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 provide 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 environment can have a large effect on voiding. Today I will dig into this area a bit further and talk about how we can minimize large ground plane solder voiding in electronic assembly with differences in the manufacturing environment.
The manufacturing environment is often overlooked as a root cause of issues and defects in electronics manufacturing assembly processes. It can be an easily identifiable starting point for determining the root cause of a solder paste viscosity issue, but, for voiding under bottom-terminated components, the environmental conditions are frequently disregarded and forgotten about.
This is especially the case in lower-cost manufacturing, where the cost of implementing controlled HVAC systems and process controls is not monetarily feasible. Temperature and humidity variations outside the optimal range can wreak havoc on assembly materials like solder pastes, conformal coatings, glues, potting materials, fluxes, and underfills. In a scenario where you are trying to reduce voiding under a bottom-terminated component (BTC), the temperature and humidity can certainly play a role. For example, if the temperature in the manufacturing environment is too high the viscosity of the solder paste could decrease, causing slumping to occur prior to reflow. We have seen a similar scenario from the effect of stencil design/thickness - an increase in the component standoff tends to decrease BTC voiding. Conversely, if the solder paste slumps and the standoff between the board and the component is decreased, the voiding percentages under the component may increase.
Higher temperatures and prolonged exposure times can also speed up the oxidation process of the solder paste. Increased oxidation levels can increase voiding percentages. Poor storage and handling procedures, excessive stencil life, and/or prolonged room-temperature storage can affect both oxidation and viscosity characteristics of the solder paste.
I have even observed that different locations in the world can have an effect on BTC voiding as well. Some solder pastes, especially water-soluble solder pastes, can perform differently, depending on the time of year. Winter months tend to be dryer and cooler than summer months here in the northeast USA. This can vary greatly from a place like Malaysia where it is hot and humid all year round. We have had customers with identical process set-ups and materials - in facilities in different locations of the world - who have experienced different voiding results in controlled experiments. We also had one customer do a similar study with voiding at different altitudes. The results of the study showed a significant difference in voiding from one altitude to another with all other variables controlled extremely well.
Always remember that the simplest things (that we often take for granted) can have a big effect.
Next time I’ll talk about the area that is always the focus when people have solder voiding issues: SOLDER PASTE!