In-circuit test

In-circuit testing is primarily a diagnostic tool. It verifies the functionality of the individual components of a subassembly. Each device is checked and failing parts are identified. In-circuit testing, while valuable for detailed component checking, does not operate at the clock rate of the subassembly. Propagation delays. 

Testpoint analysis SMT components need to be tested by in-circuit test (ICT).These analysis check testpoints to vias, surface features, tooling holes, and electrical nets. See Table 20.6d for a typical checklist for testpoint analysis. 

Probe-ability This is the ease with which conventional in-circuit test probes are able to penetrate surface oxides or solder flux residue. 

The ideal flux would be one that is potent, requires only a small quantity, leaves little or no residue (and what residue does remain would be thin, transparent, nonreflective), easily penetrated by in-circuit test probes, electrically insulating, and chemically inert. If not inert, it should be easily and completely removable in an inexpensive solvent. However, this is far from actual materials and process reality. 

FIGURE 46.5 ICT probe noncontact (a) If there is flux on a test pad, it will harden after soldering and may prevent the test probe from making electrical contact to the test pad. The test result may be interpreted as an open circuit, (b) As the board is fluxed in preparation for the wave solder process, the liquid flux may be drawn by capillary action between the wave solder pallet and the PWB. When this occurs, the in-circuit test pad (the target for bed-of-nails testing) may become fouled with solder flux. The flux residue inhibits probe contact. 

During wave soldering, most of the flux from that process is consumed or washed off the bottom side of the board when in contact with the molten solder wave. However, if no-clean flux residue seeps between the pallet and the board, it will not be removed by the wave process. Any remaining residue, if covering in-drcuit test pads, may be an impediment to proper in-circuit test probe contact. Care should be taken to minimize flux deposition for wave soldering. In terms of board design, test pads should be moved as far from intended wave-solder pallet openings as possible. 

A common defect associated with no-clean wave soldering is the seepage of flux between the pallet and the board. The resulting flux residue may inhibit in-circuit test probe contact. Care should be taken to ensure that the board fits well into the pallet nest, is rigidly indexed within the nest of the pallet, and is adequately retained against the pallet surface. As a pallet ages, it may shrink, take on twist, or bow and even delaminate. All of these may interfere with proper board seating. 

For EGAs and other area-array devices, following a visual check, use transmission x-ray to inspect for solder bridges. Sometimes this method also reveals lack of solder. Lastly, if in-circuit testing or functional testing is done on the first-build product, then it should be applied to the reworked product also. 

The bolster plate may be orthogonal or sculpted to accommodate components in the vicinity of the bolster plate or to permit access to test points for in-circuit testing (ICT) or diagnostics. The design of the bolster plate must be such that it is rigid upon fastening and must flatten out any local bow or warpage in the vicinity of the LGA contact field on the PWB to ensure that all electrical contacts are made and mechanical stability is imparted to the LGA stack. 

A fault is a manifestation of a defect An example is a digital device output pin that does not toggle correctly. For simphcity, think about a two-input OR-gate whose output is stuck high. This is a fault and is a manifestation of a defect. The causing defect can be any one of several, including a defective component, an incorrectly placed component, an open input pin, or an open output pin. The fault class is a subset of the defect class. Electrical tests such as in-circuit test (ICT), boundary-scan, built-in-self-test (BIST), functional test (FT), and system test mainly detect faults. 

Once a tester has access to the nodes of a board, it can perform in-circuit (also called in situ) tests. The idea is to test components as if they are standing alone, while they in fact are part of a board circuit. The actual electrical processes used in in-circuit testing are explained in Sec. 55.5. 

The problem of in-circuit testing can be subdivided into two main categories, analog in-circuit testing and digital in-circuit testing. 

Analog in-circuit test addresses testing for shorts in the printed wire circuitry analog components, passive devices such as resistors, inductors, and capacitors and simple semiconductor components such as diodes and transistors. Analog in-circuit testing is conducted without applying power to a board that is, it is an unpowered test methodology. 

Complex analog devices such as analog or mixed-signal ICs are not very amenable to analog in-circuit testing because they require power to be apphed to the board. Simple diodes and single transistors can be tested by in-circuit stimulus that essentially examines the characteristics of their semiconductor junctions. 

Digital in-circuit test focuses on the digital components residing on a board, and requires that power be applied to activate the digital logic contained within the ICs. Just as anaiog components... 

A great advantage of digital in-circuit testing is that it is performed directly on the inpnts and ontpnts of a targeted device. If the device should fail testing, this is seen directly, rather... 

Another major differentiator is the ease— indeed, automation—of test programming that is possible with digital in-circuit testing. Tests for ICs can be prepared as if the ICs were standing alone, stored in a hbrary, and recalled from the library when needed. Modem digital in-circuit testers may have hbrary tests for tens of thousands of devices. For custom, one-of-a-kind ICs for which a hbrary test may not exist (e.g., ASICs), it is still substantially easier to create a test for just the one device than it is for a collection of ICs. 

The workhorse of the electronics industry is the general-purpose in-circuit tester that merges support for analog and digital in-circuit tests. An example of a widely used system is shown in Fig. 55.10. It contains power supplies for powering boards and often contains sophisticated... 

FIGURE 55.10 Example of a commercial in-circuit test head and operator terminal, with a printed circuit board mounted in the testing position on top... 

In situations where a manufacturing line has a variety of technologies in production, a need may exist for both functional and in-circuit testing. Thus, hybrid functional/in-circuit testers (commonly called combinational testers) exist. These machines give test engineers a full... 

V. R. Harwood, Safeguarding Devices Against Stress Caused by In-Circuit Testing, Hewlett Packard Journal, vol. 35, no. 10, October 1984. 

Monotonic (bend-to-failure) testing is designed to simulate the assembly conditions to which a PCA is subjected in manufacturing. A variety of assembly steps impart loading conditions that are similar to a monotonic bend test, such as heatsink attachment, connector attachment, in-circuit test (ICT), handling, and card-cage installation.These assembly steps could mechanically... 

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