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PCB007-Feb2021

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FEBRUARY 2021 I PCB007 MAGAZINE 85 cept a scratch? How many scratches, and how deep? e immediate answer is that IPC does not specify what is acceptable or not. e only requirement we have is the workmanship rule that says a scratch is an imperfection that is not allowed. So, what is the solution? We need to talk to- gether and agree upon reasonable inspection criteria. 2. Foreign particles in solder mask e processing of solder mask today is in a much cleaner environment than before. e clue is cleanroom areas that enable the fac- tory to deliver a clean solder mask, free from unwanted particles that mostly come from the application room condition and workers' clothes. Still, we are not yet at a level where we can guarantee the solder mask will be 100% clean from unwanted particles. Such room conditions are costly and many factories trade between a defect free solder mask and the cost of such conditions. But what are the requirements? Even though not written specifically in IPC-6012 series, an unwanted particle can, in a worst case, lead to a defect in the application. And, with the min- iaturisation of PCB designs, existing clearance rules may not be sufficient. Again, the rule is such that unwanted particles are not allowed. We know it happens, so we need an inspection criterion just like the solder mask scratch issue. Discuss and Accept Acceptability Criteria for Both Parties Both of these issues represent a condition not allowed according to the workmanship specifi- cation. ese issues happen daily, and in most cases, the PCB supplier will handle it as a cos- metic issue and therefore the customer shall ac- cept it. But that is not what IPC says. It is impor- tant to know the standard and understand the requirements; even then, you will have a grey zone situation where the only solution is to talk together and discuss acceptability criteria that is satisfactory to both parties. e user should also understand that the workmanship standard may have different effects on different base ma- terials, and between a prototype or small vol- ume order compared to volume manufacturing. Conclusion What can we learn from this? e most im- portant thing to know is the standard and how to use it. Most people read the measurable re- quirements, while a requirement like the work- manship rule is le out. It is hard to deal with because it requires at least two parties to find a solution that works for both. is, however, leads to my next article, which will focus on the term AABUS (as agreed between user and supplier), meaning these open requirements shall be discussed with the supplier, be part of the article specification, or agreed in a general procurement requirement. Until next time… PCB007 This installment of The PCB Norsemen was written by Jan Pedersen, senior technical advisor at Elmatica. To read past columns or contact The PCB Norsemen, click here. Figure 2: A cosmetic defect in the solder mask highlighted by the customer. A typical example of a defect not specified in the standard, but still covered by the workmanship rule.

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