PCB007 Magazine

PCB007-Mar2021

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76 PCB007 MAGAZINE I MARCH 2021 Introduction Troubleshooting process-related defects is not as simple an exercise as we would like to believe. e printed wiring board fabrication process is a complex set of mechanical and chemical processes containing multiple steps. When even one of the process steps is not in control, end results can be disastrous. Further confounding the engineer responsible for solv- ing these issues is oen the over-reliance on the belief that the defect noted in a certain pro- cess step had its genesis in that step. And this assumption is oen incorrect. Experience has taught us that while the defect may be detected only aer a particular process step (e.g., voids in the via aer electrolytic plating), the origin of that void may have been in the PTH process or the result of poor drilling. ese are just a few examples. To be successful in determin- ing the problem and root cause identification, one must look at the defect holistically. In a future column, I will pres- ent a few suggested approach- es to problem solving and de- fect mitigation. For now, I pres- ent a view of some defects where at first glance the origins are not obvious. The Case of the Etch Resist Etch-out When voids in the PTH are detected aer etching, it is quite easy to assign root cause to the etching opera- tion. While there are no complete voids seen in Figure 1, one can surmise that the thin areas will eventually form a void or voids. While it is true that the etching operation is what is in ac- tion here, the etching process was deemed in control and there were no etch-out voids seen on other part numbers. Take a closer look at the cross-section. What is noted first on the pads? ere is a noticeable tapering of the copper indicating that the tin etch resist did not hold up to the etchant. True, the tin either was absent initially, or was of in- sufficient thickness to withstand the action of the etchant. In addition, there is a thinning down of the copper in the plated through-hole indicating the same. So, why can't the engineer Process Defect Anomalies, Part 1— The Case of Etch Resist Attack Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Figure 1: Note the ragged copper where the tin etch resist has been removed by the etchant.

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