414
22 PCB007 MAGAZINE I MAY 2025 building. If I am dealing with RF or HDI mate- rials, I would want to consider a photoresist with the best adhesion promotion I could find. Many modern RF-style materials are designed with signals/frequency in mind. is means from a surface preparation perspective, you would need a low- or no-etch chemical process as we do not want to lose the integrity of the signals or frequency. Having some "tooth" on the copper always provides a good surface for photoresist to conform and adhere. With these materials having a smoother copper surface by comparison, a resist with superior adhesion would help overcome the inability to apply a tooth to the copper surface. What are the most common defects you encounter during the photoresist process, and what are the typical causes? Krick: e most common defects in my expe- rience, in terms of pure volume, are opens and shorts. I do not think I have ever visited a shop where the top two defects were anything other than opens and shorts when speaking about photoresist failures. When defining opens and shorts, we need to always determine whether we are talking about inner layer processes or outer layer (plating) processes first. I always ask customers to separate these into two groups as making too many universal changes in the pro- cess can just make things worse. Many inner layer opens can be attributed to the cleanliness of the imaging and lamination areas, and some pre-clean processing lines. True cleanrooms with cleanroom equipment, protocols, and proper attire–including boo- ties, gowns, caps, and beard covers are a must for much of the technology demands today if you want consistently high yields. Tacky machines or hand rollers are ideal for cleaning before lamination as well as for keep- ing the mylar free of debris aer lamination and before exposure. Keeping your exposure units and their processing tables/trays as clean as possible is also huge. Base laminate coming in and being used should always be inspected to make sure no major dents or scratches are present. ese dents and scratches can pose issues with the resist conforming, thus lead- ing to primary failures in the form of adhesion loss, which can lead to an open or a short. Shorts on inner layers can result from many things—under-etching or under-developing, which leads to poor etching. ey can be caused by debris on the surface or under the photore- sist causing exposure issues that lead to adhe- sion loss or resist liing and falling where you do not want it, and of course, handling damage or conveyor damage can also cause the resist to li and move. Outer layer opens and shorts are not always cleanroom dependent but cleanroom stan- dards should be considered just as important as they are with inner layers. Adhesion loss on outer layer processes can lead to plated or hard shorts, while poor filtration on the pre-plate clean can lead to debris re-depositing on the surface, leading to shorts or opens, and again, handling damage can also pose issues. Resist stripping shorts or entrapped/pinched resist will be seen as shorts due to resist-related issues, but usually have more to do with plat- ing and resist stripping or chemical level issues that just happen to affect the photoresist. How long the photoresist is le on the copper (rule of thumb: We like to see the resist laminated Tim Blair