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JULY 2018 I SMT007 MAGAZINE 43 true level of performance, especially when too many data points come from locations where it is easy for all pastes to perform well. Other factors related to print performance that can also be included are slump perfor- mance, performance after pauses in production, and stencil life. That list is certainly not comprehensive and any factor that is impor- tant to an assembly process is a candidate for testing during an evaluation. Smaller evalu- ations may be able to rely on the testing performed by the manufacturer to standard- ized test methods such as hot and cold slump, where eval- uations with higher resource allocation may desire to repli- cate these tests during evalu- ation. Larger evaluations can also develop unique tests or test vehicles to reflect specific issues encountered or unique needs of their application; the limits to evaluation test development are imagination and resource availability. Reflow is another area where quality measures can be applied as an evalu- ation test. Voiding is prob- ably the most obvious and relevant quality test that can be performed after reflow. There are also means to test the wetting and spread of a paste, resistance to graping, solder ball performance, and head in pillow and non-wet open defects. The focus during test develop- ment should be to identify the key outputs from reflow that are pertinent to the process, just as was done for print quality and with an eye towards resource limitations. Reliability is the second major factor that any evaluation should include. Reliability is defined by ASQ as "The probability of a prod- uct's performing its intended function under stated conditions without failure for a given period of time." In the case of solder paste, there are two areas of focus: mechanical, which is driven by solder alloy, and electro- chemical, which is driven by flux chemistry. Reliability testing re- quires an understanding of the service environment of the end product, which is why the proper tests depend very heavily on the design of the assembly and how the customer uses it ("perform - ing its intended function"), where the customer plans to use it ("under stated conditions"), and the length of the warranty or customer's expected product life ("with - out failure for a given period of time"). This is defined by the design function during product development, so these factors are generally easily determined by any organization performing both design and manufactur- ing. Contract manufactur- ers, on the other hand, rarely have visibility to these prod- uct factors and either need to choose representative tests or consult with their customers during development of any reliability test plan. Unfortunately, reliability tests are neither inexpensive nor fast, so this is an area that can be tempting to cut out in smaller evaluations—but at the peril of those who choose to do so. Reliability factors are ones that cannot be easily observed at the time of manufacture, but manifest themselves as poor customer satisfaction over time, long after the decision to adopt a new material has been made. The Microsoft® Xbox 360 "Red Ring of Death" is an excellent example of a reliabil- © Alpha Assembly Solutions Inc.

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