Troubleshooting part 1

Troubleshooting the Printed Wiring Board Fabrication Process - it's funny, but it's no joke Part 1 in the series "Trouble in your tank?" by Michael Carano	Back to articles index

Introduction The printed wiring board fabrication process is an intricate maze of interrelated steps, both chemical and mechanical. A thorough understanding of each of the process steps is critical in minimizing or eliminating non-conforming defects - the ones that cost the fabricator money and can lead to lost customers. Over the coming months (the column will run every other month) we will explore the pwb process in general, with particular emphasis on plating defects. Certainly we will present other defects as well, especially when the defect or defects have their origins elsewhere in the process. That is the difficult thing about troubleshooting - the defect is often blamed on the process (such as electrodeposition of copper) because that is where the problem is first "discovered." However, this is often false, as the origin of the defect may have had its humble beginnings in a previous process step. This will be a recurring theme throughout these columns, as I am a firm believer in the interrelatedness of processes. To be successful at trouble shooting a problem, common sense usually applies. Basically one must first: Identify the problem or problems (be as specific as possible) Determine possible causes (look for links to those other less obvious processes) Methods and procedures to test to see which causes apply Test the assumptions Implement corrective action While this sounds like an oversimplification, this approach is required to identify properly and attack the problem at hand. A structured routine is really what is required. In these columns we will attempt to provide some insight as to the cause or causes of these non-conforming defects and the potential solutions. We will discuss process parameters and the importance of control of the processes. Let's discuss the step by step methodology for troubleshooting. Identify the problem or problems First and foremost, you must have a clear sense of what you are looking at. In troubleshooting, I firmly believe in the team approach to solving the problem. However, the team must agree on what the defect is: is it hole wall pullaway or resin recession, is this an interconnect defect (ICD) or simply a line of demarcation? These are just a few examples. But the wrong call will lead you down the incorrect path as you attempt to tech out of the problem. Determine whether or not proper operating procedures were followed. Did the process change in some way from the standard? Compare the defective product to a non- defective one. Was the process operating within the specified parameters? Don't take anyone's word for it. Check this out thoroughly! On way too many occasions, significant loss has been incurred because the process or processes were run out of spec. Simple analysis or control procedures would have prevented such an occurrence. (In a future column, we will discuss process controls.) If the analysis shows that the chemistry, dwell times, operating temperatures, etc. are not in control, then bring the operation to a halt until the corrections are made. Then run some tests to see these corrections solve the problem. If this does not correct the problem, then the team must sit down to determine other possible causes for the defect. Possible Causes The rule of thumb here is to keep the troubleshooting project as manageable as possible. Begin to brainstorm on the linkages in the up and downstream processes and potential effects of process variation in these process steps. Gather all pertinent information including SPC charts, temperature logs, analysis records (including record of calibration and analytical standards) and the like. Then develop a cause-and-effect diagram. Fish-bone diagrams serve this purpose well. At the risk of having hundreds of factors to investigate, only the most likely causes should be investigated first. This will serve to weed out those processes that are not contributing to the defect. A process audit is a must in this situation. Hopefully, you have a reliable supplier or suppliers who work with your company in close partnership. On-going process audits jointly and separately performed by your supplier and designated individuals in the fabricator's facility should be standard operating procedure. Process audits alert the manufacturer if a process is "drifting" out of the control window. Once the team has set up its test plan based on a narrowing of potential causes, the divide and conquer approach will aid in the efforts. For example, if one suspects that thin plating of copper in the hole is caused by problems associated with the electrodeposition process, simply processing the pwb in the acid copper plating solution for the required time and current density should yield whether or not the copper plating process or the equipment (copper plating anodes, rectifier, electrical connections, etc.) are the cause. If not, then one must examine the previous steps. Are there discontinuities in electroless copper deposit or direct metalization process causing thin plating? Are there voids one cannot see? These are just some of the questions to be asked. Only a systematic approach will help solve problems expeditiously. Above all, after selecting which processes and test procedures will be implemented, test these assumptions with vigor. Time is money. Hopefully, these actions will identify the suspect cause(s) of the defect. When the problem is located, corrective action must be implemented. Determine what measures will be taken to insure that the problem does not reoccur. Redefine the process control window. Set up permanent controls in order to keep the process within a tighter operating window if necessary. For example: Set up process audits at specified times Implement process control procedures and measurements Establish preventative maintenance on equipment Train operators Maintain or establish close ties with your suppliers Certainly, this is only a guideline for troubleshooting. The key is to get to the root cause of the problem as soon as possible without taking shortcuts for the sake of time. There is no substitute for a systematic approach backed by solid knowledge and understanding of the process parameters and the influences of up and downstream processes. Remember, the process in which the defect manifests itself is not always the main cause. Look carefully. In the next article of "Trouble in Your Tank" we will tackle the issue of voids in the plated through hole.