Welcome to our latest Defect of the Month – Solved, this month we illustrate solder voids. My name is Bob Willis, now a picture is worth 1000 words and if that is true then a video is must be worth a million. Hence the creation of Defect of the Month videos for engineers back in early 2010. This is just one of the Defect of the Month videos previously featured by IPC, SMTA, SMART Group and Circuits Assembly magazine helping to provide better understanding of process issues and help support training of younger engineers. We have recently celebrated being a 100, that is the number of videos produced not Bob’s age Bob
We know that during reflow soldering voids can occur. But is a 5%, 10% or 30% void within a joint acceptable, particularly under bottom termination components really an issuer to you. If it’s not an issue, then don’t worry about it it’s not going to really affect the reliability of the circuit board assembly in many applications.
Generally, people tend to look to achieve less than 20% voiding in a particular area however you’ve got to decide on what’s acceptable for the product application. Now we can reduce voids by looking at the design of the stencil, the solder pastes the design of the printed circuit board if it’s the board or component actually causing the problem in the first place. Please don’t assume it’s the paste, stencil or profile. PCB and component outgassing, poor via location poor via filling are other areas I have demonstrated in the past.
However, if you have a problem board or a problem component or you can’t change the solder paste or any other parameters then you can consider vacuum soldering basically with a vacuum soldering, you’re sucking out the gas trapped within the void effectively and this is done when the soldier is in a liquid state. When the soldier goes into a liquid state, we slowly pull a vacuum within a sealed chamber where the boards are sitting and slowly the void disappears from the bulk of the joint and we’re left with no voids. The key point here is slowly pulling a vacuum, suck the gas out too fast you will have a mess. I have successfully used vacuum with customers for QFN, BGA and larger SO packages, even if I did not feel it was necessary.
The images above show voids in through hole, BGA and standard SMT joint
Using vacuum soldering maybe a slower process of course you’ve got to look at the equipment issues, cost of using vacuum soldering. There are many manufacturers of solder paste that really do have extremely low voiding capability so that’s your first port of call but as an alternative if you really need to go to as close as you possibly to zero voids vacuum is an option
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