When Should You Use Underfill?

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, also known by day as consultants for ITM Consulting. We are coming to you from Board Talk Headquarters high above Mt. Rialto. We're here to answer your process questions, and I believe we have a very interesting one today.

Jim
This question comes from T.S.Y., and the question is, "When should we use underfill? We have a circuit board assembly with a wafer level chip scale package. The package is 2 x 2 millimeters with 20 balls. The diameter is 20.2 millimeter, and the standoff is .16 millimeter."

I would assume that's with a .2 millimeter ball, that that's probably a .4 millimeter pitch, but that's a guess. But the question is, "Under what circumstances should we use underfill? What type of underfill material do you suggest?"

When should you underfill?  - - - When the board without underfill does not meet your reliability requirements either through thermal cycling, or vibration or shock; whatever you use to validate this product. If these very small solder joints from the 20.2 millimeter balls are cracking, or not holding up under the service life of the product, then you can use an underfill material to improve the structural integrity of those joints and hopefully meet the reliability requirement.

Underfill is continuously evolving science, new materials are being brought out every day. I suggest you talk to the major suppliers such as Henkel Loctite and give them your specs, the size of the component, and standoff and so-forth.

Look at the type of process you're going to use to apply the underfill, the type of dispenser, whether or not you want to use heating during the process while you're dispensing, the cure cycle, and chose the best material that's available specifically for your application. I wouldn't attempt to give you a specific recommendation. How about you, Phil?

Phil
The only other thing I'll say is typically we found that chip scale packages using elastomer interposer, such as those covered by the Tessera generally do not require underfill, but I can't tell you if I'd take that to the bank, so you definitely want to check.

The bottom line is reliability. What are you seeing in your reliability testing? So, we hope this helps answers your question. 

Jim
Typically underfill has been used for flip chips and not for advanced of packages that have more robust soldered joints.

As we're shrinking packages into wafer scale or chip scale packages, the solder balls and solder joints are getting smaller and smaller, we're converging with the tiny soldered joints on flip chips and experiencing some problems. The joints are just not strong enough to survive all the surface conditions in the normal product life, and underfill is a viable solution for some. I can't say it's a panacea.

Phil
Beyond that, this is Phil Zarrow and Jim Hall, the Assembly Brothers, saying...

Jim
Don't solder like my brother whatever you do.

Phil
And don't solder like my brother, and keep the kids away from that underfill. You don't know what's in it.

Jim and Phil, Good talk. I think you hit the nail on the head here, you use underfill when the product can not meet the reliability requirements. There are a variety of underfill materials, each designed to provide additional reliability for different conditions. For example, a cornerbond material for bend/twist conditions, a low Tg flexible material for drop/shock, or a low CTE material for rigorous thermal cycling. New advancements have allowed us to now have high Tg reworkable materials - providing additional TC reliability and allowing rework.

An alternate approach is to use a constraining layer to reduce the CTE of the PCB. This approach has been used for decades with Copper-Invar-Copper. A new generation of constraining materials include an array of composites, such as carbon fiber-reinforced polymers, aluminum and copper, among others.