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Excerpt from: The Printed Circuit Assembler's Guide to... Process Control, Chapter 3
Process Control
As established, the SEC technique can be used as a process control tool. First published over 50 years ago, the SEC test was used in defining the ROSE test, that is IPC-TM-650 Method 2.3.25. There, the concept behind the test method is that of a cleanliness test, rather than a process control test. (We have already discussed its deficiency as a cleanliness test.)
GEN3 undertook a total rewrite of this test to render it suitable for optimised process control in order to fulfill Six Sigma requirements in electronic manufacturing4. The new method can be found in IEC 61189-5-504 but, given how few companies have acquired a copy of this test standard, we will deal with the extant deficiencies between PICT and ROSE. In addition, we will discuss the two commonly encountered equipment arrangements of SEC. These two variants are identified as closed loop and open loop and are shown schematically in Figure 3.1.
In IPC-TM-650 Method 2.3.25, the use of the terms static test and dynamic test are misleading. They came about originally to bypass patents that existed at the time. In truth, the test should be regarded as closed loop (static) and open loop (dynamic):
Closed Loop. The test solution in the test chamber is pumped via the ion exchange column, or regenerating filter, until it reaches a predetermined level of conductivity. A valve closes and isolates the ion exchange column from the circuit, and now the solution only circulates through the tank. The test specimen is introduced into the test chamber and the test solution begins to recirculate, bypassing the filter.
Open Loop. The test solution in the test chamber is pumped via the regenerating filter until it reaches a predetermined level of conductivity. The test specimen is introduced into the test chamber and the test solution begins to recirculate, through the filter.
Using the PICT test as a process control tool was discussed briefly above. An OE exercise using the SIR test will qualify a material set and produce the necessary SIR data that forms the OE. The material set in this context encompasses all the various materials: PCBs, solder masks, solder pastes, adhesives, coatings, etc. However, the SIR technique cannot be used to characterise the final product: board assembly. Products will include specific components each with their own unique materials, surface roughness and finish, and residue entrapment potential. Many components are incompatible with SIR testing as the internal connections short out the SIR measurement—hence, the use of dummy components in the standard test vehicles.
Some SIR testers advocate a snap-off area from the product that can be used for a quick SIR test. This sample board will not be representative of the product; it will not contain the same component set. Therefore, changes in the product, in particular component material details, will not be reflected in the snap-off sample. This snap-off board cannot track all changes in the product; hence, it is insensitive and unreliable in following changes in the product.
