Testing the Reliability of BGAs

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BY Ching-Mai Ko, Ming-Kun Chen, Yu-Jung Huang, and Shen-Li Fu, I-Shou University

Experimental analysis of failure site conditions for the final testing of BGA packages, focusing on touchdown-times-related worn-out changes in BGA sockets, is presented. Electrical characteristics BGAs are generally tested using a pogo-pin socket. A low and stable contact resistance must be maintained between the pogo pin and the solder ball of the BGA package. In conventional final test, BGA sockets are punctured against the solder ball to obtain a low contact resistance by mechanically breaking down the interfacial oxide layer on the solder ball. Here, we provide an experimental procedure for investigating the effect of particle contamination and the worn-out of crown tip in final testing of BGA packages.

Demand for BGA packages continuously grows because of their low cost and high pin-count density. As product quality and production capability requirements increase, BGAs present a popular packaging alternative for high I/O devices. Having no leads to bend, the plastic-overmolded BGA (PBGA) has greatly reduced coplanarity problems and minimized handling issues. BGAs are available in a variety of other types as well.1,17 BGA testing ensures performance and quality, but it also makes up a large portion of production cost. Significant loss is generated when circuits are proven faulty after the packaging processes. BGA test sockets are separable interconnection between BGA-type packaging components and a loadboard. Establishing temporary connectivity to a solder ball on a BGA pin is becoming increasingly difficult with decreasing feature size, increasing operations frequency, and new technical concerns. The greatest challenge of BGA sockets is to avoid damage to the solder balls while maintaining a contact force and a stable contact interface.3 To ensure accurate measurement results, it is essential that a low and stable contact resistance is maintained between the pogo pin and the BGA solder ball. In general, contact resistance of less than 20 mΩ is desirable, depending on the particular application.1,2,3

60572-th_testing 0709 01.gifSchematic cross-section of the BGA package and test socket structure.

In conventional final testing of BGAs, the socket pins puncture the solder balls to obtain a low contact resistance by mechanically breaking down the interfacial oxide layer. However, variations in the magnitude and stability of the contact resistance are observed following repeated contacts between the pogo pin and the solder balls, as a result of interfacial phenomena between the crown tip and the oxide layer and the accumulation of particles from lead-free solder balls on the crown surface. To maintain a low contact resistance and a stable electrical contact, increase the drive force concurrently with the pogo pin contamination, and clean the crown tip on a regular basis. However, an excessive drive force creates large puncture marks on the solder ball, which not only increase the risk of permanent pogo pin deformation, but may also adversely affect package performance and reliability. Furthermore, frequent abrasive cleaning reduces the socket’s service life. An appropriate tradeoff is required between contact resistance and the number of allowable touch contacts between cleaning operations.

This article covers the behavior at the contact surface via surface failure analysis. An experiment was carried out to clarify the effect of the contact failure on the IC testing. Test sockets touch down and contact the solder ball of the BGA package. At the same time, the power supply and input test pattern was applied to the device under test (DUT). Then the relationship between the contact force and the vibration was active during final testing. The contact zone of BGA socket and the surface-flattening effects on the crown tip are assessed using optical microscope (OM), scanning electronic microscope (SEM), and energy dispersive X-ray spectrometry (EDX) techniques after test, and the influence of these changes on the contact resistance is correlated. Conclusions about the instability effects on the contact pin phenomenon and the influence of the contaminations on the socket are based on the experimental results.

Design of Experiment (DOE)

The commonly used 32-mm2 356-ball PBGA package is the assembly vehicle for this study. The package can match a mass production impact on high-speed applications. The substrate core material is a four-metal-layer copperclad BT laminate. Its solder mask thickness is 70-µm and BT resin core thickness is 150 µm. Its prepreg thickness is 100 µm with the inner layers’ copper trace at 37 µm thick compared to the outer layers’ 30-µm-thick copper trace. Eutectic Sn/Pb solder comprises the 0.63-mm-diameter balls on a 1.0-mm pitch.

Online at smtonline.com

Read the full technical article online! Visit smtonline.com and search for “Testing the Reliability of BGAs“ to finish reading this feature. The second half of the article includes information on experiment materials and procedure, and the results discovered by the BGA testing team. SMT



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