Key Benefits of In-Circuit Test in High-Yield PCB Manufacturing

An in-circuit test checks the parts on an assembled board one at a time, through spring pins pressed onto the copper. A whole board takes seconds. It does not care whether the firmware boots, because it asks a narrower question: is every part the right value, the right way round, and soldered to the right nets? A failed functional test tells you the board is dead, whereas a failed in-circuit test names the joint: R14 reads open, or two pins of U3 are bridged. Each of those measurements needs a pad that the probe can reach.

Figure 1: An assembled PCB pressed onto bed-of-nails spring probes

So the coverage you get is decided in layout, long before anyone builds a fixture. This guide covers how ICT runs in production and how bed-of-nails fixtures compare with flying probes. It also sets out the defects ICT catches and the test-point rules that determine its coverage.

Fundamentals of In-Circuit Testing

In-circuit testing is a structural test; it verifies how the board was built rather than what it does. That distinction decides which faults it catches, how it touches the board, and which machine type suits your volume.

What Is PCB In-Circuit Test (ICT)?

PCB in-circuit test, or ICT, is an electrical test of an assembled board. Probes contact a test point on each net, and a tester measures the parts between them. It checks for shorts, opens, incorrect values, and incorrect orientation, part by part, so the failure report names the exact component and pin.

Thus, the coverage received is determined in the layout and long before the fixture is built. This guide will discuss the operation of ICT in production and compare bed-of-nails fixtures with flying probes. It also provides a list of defects detected by ICT and test-point rules, which define its coverage.

Fundamentals of In-Circuit Testing

In-circuit testing is a structural test; it checks the integrity of the board, but not its function. That difference determines the type of faults it detects, how it interacts with the board, and what machine type is appropriate for your volume.

What Is PCB In-Circuit Test (ICT)?

PCB in-circuit test (ICT) is an electrical test of an assembled board. Probes touch a test point on every net, and a tester tests the parts between the test points. Checks shorts, opens, bad value, and bad orientation one component at a time, and reports the failure in the component and pin report.

The "in-circuit" aspect is the difficult one. A resistor is located in parallel with each other path between the two nets on the board. The measurement of a 10 kΩ component connected in parallel with a 4.7 kΩ path is approximately. Thus, it is the duty of the guard to cancel those paths before the tester can judge the part.

How In-Circuit Test Works in Production

The board is protected by a production ICT cycle that has a fixed order. No power is powered until the tester has demonstrated that there is no short in the power to find.

Figure 2: The seven-step ICT production cycle

• Program generation: A program reads the CAD data and netlist, assigning a tolerance to each part and a probe to each net.
• Fixture contact: The board is pulled down on the spring probes by a vacuum or pneumatic press.
• Contact check: Each probe is checked for contact with copper and not scored as open due to a dirty pad.
• Shorts and opens: All nets are tested at a low voltage to check for shorts to other nets or opens.
• Unpowered component tests: Resistors, capacitors, inductors, and diodes are tested individually.
• Powered tests: Tests of regulator outputs, of digital pins, and of boundary scan chains applied with power on.
• Fault report: The ticket specifies the component name, the pin names, and the measured value for repair.

The reason why the low voltage of step 4 is important is that a silicon junction begins to conduct at approximately 0.6 V, so as long as the stimulus is well below that, every diode and transistor is off, and no current flows. Then the tester only observes copper and solder, and unpowered steps can safely run first.

Bed-of-Nails Fixtures vs. Flying Probe

A flying-probe tester has a small number of probes that move from test point to test point; a bed-of-nails fixture has a probe for every test point, touching them simultaneously. The fixture is a plate with hundreds of spring probes drilled into it for a single design, and a test can be performed in seconds after the fixture is built. A flying probe, typically 4 to 8 moving probes, is a tool-less and time-consuming solution as described in "How Flying Probe Testing Works".

