SMT007 Magazine

SMT-June2015

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68 SMT Magazine • June 2015 In-circuit pin testing (ICT) is a common method of inspecting electronic assemblies to measure the effectiveness of the assembly pro- cess and to predict electrical functionality. Test probes are put in contact with dedicated test points along the surface of an assembly, checking for electrical functions such as resistance, capaci- tance inductance and signal timing. Some circuit assemblers use functional testing for quality as- surance as a substitute for ICT. In this example, the entire assembly either works or it doesn't. Unlike printing paste, placing components and reflowing solder, ICT is considered a non- value added process. In fact, ICT increases the cycle time of the assembly process. If the ICT is a fail, but the circuit being tested is actually good (known as a false negative), even more time is wasted trying to determine whether or not a good assembly will function properly. The two basic types of ICT are commonly re - ferred to as clamshell and flying probe (Figures 1 and 2). The clamshell ICT simultaneously tests doz- ens of points on a single board. One laboratory type of clamshell ICT device uses three types of pins and four different forces. Crown, spear, and blade pins are shown in Figure 4. A test vehicle is placed in the clamshell fixture. The clamshell is closed, and electrical resistivity is measured at each of the test pins. Although clamshell devices are commonly used, they are somewhat impractical for testing solder paste and flux's ability to be probed. Typi - cally, there can be a 15-minute cycle time per test. Pin residue build up generally occurs after hundreds of tests. Cleaning flux build-up from pins is time-consuming, but necessary to mini- mize false negative readings. Clamshell fixtures are quite expensive. A customized fixture must be fabricated for each unique assembly. This cost is only justified for the testing of high-value or high-volume assemblies. Flying probe testing is also commonly used for lower value or lower volume assemblies. Fly- ing probes test sequences are determined by eas- ily changeable programming inputs. Clamshell fixtures are not required. Therefore, this type of test method is more suited for high mix circuit assembly applications. In a common laboratory flying probe proce- dure, one pin type and one force are used, but the test runs for a total of 4,000 strikes. Flux build-up on the pins is an important part of the by Mitch Holtzer AlPhA ACROSS THe BOARD In-Circuit Pin Testing: An excellent Potential Source of Value Creation ColuMn figure 1: clamshell icT device. figure 2: flying probe icT device.

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