ICT Test and Functional Test Services for PCBA

Assembly quality and product quality are two different questions, and the inspection that follows reflow only answers the first of them. A board can be soldered perfectly and still fail, because a component is the wrong value, because a joint that looks acceptable has not actually connected, or because a device is orientated correctly and configured wrongly. Testing is the stage that asks whether the board does what it is supposed to do.

What Each Test Type Answers

An ICT test places probes on defined points of the board and measures the electrical parameters of the components and the connections between them. Resistance, capacitance, inductance, diodes, transistors and integrated circuits are checked for value and for the state of their joints, and the same programme detects opens, shorts, wrong parts, missing parts and parts fitted the wrong way round.

The strength of in circuit testing is that it is fast and that it localises a fault to a specific reference designator rather than to a board. That combination is what makes it suited to a product built in volume, where the diagnosis has to be made quickly and by an operator rather than by an engineer.

A functional test applies the inputs the board expects in normal operation and measures the outputs against the design requirement. It is closer to the way the product will actually be used, and it catches the failures that a component measurement cannot see, such as a firmware problem, a timing issue or an interface that works with one peripheral and not another. The test method is normally supplied by the customer, or agreed with the customer, because only the customer knows what the product has to do.

Beyond those two, other stages are added according to the product. An aging test runs the unit at elevated temperature or at room temperature for a defined period so that the early failures appear in the factory rather than in the field. Environmental testing adds temperature cycling, humidity and vibration where the product will meet those conditions. Programming can be carried out during the test sequence so that the firmware and the parameters are written and verified in the same flow, and the appearance is confirmed by combining automated optical inspection with a visual check.

Fixtures and the Cost Behind Them

An in circuit test needs a test fixture built from the Gerber data and the bill of materials, with probes positioned on the nodes the programme will measure. The accuracy of the probe positions and the coverage of the points decide how much of the board can be tested, and a design that leaves no room for probe access limits that coverage no matter how good the fixture is.

A functional fixture is designed around the product behaviour instead. It supports key presses, display checks and signal inputs and outputs, and the sequence can be arranged to match the customer’s requirement. Where the product is complex, the fixture becomes a small system of its own.

The cost of a fixture is driven by the number of test points and the size of the board, and it is a fixed cost that is then spread across the boards it tests. That is why the economics change with volume: on a short run the fixture can be a significant part of the unit cost, while on a long run the same fixture contributes very little. Writing the test programme is a separate engineering task, and automation and data recording are usually included so that the results are captured rather than written down by hand.

The Three Inspection Gates Before Testing

Testing does not substitute for process control. SPI after printing measures the thickness, the area and the offset of the paste deposit and catches the printing defects while the board can still be cleaned, which is the cheapest point at which to correct them.

Optical inspection after reflow compares each board against a reference and identifies missing parts, wrong parts, offset, tombstoning and bridging. It runs on every board, and its usefulness depends on the programme being tuned for the product, because an inspection that returns false calls teaches the operator to clear them without looking.

The third gate covers what cannot be seen. Where a ball grid array or a quad flat no lead package hides its joints under the body, X-ray inspection is applied on a sample or on every board according to the requirement, and the functional test follows to confirm the behaviour of the finished unit. A high reliability product may then continue into ageing as well.

Designing for Testability

The decisions that determine how testable a board will be are taken at the design stage. Access for probes needs to be left on the nets that matter, and a net that is buried under a connector or covered by a tall component will not be reachable however the fixture is designed. Test points have to be separated from one another so that the probes do not interfere, and they should be on a grid that the fixture can use.

Where a board is designed with testing in mind, the fixture is simpler, the coverage is better and the test time is shorter, all of which show up in the unit cost. Our PCB design and layout group raises these points during the review rather than after the fixture has been quoted.

How the Result Is Handled

Boards that fail are routed to repair rather than scrapped, and the fault is recorded against the reference designator so that the pattern across a batch becomes visible. A single failing board is a board problem; the same failure on many boards is a process problem, and the record is what distinguishes them.

After repair the board is tested again with the same programme, so that the original fault is shown to have gone and the neighbouring circuitry is shown not to have been disturbed. Where the cause turns out to be a design issue or a component issue, the customer is informed with the evidence rather than only with the failure count.

Our quality management system holds the test records, and the whole flow from placement to test runs in house, which keeps the delivery date under one schedule. The PCBA testing team handles the fixture development, the programme writing and the operation of the test area.

Choosing the Right Level of Test

There is no single correct answer to how much testing a product needs, and spending more than the product requires is as much of a mistake as spending too little. The questions that decide it are the complexity of the board, the volume being built, the value of the unit, the consequence of a failure reaching the customer and the amount of test content the customer is able to define.

A low value board built in a small quantity is usually covered by paste inspection, optical inspection and a visual check, because the cost of a fixture would exceed the cost of the occasional failure it prevents. A complex board with a fine pitch device, a large quantity or a demanding application inverts that calculation, and the fixture pays for itself because it catches a fault in seconds that would otherwise take an engineer an hour to find.

Where the customer has no test specification, the discussion starts from the function of the product and works backwards to the measurements that show it. That is how a test plan is agreed rather than inherited, and it is where an experienced engineering group earns its place.

FAQ

Is in circuit testing always necessary? No. A simple, low value board may be covered by optical inspection and a visual check. A complex board, a large batch or a demanding product normally justifies the fixture.

Can the factory perform the functional test? Yes, given the test specification and the acceptance criteria. The fixture and the operator time are quoted from those.

What happens to a board that fails? It is repaired and retested. If the failure is caused by the design or the component, the analysis is passed back to the customer.