Figure 3: Bed-of-nails fixture and flying probe tester compared

Top Advantages of In-Circuit Test for Quality Control

It has one property that makes it advantageous – it is a direct measurement of each part. It should never have to assume that the fault is with the entire board, and that saves time, less expensive repairs, and wider electrical coverage.

Rapid Defect Detection and High Accuracy

A bed-of-nails tester can perform hundreds of tests in seconds on each board. All the probes are already in place, and the tester uses relays to switch between them. Since there are no moving parts, it is feasible to test all the boards rather than sample a few.

Figure 4: Guarding isolates one resistor from a parallel path

The accuracy is derived from the guarding: to check a resistor in a network, the tester sends current to one end and detects it at the other end. It maintains the middle node voltage of each parallel path at the voltage at the measuring end. Thus, the current flows in parallel into the guard, not the meter, and the meter reading is only for the target part.

Very low values require one additional step: A 10 mΩ current shunt will be smaller than the resistance of the probe and wiring in series with it, so a two-wire reading is a measurement of the fixture. The four-wire Kelvin testing technique supplies current through one pair of probes and measures voltage through another pair, thus requiring two test pads per net for such parts.

Lower Repair Costs and Higher First-Pass Yield

A defect has an increasing cost the further down the line it goes. The rule of ten is the usual estimate, and this rule proposes that the same fault will cost about ten times more at each stage. ICT detects the assembly error before it gets to the point of one joint and a soldering iron.

The majority of repair costs are for determining the problem. An unbootable board can be an hour of probing for a tech, while a ticket that says "C22 missing" is a few-minute job. The reworked board can be returned to the fixture immediately for retest, as the fault has been identified. First-pass yield is the percentage of boards that pass all tests without rework, an improvement based on feedback from ICT. If a failure occurs on board after board and the same pin fails, the issue is located upstream in a stencil aperture, a placement offset, or a reflow profile. Correcting this cause prevents the defect from accumulating.

Comprehensive Electrical Inspection

ICT checks electrical facts; that is why it detects faults that cameras can't. A camera reveals a resistor that appears to be OK, but only a measurement reveals it's 1 kΩ - the BOM calls for 10 kΩ.

DFM Requirements for Successful ICT Coverage

ICT coverage is determined during layout, since the tester can only measure the net it can touch. Those three things are the location of the test points, the space around the test points, and whether the board will remain flat while under the fixture.

Strategic Test Point Placement

A test point on each net is required for full coverage; a net without a test point will not be measurable. Four-wire parts require two points per net. Several are required for the main power and ground nets to distribute the current across multiple probes, and to keep contact resistance low. Consider test points to be part of the PCB design, rather than an add-on.

A one-sided fixture is less expensive and easier to align; therefore, all test points should be on one side, usually the bottom side, of the fixture. All points on both sides are used to achieve a clamshell fixture with probes above and below. It's often worthwhile to move a couple of traces to avoid that. Even for those who design regularly, there are some common pitfalls for loss of coverage. Each of them contains a straightforward layout fix:

• The mask of a via for probing conceals its copper, so open the mask and keep the drill ≤ 0.36 mm (14 mil).
• When pressure is applied to the probe, the probe pressure can be used to close a cracked joint using the component pad as the test point; place a dedicated pad somewhere on a short trace.
• When making a test point on a power rail, multiple test points should be used to distribute the test current across them.
• Make sure there is no ink on any test pad or silkscreen that will be printed over them, and isolate the copper from the probe.

Component Clearance and Board Layout Rules

A probe tip is small; however, the barrel or receptacle is not. The clearance thus increases with component height, as probe plate relief pockets are required around tall components. Two tooling holes, which are not plated, drop over guide pins, registering each probe simultaneously. Pads close to the edge of the board are prone to landing on the seal of the fixture, so they are pushed back off the edge.

