Brittle Fracture of BGA Caused by Mechanical Shock

Lead-free solder has become increasingly popular in BGA packaging. This is especially true for portable devices. When mechanically impacted, this packaging method is prone to brittle fracture failure between the solder pad and the ball. It is unacceptable to have a brittle crack at the interface of the solder ball with the bonding pad on the package substrate.

In principle, the reliability and durability of solder joints are determined by board-level drops tests. However, this test has several flaws. Drop tests require many solder joints and packages, which can result in significant costs. If the system is not equipped with high-speed real-time data collection, it may be possible for cracks to close in the solder joints after impact. This makes potential failures impossible to detect. The data analysis is also time-consuming, and expensive. There is a need for an alternative method of evaluating solder joint strength under mechanical shock loading conditions.

The study compares high-speed testing with board-level drops testing for BGA packages, using different surface finishes and lead-free solder ball.

The study of BGA structures was conducted using a variety of combinations including solder alloys and surface finishes. Also, different solder ball sizes were used. Typical test devices included 316 PBGA structures (27 mm x27 mm), using Sn 4.0%/Ag 0.5 %/Cu (SAC 404), and various substrate surfaces including electroless Nickel Immersion Gold (ENIG) or organic solderability preservers (OSP).

316 PBGA sample use standard 0.76 mm spheres. The package substrate is BT laminate of thickness 0.36 mm. The mask layer defines the solder mask pad, which has an opening of 0.635mm. In a hot-air convection reflow oven with a lead-free soldering temperature profile of 150degC+-2degC and a maximum reflow of 260degC the solder balls will be attached to substrate.

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