Curve-trace testing can reveal whether an incoming batch of ICs are bogus without resorting to destructive inspection measures.
Alan Lowne | Saelig
Making fake “lookalike” integrated circuits which resemble real ones takes minimal skill. It may simply require finding cheap parts in the same package and applying new markings. This problem has arisen due to the high value of electronics parts, and the whole manufacturing chain from assembly house to end-user is vulnerable.
There are several methods the counterfeiters use to produce their fake goods. Consider just one means, salvaging ICs from old circuit boards. ICs recycled from old PCBs are often so old that they contain lead or other materials now banned by RoHS. Moreover, those built to use tin-lead solder were built to use relatively low peak solder reflow temperatures (typically below 235°C). Today’s lead-free IC packages now see peak reflow temperatures as high as 260°C. Manufacturers that mount what they believe are lead-free ICs can unknowingly induce major reliability hazards such as cracking or delamination of the package.
Conversely, counterfeiters may mark lead-free packages as lead-bearing to meet demands for legacy lead-bearing packages. This may cause tin whiskers to form between adjacent pins and solder balls on ICs.
To better mimic original parts, counterfeiters now largely mark IC packages with lasers rather than ink. The problem is that counterfeiters usually do not know the depth of bond wires in plastic packages, especially when they have thinned these packages by chemically or mechanically removing the original package markings. So the fake laser-marking process sometimes partially melts bond wires.
Bad laser marking can also compromise hermetic packages. In one case, counterfeiters laser-marked iron-based IC lids plated with nickel and gold. The laser fully removed both layers of plating to expose the underlying iron. Prolonged exposure to moisture would corrode away the iron, allowing moisture ingression.
Also in attempting to make old components look new, counterfeiters typically can use acids on package pins and solder balls. These acids may be incompatible with the package materials, and sometimes counterfeiters don’t wash them off completely. Particularly if the counterfeiters cause package delamination with their antics, acid residues can corrode active die circuitry after months or years of use. Some of the same problems can arise when ICs are new but have been rebadged, remarking cheap ICs to look like more expensive versions.
Technical measures to detect counterfeits have previously included visual inspection of devices for marking errors – which needs a trained eye for all possible variations in marking. The x-raying of incoming parts is another technique. Non-destructive imaging techniques such as radiography can generate 2D images showing internal chip features, and computational tomography can generate 3D representations from multiple 2D projections. More sophisticated non-destructive techniques include energy disruptive spectroscopy, which involves high-energy x-rays, and terahertz time-domain spectroscopy. There are also destructive methods that destroy the IC. For example, a complex decapsulation system can be used to visually inspect IC die samples. However, all these procedures are expensive and time consuming. They require skilled operators and expensive equipment.
Some distributors have advertised screening services for verifying components with a turnaround time of “as little as two days.” That time frame is unacceptable in many cases. These companies offer techniques such as x-ray, x-ray fluorescence analysis (XRF), decapsulation, heated solvent testing, visual inspection, and solderability testing. These tests result in detailed reports – when all that is often required is an answer to the question, “Is it a good part?” In reality, this approach is only viable for military or large volume production runs.
Another approach for detecting counterfeits is to perform a functional test on a sample of the ICs; logic I/O conforming to a truth table is an example. This technique is faster and less expensive than typical destructive and inspection tests. It will detect gross problems, such as a incorrect logical function, or no function at all, but will miss the subtle ‘out of tolerance’ issues – tell-tale signs that a component is counterfeit. With older-technology IC families, different speed variants are available. Conventional testing equipment with this level of speed test capability is extremely expensive.
There is a different kind of non-destructive electrical test used to detect counterfeits. Called a curve-trace test, the idea is to apply a voltage and current to pairs of pins on the IC to gauge the V-I behavior. The stimulus signal can be a sine wave, a triangle wave, a ramp, and so forth. The stimulus is applied to all pins on a known-good part, then the resulting responses serve as a template for what to expect from ICs that aren’t counterfeit. This response is directly related to the device characteristic, its internal structure, and its manufacturing processes. These types of tests can detect defects such as missing or broken bond wires, cracked die, and damage to hermetically sealed chip packages.
