Redesigning the ground pad to increase standoff height resolves PCB cleanliness issues in both no-clean and water-soluble flux environments.
Flux residues under component bodies on a printed circuit board can cause reliability problems for manufacturers. This article explains an approach that reduces flux residues and ionic contamination trapped beneath PCB components.
When producing electronic assemblies featuring low-standoff components such as QFNs, dealing with flux residues can be tricky. Different kinds of flux are recommended by component manufacturers in response to this issue, but each is accompanied by distinct disadvantages. In the end, the type of flux to use isn't as important as you might think.
Some component manufacturers recommend no-clean fluxes, as it has been proven that residues are difficult to fully remove. The thought process behind recommending a no-clean flux is that any remaining residues will be rendered near benign and not cause issues; however, we routinely see instances of the opposite. As no-clean flux activators outgas, with insufficient clearance between the bottom of the component body and the PCB surface, the gasses can become entrapped. Many companies are attempting to clean no-clean fluxes which can increase the risk of promoting electrical leakage versus not cleaning, due to incomplete removal of the residue. When the outer-layer of no-clean flux is removed, the contaminants that are normally entrapped will become exposed. This is like ripping a bandage off and leaving the wound exposed.
Alternatively, when using a water-soluble flux, any material left behind will be both conductive and corrosive. These residues may absorb moisture and facilitate electrical leakage and/or electrochemical migration on tightly-spaced architectures. When processing with a water-soluble flux, the standoff height creates a dam that is difficult to penetrate with inline or batch washers.
Redesigning the Ground Pad
One option for increasing standoff height is redesigning the ground pad, which can create avenues for no-clean fluxes to outgas or for wash solution to better penetrate under the part. The photos below show alternatives to solid ground pads on the PCB.
You can see in the next image that without pad redesign the standoff height is approximately 0.8 mils, but after pad redesign it is approximately 3-4 mils.
.80 mil lift with solid ground pad
3-4 mil lift after pad redesign
Cleanliness data before pad redesign:
Cleanliness data after pad redesign:
Ion chromatography (IC) analysis was performed on two groups of samples, one utilizing the standard pad design and one utilizing a redesigned pad. IC data for the group utilizing the standard pad design exhibited elevated levels of weak organic acids (WOA), well above Foresite's recommended cleanliness limits. Conversely, the group of samples with the redesigned pad exhibited low levels of ionic and organic residues, which were well under Foresite's recommended cleanliness limits. These ion chromatography results show a vast improvement in the post-redesign cleanliness data. This is specific to no-clean flux, but the same principle applies to water-soluble, as the avenues created allow effective cleaning under the part.
While the low standoff height of bottom-terminated components can present a challenge for assemblies, there are proven ways to combat the issue and produce reliable hardware—such as redesigning the ground pad.