Material | September 02, 2008

Head-in-Pillow: The Defect that Caught Us Napping

Although the head-in-pillow defect is not totally new, the high frequency of occurrence is. This defect is a direct result of the convergence of product miniaturization and the transition to Pb-free solders.
The head-in-pillow defect is an open solder joint in a BGA or CSP where the solder paste deposit doesn’t coalesce to the ball on the component. The result is an apparent solder joint with a hairline gap between the solder reflowed to the PCB paste and the solder ball itself. Often, the BGA ball deforms the solder on the pad to look like a head resting on a soft pillow. This defect is particularly troublesome because it is difficult to detect even with X-ray inspection. In addition, sometimes the mechanical contact between the two solder deposits allows it to pass functional testing but fail quickly in the field.

The root cause of this defect is most often associated with the warping of a BGA component during reflow. As the component warps, it separates from the solder paste and both the ball and paste reflow independently of each other. As electronics products continue to miniaturize, BGAs and CSPs are becoming smaller and lighter, making them highly susceptible to warping. Additionally, the higher the reflow temperature, the more the components will warp. That is why the transition to Pb-free solders with significantly higher reflow temperatures is a critical factor in the increased frequency of head-in-pillow defects.

Although warping is usually the root cause, paying close consideration to three areas of the assembly process can mitigate the occurrence of head-in-pillow defects; choosing the correct solder paste, consistent printing, and a slow ramp rate.

The choice of solder paste has been shown to have a profound effect on head-in-pillow defects. The three characteristics of a solder paste that are essential to reduction of this defect are:

• Ultra low slump: It is critical that the solder paste stays in contact with the BGA ball as long as possible. A solder paste that slumps will drop away from the ball even with very little component warping.

• High tackiness: A solder paste with high tackiness allows it to “stretch” with the component as warping occurs, allowing the solder paste to remain in contact with the ball for a greater part of the reflow process.

• High activity and oxidation barrier: The fundamental reason that the two separated solder deposits don’t coalesce back together during cool down is that there is an oxide layer on the two molten solder surfaces. Activators that remove oxide and oxidation barriers, such as rosin, can prevent further oxidation from occurring.

When looking at the print process, it is essential to get a consistent amount of solder paste onto each pad of the BGA. If there is significant print volume variation, the pads with insufficient paste will be more prone to head-in-pillow defects (similar to the problem you would see with paste slumping).

In reflow, the key is to utilize a slow ramp rate and as low a peak temperature as is practical. The slow ramp prevents solder paste slump and the low peak temperature minimizes component warp and oxidation of the solder.

With proper preparation and process optimization, the head-in-pillow defect can be kept to a minimum.

This article was issued by Indium Corporation.


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