Figure 5: ICT test point size, pitch, clearance, and edge setback

Fixture Stability and Mechanical Stress Control

The pushing force on each spring probe is about 1-2N, and hundreds of spring probes are mounted on a fixture. The board is rated at 400 probes at 1.5N per probe, totaling around 600N - equivalent to the weight of a 60kg person. That load will bend the board if it is not evenly supported, and bending can cause parts to crack.

The first components to fail under flex are ceramic capacitors and BGA joints. A multiple-layer ceramic capacitor may crack internally, and this crack may not fail for weeks. BGA pads may tear the resin (under the pad), termed as pad cratering. Thus, the fixtures position their push-down fingers directly over the probe clusters, where the forces are canceled, so they do not bend the board.

Our Professional Testing Capabilities for Zero-Defect Delivery

The realistic goal is layered inspection; no single test will give a zero-defect shipment. At JLCPCB, this layering begins even on the blank board and persists throughout assembly, with each layer capturing what the others missed.

Customized ICT Fixture Design and Engineering

A custom ICT fixture designed from the same CAD information used to create the board. The probe map is drawn from the test-point coordinates; the plate is drilled to accommodate the probes; and each probe is connected to a tester channel. This implies that if a pad is revised, so is a fixture, as it's dealt with in bed-of-nails testing with high-quantity PCBs.

JLCPCB conducts electrical testing in the bare-board stage. It uses flying-probe testing for prototypes and small-volume products and bed-of-nails fixture testing for large-volume products. Assembled boards are inspected using solder paste inspection, 2D/3D AOI, X-ray inspection, flying-probe testing, and functional testing (FCT).

Multi-Stage QA: Integrating ICT with AOI and AXI

Different types of faults occur in each automated check between reflow and functional test. They are normally arranged on a typical line as follows:

Figure 6: A multi-stage PCBA inspection line from SPI to functional test

• Because AOI (automated optical inspection) requires no contact, it is the first process performed. Cameras are used to determine the presence, position, polarity, and visible fillets, but cannot determine the presence of an invisible joint or the value of a resistor.
• AXI (automated X-ray inspection) examines BGA, QFN, and LGA bodies and inspects the underlying joints. It indicates missing balls, bridges, and voids through density, but it can't determine if a well-soldered part is the correct value.
• ICT records the electric outcome. It is a proof of the "current flows where the netlist says"; the other two questions of value remain.

JLCPCB has 2D and 3D AOI during assembly (how AOI detection works), and uses X-ray for bottom-terminated packages. If the ICT is done in those steps, the only question that remains after the functional test is whether the product works.

Strict Turnkey Manufacturing and Traceability

Turnkey means that one supplier will fabricate the board, source the parts, and assemble them. These are then all the records for one particular build, and if the value is flagged as incorrect by ICT, it is easy to identify the reel that fed it. JLCPCB provides parts from its component library; therefore, each part placed has a known part number.

Board-level traceability requires the identity to be printed on the board. JLCPCB can add a 2D barcode, a serial number, or both, with the barcode as a QR code or Data Matrix in one of three sizes: 5 × 5 mm, 8 × 8 mm, or 10 × 10 mm (197, 315, or 394 mil square). If your design already has testpoints and a clean netlist, it's ready to be built. JLCPCB allows you to upload your Gerber files to the website and instantly receive a quote for your Gerber files, assembly, and inspection.

FAQs About In-Circuit Test

Conclusion

The advantages of using an in-circuit test are that it measures each part in place and it determines exactly where the fault is. It repairs quickly and allows us to address issues at the source. It is established prior to the board going to the fixture: test point on each net, clearance around high elements, and fixture designed so that probing forces never bend the board.

ICT is no longer a standalone solution, but, as boards become denser, it is used in conjunction with boundary scan and X-ray. Designs that maintain test access without incurring significant testing cost. When preparing a board for production, JLCPCB's fabrication, assembly, and inspection services allow you to produce the board with that test plan in place.