When I was first introduced to QFN voiding, it appeared to be a straightforward issue: outgassing from flux creates pockets of entrapped air causing reliability problems. But I’ve since come to learn so much more about voiding from the many fantastic resources here at Indium Corporation. When we dive deeper into exploring the types of voids and how they occur, more complex interactions between material and process variables begin to appear.
The Ishikawa Diagram ifrom Chris Nash's post is a great tool for diagnosing the root causes of voiding, but we can also learn a lot from the size, shape, and location of the void in question.
Typical voids from outgassing, also known as process voids, can be found in the bulk of the solder joint and have a large, bubble-like appearance. These are the voids we mentioned above. They form because the flux volatile becomes trapped during outgassing. These types of voids can often be reduced by adjusting the reflow profile.
Voids located at the pad are typically wetting-related, meaning that there is an issue with solderability. Adjusting the reflow profile might not be enough to to fix this issue. Often voiding at the pad is caused by solder starvation, or because an inadequate solder volume is present. Starvation is often a result of the reduction in stencil thickness. Why? In simple words there isn’t enough solder volume present to fully wet the pad.
A thicker stencil should result in less voiding. In addition to increased solder volume, when the standoff of a particular component is higher (due to a thicker solder paste deposit, due to a thicker stencil), the flux volatiles have more room to outgas and escape confinement from under the component.
These factors are important because there are serious consequences to starvation voids, including:
- Weak solder joint strength
- Open solder joints
- Intermittent short circuits
- Reduced first-pass yields
- Increased inspection
- Increased rework
- Field failures
- Damage to your company's brand and image
- Reduced sales and profits
There are many variables that can help reduce voiding. One simple solution is to use Indium8.9HF Solder Paste. Additionally, you should look at the reflow profile. Indium8.9HF is enhanced by using a high peak temperture profile to dramatically improve wetting. With those first two variables covered, the next step to reduce voiding is to use an appropriately sized stencil. Once the solder paste, reflow profile, and stencil size are set up correctly, the bare areas on the pad where the flux remnant could pool are minimized, and the surface tension of the molten solder is lowered, enabling the volatiles to escape easier — thus reduced voiding.
For more information, or to talk about how to adjust your variables for reduced voiding, reach out to our Technical Services team or contact me at chotvedt@indium.com