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 demonstrated that solder paste 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 electronics assembly by optimizing your solder paste.
In today’s electronics manufacturing world there is a multitude of different solder pastes for different applications. Let's briefly explore how that affects voiding.
Solder paste is roughly 50% flux vehicle and 50% solder powder by volume, and, typically, 88-90% metal by weight (depending on the alloy and application). When it comes to voiding, the biggest difference between any solder pastes is the flux vehicle. There are many different flux vehicle options on the market; some are no-clean, some are water-soluble, and some are classified as RMA. These formulations, all designed to meet different requirements and challenges, vary significantly. Consequently, some solder pastes perform better, with regard to voiding, than others.
The greater truth (and the reason why engineers are needed) is that voiding is not the only issue that a process engineer manages. We must also concern ourselves with solder paste performance with regard to: fine feature printing, high-warpage components, NWO and HiP mitigation/elimination, high ECM reliability performance, and ICT first-pass yields. In other words there is not one single golden bullet to solve the comprehensive list of challenges that manufacturers face on a daily basis.
Digging into solder paste even further - there are factors that have a secondary effect on voiding, such as; viscosity, metal load, alloy, and metal mesh size (particle size distribution).
Higher viscosity materials tend to void more than lower viscosity materials. Imagine a scenario of air bubbles trying to escape from honey vs. air bubbles trying to escape from water.
Metal load affects the viscosity of the material and, therefore, affects the BTC voiding performance.
Alloy type affects wetting energy. Pb-free materials are not as straight-forward as they once were - when SAC305 was the definitive choice of many assemblers. We now have SAC405, SAC305, SAC105, low-Ag options for cost savings, and SAC alloys with dopants to increase solder joint reliability. The wetting energy, or the wetting force, of the alloy can have a dramatic effect on voiding performance. In essence, the higher the wetting energy the better the resultant BTC voiding results.
Particle size distribution can also have an affect.
It is important to consider all aspects and characteristics of a solder paste simultaneously when determining how to reduce voiding in your large ground plane component assembly. It can get pretty complicated. The good news is that our technical support engineers here at Indium Corporation have thoroughly researched these topics and can recommend the appropriate solder paste to enable you to reduce BTC voiding in your assemblies. And, our formulation scientists have developed Indium10.1 and Indium8.9HF solder pastes, two of the best Pb-free, no-clean solder paste flux vehicles on the market for this challenge - very worthy of your evaluation.
Next time we meet I’ll dig into how the Printed Wiring Board (PWB) design can effect BTC voiding levels. Dontcha' love it!?!