In a previous post I spoke about Large Ground Plane Voiding in Electronics Assembly and referred to a statistical tool called an Ishikawa Diagram. This tool helps map out a process and provides an excellent visual aid that helps show the potential defect causes and the effects the process variables can have. This particular Ishikawa Diagram displayed that surface finish can have a large effect on voiding. Let's talk about how we can minimize large ground plane solder voiding in electronics assembly with differences in printed wiring board (PWB) surface finish.
Studies have shown that there is a correlation between voiding percentages and different PWB surface finish metallizations. ENIG, Immersion Sn, and Organic Solderability Preservative (OSP) tend to be the most common PWB surface finishes in today's Surface Mount Technology (SMT) industry. When comparing the voiding percentages of these three popular board metallizations, under bottom-terminated components, we typically see ENIG being the best (lowest level of voiding) followed by Immersion Sn, and then OSP.
There are different OSP versions available commercially, depending on the board house from which you are sourcing boards. Some OSP finishes may deliver better voiding performance than others.
Likewise, not all board houses have the same quality. So, solder voiding performance will likely vary from one board house to another. It is important to do your due diligence at the design stage so that you choose the right board metallization for your application and minimize the potential for voiding under bottom-terminated components.
The age and storage conditions of the boards also plays a role in voiding levels. An aged surface finish will tend to oxidize unless stored in an oxygen deprived environment. Oxidation is cleaned by the solder flux. A thicker oxide layer can make the cleaning process more challenging. It is possible that the flux may not fully clean portions of a heavily oxidized board pad/metallization, making it difficult for the solder to wet to that particular location. Voiding caused by a wetting deficiency will be the typical result.
Unused/virgin PWBs are often stored in nitrogen dry boxes to protect them from oxidation, but these storage boxes also protect the boards from moisture. If the boards are exposed to moisture/humidity they can absorb moisture from the air which could create solder voids during the reflow process when the moisture transforms to a gas. If the gas does not escape the solder joint before solidification, a void will form.
Picking the right board metallization and then storing and handling the boards properly once you receive them is vital to minimizing voids in your electronics manufacturing process. Next time I’ll talk more about how the environment plays a role in solder voiding under large ground plane devices.