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62 The PCB Magazine • April 2015 by Todd Kolmodin GarDIen SerVICeS testiNg todd Electrical Test: Surface Finish vs. Water Marks Column After a brief time off and a successful IPC APEX EXPO, I'm here to field your questions and get back to business. Thank you to all that stopped by to see me at the show. It was a good year in San Diego and the weather (as usual) mostly cooperated. Looks like we'll all be head- ing back to Vegas next year! This month we will look at the ballet of sorts, performed by electrical test vs. surface finishes— namely witness marks. Over time, new finishes have come to market. Some allow better conduc- tivity while others reduce overall cost of precious materials. Regardless of the finish, electrical test must be performed on these circuits. With that comes the caveat of how much of a witness mark can be left on any given landing pad and still be acceptable to the CM or the final OEM user. First, we must understand what a witness mark actually is. In "down home" terms, when build- ing a fence the old-fashioned way, you typically hammer the planks of the fence to the support- ing framework. When the hammer hits the nail, the hammer leaves (unless you are really good) a round indentation into the board you are attach- ing, a.k.a. a witness mark! This is a mark indicat- ing the historic process that was applied. Witness marks in electrical test are much the same. When an electrical test is performed, a physical contact is made from the machine to the PCB. This works the same whether it is from a translator fixture on a bed of nails or the direct contact from a fly- ing probe. The combination of the surface finish being used and the type of electrical tester being used can produce a wide array of witness marks. Most of these marks are benign and acceptable, but in severe cases they can be destructive result- ing in costly rework or even worse, scrap! Some finishes are more critical than oth- ers. These range from HASL, immersion tin and ENIG, to the critical finishes such as immersion gold, immersion silver, soft gold and the ulti- mately delicate wire-bond. Customer require- ments and industry specifications have guide- lines regarding what acceptable witness marks may be. In the general specification for the man- ufacture of PWBs, the specification references the Figure 1: Pin damage. Figure 2: Pin damage.

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