Instruments that carry out curve-trace tests of this sort were originally used to check out discrete transistors and diodes. But these first curve tracers generally could only test one discrete semiconductor device at a time. Their modern counterparts are designed with multiple channels to simultaneously stimulate all or most of the pins on a modern IC.
An example of one such device in this category is the ABI Sentry Counterfeit IC Detector. Sentry is PC-driven and checks the validity of parts in seconds. The product is designed to be used by personnel in a receiving department. The analysis takes place in the background and the operator only sees a simple “Good Device,, “Blank Device,” or “Fail Device” message, with the option to produce a detailed report to send to the supplier.
The ABI Sentry is a benchtop device that uses an advanced form of VI testing to determine an IC’s electrical characteristics or signature. The Sentry’s VI Matrix Test exercises every possible pin combination on the IC under investigation. This provides great insight, more than simple systems that are restricted to testing between pins and ground. The Sentry’s Matrix VI Test can reveal differences between devices with different functionality but similar technology. For example, it can detect a relabeled chip with the same input/output pinout. So the Matrix VI Test yields much more useful data than the more limited pin-to-ground test.
The VI characteristics captured by Sentry are called PinPrints and are the unique signature for a device. In operation, technicians would first use Sentry to test a known-good device and obtain its “gold standard” signature. They would then compare subsequent signatures of incoming, unknown chips with the known-good version to check for discrepancies. Small variations are likely to indicate that the chips are from different manufacturers, or possibly different batches from the same manufacturer. Larger differences, however, suggest that the chips are faulty or counterfeit.
Sentry can be customized for each IC type by setting tolerances that define the point at which a tested device is deemed bad. If no reference devices are available there are two alternatives. Reference data can be imported into the Sentry database from other machines or libraries. Alternatively, and not quite as good, testing can be done across a batch; if there is any variance then the whole batch becomes suspect and should be rejected.
A package with no internal die is easily detected – all pins will show the straight line ‘null response’ of an open-circuit. Sentry uses a comparative technique to rapidly analyze and learn new components, and then test the unknown parts. A known-good component is locked into a ZIF socket while a test pattern is applied across all its pins. The component’s response to this test pattern is automatically measured and stored as a benchmark.
Sentry uses a combination of electronic parameter settings (voltage, frequency, source resistance and waveform) to generate the “signature” for each pin of the IC it checks. It then compares the unique electrical characteristics of known components and with suspect components. The testing between every possible pin combination maximizes the chances of capturing internal fault conditions. Sentry can quickly detect missing or incorrect dies, lack of bond wires, inaccurate pin outs and pin impedance variations. Simple pass or fail results are returned after testing.
Controlled via USB using PC software, Sentry’s device library can be built up by adding specific known good devices. The device database can include documents such as photos of device markings, data sheets, and other documentation to further help in confirming the integrity of a device. In the same vein, Sentry can improve quality assurance programs via detailed reports saved to provide quality control traceability.
There are efforts afoot to curb various forms of IC counterfeiting. For example, the IEEE P1735 standard spells out ways to encrypt electronic-design intellectual property in the IC hardware and software so chip designers can protect their IP and prevent nefarious manufacturers from copying the chip design. But vulnerabilities have been uncovered in the standard itself. Indications are that the cat-and-mouse game between counterfeiters and manufacturers will be with us for a long time to come. Thus it’s likely that counterfeit detection measures such as electronic testing and package inspection will be necessary for a long time to come.
SENTRY contains all the hardware required to analyze the electrical characteristics of ICs with up to 256 pins. 256 pins+ devices can also be tested by rotating the device (BGA, QFP) to allow all pins to be learned and compared. SENTRY contains four 48-pin dual-in-line (DIL) zero-insertion-force (ZIF) sockets; these can be used directly for older DIP components but can also be used to accommodate a variety of additional socket adapters available for different package types. The socket adapter can contain multiple IC sockets to allow testing several ICs simultaneously or comparing one IC with another. An expansion connector allows custom socket adapters with up to 256 pins to be attached.