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Today I will be discussing a customer scenario that I have been seeing pop up more and more lately; voiding under bottom terminated components… but with a twist. This isn’t your standard voiding defect. I had a customer come to me wanting to lower their voiding percentages under a few select QFN style components. They were getting a significant amount of voiding; greater than 50% under the ground pad of this QFN. The first thing I looked at was the reflow profile. The reflow profile was solid and within specification for this particular material, so what could the problem be? A quick area calculation based on the aperture design yielded the conclusion that there was not enough solder paste on the pad.
When I calculated the solder deposit area in relation to the pad area, I also noticed that the ground pad design had a plated via. I asked the customer if this via was filled or open; he replied that it was open. I had an epiphany: of course there is a large percentage of voiding; the solder paste is flowing from the pad down the via creating starvation voids. Starvation voids are formed when there is an inadequate amount of solder to form a joint that would otherwise be adequate. So by coupling the open via and the initial low solder area deposit, I recommended increasing the aperture sizes or stencil thickness in order to transfer more solder volume to the pads, fill the via, and ultimately solve the customer’s voiding issue. Problem solved, right? Not quite. This customer has an extensive stencil library and it would be very costly to change their designs. Of course, they also had this via design on other builds that had to be built. So what could be done? How do we add solder without increasing the apertures or stencil thickness?
Solder fortification preforms!!
What are they?
My teammate and fellow Technical Support Engineer Miloš Lazić explains what the difference is between solder fortification preforms and “regular” solder preforms. I recommend you read the blog here in its entirety. For this post, I will quote Miloš, “Solder Fortification® preforms are rectangular-shaped preforms, and typically come in the same sizes as passive components: 0201, 0402, 0603, and 0805.” They add additional solder volume without adding any additional solder paste.
My next blog post will explore why we recommended using Solder Fortification® preforms to fix some customer problemsUsi. I will also share a new tool that I am developing to help with solder starvation voids caused by vias and how to fix them.
‘Til next time,
Adam
P.s. Let me know what you think in the comments below! Are you experiencing similar issues that this customer is facing? If so, feel free to contact any of our Technical Support Team; we are Global and we are here for you.