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June 2015 • SMT Magazine 71 In-CIRCuIT PIn TeSTInG: continues aCross the Board test. In addition, one common laboratory test vehicle contains four different types of pads. In this example, pad A is a 40 mil (1 mm) square pad without vias, pad B is a 40 mil (1 mm) square pad with 13 mil (0.33 mm) vias, pad C is a 28 mil (0.7 mm) round pad without vias, and pad D is a 28 mil (0.77 mm) round pad with 13 mil (0.33 mm) vias. (Figure 5). There are 1,000 opportuni - ties for each pad type. Pads with vias tend to collect more flux resi- due and generally have lower yield for a given paste or wave soldering/selective soldering flux. Often these pads are the source of discrimination between high performing paste and flux, because the first pass yield is somewhere between 0 and 100%. Tests that show 0% or 100% yield are nev- er good tests for discriminating between the ef- ficacy of material sets. Having said that, value is created when the first pass yield of the ICT approaches 100%. This means that the pin probe is able to penetrate flux residue and make positive contact with the test point in the circuit. Value is lost when the probe does not make a low resistance reading with a fully functioning test point, but the test shows an open or high-resistance circuit. This is a false negative. Even though the assembly is perfectly functional, re-testing and extra handling are re - quired, increasing the assembly cycle time. Based on actual test data, a value-in-use mod- el has been created to estimate the value of in- creased first pass ICT yields. A leading competi- tive paste has been tested using both the clam- shell and flying probe method discussed above. In-circuit paste testing accuracy is a signifi- cant competitive advantage. If the paste or flux residue is not conducive to clamshell or flying probe test accuracy, you may quickly fall behind in an increasingly competitive marketplace. SMT CliCk To View vIdeo IntervIew Bhanu Sood describes the services offered by the center for Advanced life cycle engineering (cAlce), and explains how physics- of-failure techniques are applied as a means of gaining a scientific under- standing of reliability. Open Failure Analysis Lab Assessment Service Available by Real Time with... IPC APEX EXPO 2015 Mitch holtzer is global director of customer technical service (cTS) for Alpha. To reach holtzer, click here.

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