Issue link: https://iconnect007.uberflip.com/i/1350598
56 PCB007 MAGAZINE I MARCH 2021 gy today you will not be able to perform a val- id inspection with the naked eye. It should be viewed under a scope. Now, what to inspect? e examination should cover the entire board, but significant detailed inspection should isolate on the high- density areas and small pads. ese areas are more prone to heavy witness marks as the pins are usually quite small and are more sus- ceptible to damage. Figure 2 shows an exam- ple of higher density areas that should warrant higher scrutiny. Once the inspection is com- plete it can be determined whether continu- ing with testing is allowed or intervention is required. Intervention is usually an adjustment in compression or perhaps replacement of a test pin or two. Once testing has begun it is recommended that an inspection matrix be created. How of- ten should we inspect the product during the process? Inspecting 100% of a large order is counterproductive, I understand. erefore, the matrix is necessary. Should we inspect ev- ery 10, 20, or 50? Early rules suggest first, mid- dle, and last board of an order. is can work if the order is only 10–15 boards. Worst case sce- e fixture needs to op- erate freely with no stuck pins. e largest variable contributing to damage is a bent pin or pin side-load- ing (pin sticking to the side wall of grid stripper plate), which locks the pin from free movement. Once this has been performed the electrical validation should take place. is includes a continuity and isolation test against a copper sheet (continuity) and non-con- ductive sheet (isolation.) ese tests conclude the following: 1. Continuity: All pins are moving freely and conduct correctly to the fixture tester electronics. is will also identify faulty connections or missing pins. 2. Isolation: Provides validation that no shorts exist in the test matrix, either pin-to-pin in fixture or faulty test electronics. It should only be aer these critical setup steps are performed that the product be com- mitted to test. When to inspect is now the question, isn't it? Absolutely. On fixture testers the optimum scenario is to run a test cycle on a pre-scrap board if one is available. However, many times there are none available and a live board must be used. (Personal note: If the fixture has a diary or log of past performance, do not just use the logged compression values! is can be fatal as mechan- ical changes may have been made due to mainte- nance or service repairs.) Always run at the least possible compression to provide desirable results. Once the board has been tested, regardless of the result, it should be inspected for witness marks. Do not trust your eye alone as with product technolo- Figure 2: An example of higher-density areas that should warrant greater scrutiny.