Circuits Assembly Online Magazine - Shotgunning: Not for Pb-Free

A proper test approach can reduce scrap from random repair methods.

This year in this space we have covered the impact of Pb-free on the full array of test and inspection methods, including solder paste inspection, automatic optical inspection (pre-reflow as well as post-reflow), automatic x-ray inspection and in-circuit test.

One technique we have not discussed is functional test. Of all the test and inspection techniques, functional has the lowest diagnostic resolution. Therefore, manufacturers often use a repair technique referred to as shotgunning.

Typically with this technique, a repair technician suspects one component to be faulty and replaces it. When the functional test is run again it becomes clear that was not the case. So the technician suspects another component is causing the fault and replaces it. This process often continues until the board is repaired, damaged by all the repair attempts, a pre-set number of repair attempts are reached or the technician runs out of ideas for what is causing the fault and the board gets scrapped.

Shotgunning is expected to wreak havoc on Pb-free processes. Because of the higher reflow temperatures of Pb-free solder, components and substrates will undergo added stress during repair. The number of acceptable repair attempts will likely be lower, and damage to the board and components is more likely.

It is recommended that shotgunning be reduced as much as possible. Pb-free processes should aim to minimize the defects escaping to functional test. If nothing is done, defect levels are likely to rise. Plus, higher reflow temperatures will likely increase internal component defects, resulting in higher scrap costs and warranty costs.

To catch all manufacturing defects prior to functional test, increase test and inspection coverage prior to this step. Since fewer repair attempts will be available, methods to minimize shotgunning using software-based intelligent diagnostic solutions should also be considered. If functional test calls for bed-of-nails type fixtures, the same recommendations as for ICT apply: to avoid probing bare copper or OSP (organic solder preservative) covered copper. The solution in most cases is to put solder on the test pads by opening up the solder paste stencil for these locations. The fixture contact problems at functional test, combined with bad diagnostic resolution, make shotgunning a nightmare.

In our research of defect levels and the effectiveness of test within the normal SnPb process, we have observed that many defects escape to functional test. Most could have been detected earlier and corrected. The best strategy for Pb-free is to minimize those defects. This can be done via a more rigorous test and inspection strategy prior to functional test and also a more rigorous repair implementation around AOI and AXI.

The optimal test strategy should be an economic tradeoff. The added cost of more rigorous test pre-functional test should be compared to the lower scrap and warranty costs that would result from fewer defects escaping to functional test and the end-customer.

The transition to Pb-free is a major process change and an increase in defect levels is likely for many board types. This makes optimal test strategies more important than ever. An optimal test strategy should include defect prevention and defect containment techniques. Defect prevention should be focused on pre-reflow using SPI and pre-reflow AOI. The objective with defect prevention is to identify systematic defects and the root cause of these defects, and eliminate them. With Pb-free, systematic defects will likely increase and it is important to identify and eliminate them.

Even with very good defect prevention strategies in place, random defects will still occur and good defect containment strategies should be in place. Higher defect levels, caused by the transition to Pb-free, will also increase the need for an inspection and test strategy with high defect coverage. In many cases the optimal defect containment strategy for PCBAs will include AOI post-reflow, AXI, ICT and functional test.

Expect wide variation. While some board types will offer few challenges, others – mainly higher complexity – will present significant challenges in form of higher defect levels and a changing defect spectrum. Be prepared, plan for the worst and hope for the best.

Stig Oresjo is a senior test strategy consultant with Agilent Technologies (agilent.com), 970-679-3215.