Issue link: https://iconnect007.uberflip.com/i/663184
24 The PCB Magazine • April 2016 call and complains of solderability defects (Fig- ure 5). Now the team needs to spring into action. At first glance, the fabrication team should not simply assume that the defect that manifested it- self through the assembly process was the fault of the bare board. Regardless, the assembly firm most likely will not see it that way. The assembly team argued that there were issues with solder joints not forming properly—dewetting. In addi- tion there were black areas visible across a num- ber of surface mount pads. The final finish in question was ENIG. Now there is concern about hyper-corrosion of the underlying nickel surface. It would be easy at this point to just call this black pad and allow the PWB fabricator to accept blame. But a truly experienced and effective trou- bleshooter would ask for more data and conduct additional tests on her own. One issue I sometimes encounter in cases like this is: The EMS company engineer refers the dewetting issue to IPC-610. And yes, the IPC-610 provides guidance on ac- ceptability of wetting, etc. However IPC-610 does not provide adequate vetting in order to screen for solderability concerns or even black pad. So, what needs to be done here is to take some of the defective boards and cross-section those areas. Refer to IPC-4552 and determine if there is corrosion of the underlying nickel. Keep in mind that minor corrosion spikes in the nickel are not cause for concern (IPC-4552). At the very minimum, the bare board fabri- cator must perform a thorough audit of its ENIG process including SEM analysis, gold thickness measurements and solderability testing. How- ever, the assembly company must also provide reflow profiles, information on solder pastes used, as well as testing of the solderability of the components! Leave no stone unturned. Summary: Guidelines for Effective Troubleshooting and Process Control One of the keys to effective problem solving is a structured routine that addresses key points each time a major problem is encountered. This section suggests steps to effectively find the cause of a problem and to solve it permanently. Refer to IPC-9191 for greater detail to suggested methodology for SPC. 1. Before beginning a detailed troubleshoot- ing project, use common sense in defining the problem. • Verify that there is a problem • Observe the defective product and compare it to the standard • Identify the standard process and product, and then determine any present deviation from the standard or any change in the product a proCess engineer's guiDe to effeCtively troubleshooting pWb DefeCts Figure 4: Exposed resist due to pinholes in the phototool. (Source: IPC photo archive) Figure 5: Issues with assembled PWB: dewetting and black residues.