Dr. Alan Brown, CSMA Ltd, Queens Road, Penkhull Stoke-on-Trent ST4 7LQ
Modern surface analysis techniques provide the means of rapidly solving age-old problems in the PCB industry leading to reduced defect levels.
The complexity of PCB manufacturing has increased dramatically over the last three decades, progressing from straightforward double-sided boards with 100% through-hole technology to highly-complex multi-layer PCB’s with mixtures of through-hole, surface mount and chip-on-board configurations. Board layouts have consequently increased in density with tighter tolerances and decreased distances between electrical contacts. With this increase in complexity the possibility of manufacturing defects has also consequently increased. Typical causes of failure include:
- Board delamination
- Component misalignment
- Broken metal lines
- Cold-solder joints and poor die bonding
- Surface contamination by metal and ionic residues
Industry defect levels are variously reported to be in the range of 100 – 1000 parts per million i.e. for every million solder joints made, up to 1000 may be faulty. Defects may not “show-up” at the time of manufacture and return to haunt manufacturers as “field failures” (see later). They can escape detection by normal in-house test procedures or may not even be included in acceptance specifications. Nevertheless, defects directly affect the form, fit, function and long-term performance of PCB’s and for manufacturers, suppliers and customers they represent BAD NEWS.
Many defects are related to PCB or component surfaces i.e. their chemical and/or physical composition determines performance. This article describes case studies and examples of where modern surface analysis techniques (including XPS, Imaging ToFSIMS, Dynamic SIMS and 3DP) have solved problems for PCB manufacturers and helped to improve their processes. N.B. It is important to make a distinction between the surface sensitivity of analytical techniques. XPS and SIMS methods (including ToFSIMS and Dynamic SIMS) take their information from a depth of only 1 – 5nm. The more routinely-used techniques in industry e.g. SEM/EDX and FTIR include contributions from the surface but effectively sample microns in depth. Hence a contaminant which appears only on the surface at a low but sufficient level to cause problems will be easily detected by XPS/SIMS but may not even register in SEM/EDX or FTIR analyses.