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How to prevent damage when testing thin devices

19 Sep 2014  | Taqi Mohiuddin

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At EAG, we've experimented with a variety of carrier materials and structures for protecting CSPs during stress tests, including small baskets with covers or a "top hat" that covers the device so it isn't bumped, blown around, or otherwise impacted and damaged (Figure 4). Material is important for protecting the CSP because the carrier is also exposed to extreme environmental conditions. Furthermore, a carrier must be large enough so that the CSP isn't damaged when being inserted into and removed yet not so large that it can move around inside and become cracked. Each carrier can hold up to 240 parts and is unique to the device and custom-designed for the correct size of the CSP.

 Top hat

Figure 4: This carrier includes a "top hat" that covers the device to optimise protection during handling.

Operator training—including how to properly perform inspection, the types of defects to look for, and how to handle parts (if needed) to minimise damage—is essential. If you use sockets during biased stress testing, training operators and technicians on how to insert CSPs into sockets and safely remove them is also required. Training will also be required on the use of custom fixtures, if implemented. Anyone involved with the CSP qualification process needs training, and the most valuable training is generally on the job. EAG has gone through considerable trial-and-error to develop best practices that apply to most situations, including developing the most effective screening processes, carriers and other elements.


Best practices

EAG has established a number of best practices. While no two challenges are precisely alike, there are recurring issues that must be resolved.

For instance, a major chip supplier for mobile devices had a damage issue during stress testing. Qualification lots were being invalidated at an unknown point in the multi-step process. The resulting delays became a real problem because of the pressure to hit milestones in an industry with extremely short development cycles.

Analysis showed that the problem originated at an assembly house where the devices were being diced and bumped. The devices were exhibiting cracks from improper handling. The solution was to implement an inspection process for outgoing devices at the assembly house, including 100 per cent top-and-bottom visual inspection, which revealed the source of the problem at the assembly house. EAG then trained operators at the customer site on the types of things to look for and provided guidelines for ongoing inspection. After instituting these steps, the customer is now able to screen out all damaged parts before stress tests begin.

In another example, a customer that manufactures ICs for consumer electronics devices was having problems with CSP cracking and chipping from the effects of airflow equipment during unbiased stress tests. Although the airflow was very minor, it was causing approximately 20 per cent of the tested parts to be damaged as they were moved around, invalidating the entire screening lot. EAG designed a patented, custom fixture that allowed devices to be secured properly, eliminating the problem at its source.


Conclusion

Resolving CSP qualification challenges requires experience with a number of complex issues across a wide variety of customers and situations. Best practices require the use of specialised processes, the proper choice of sockets, daughtercards or customised fixtures (including specialised carriers for biased test socket insertion), and highly trained equipment operators and inspection technicians. With the right implementation, there should be less than 8 per cent "fallout" on a good lot, ensuring that no lots will be invalidated at any point upstream or downstream in the complex CSP qualification process.


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