Production Test Strategy: Choosing Between ICT, Flying Probe and Functional

Every test method detects a subset of the possible faults and costs a certain amount per board. A test strategy is the choice of which methods to use, in which order, and what to do with what they find.

What Each Method Detects

Automated optical inspection detects the presence, position and orientation of components and the shape of the joints that are visible. It does not detect an electrically wrong but correctly placed part.

In-circuit test detects the electrical value of components and the continuity of the nets, which catches shorts, opens and wrong values. It requires access to the nets, which the designer must provide.

Flying probe test performs the same measurements without a fixture, which suits low volumes and prototypes, and it is slow per board. Our flying probe notes describe the access requirements.

Functional Test and What It Covers

A functional test exercises the assembly as the product will operate it, which means it detects faults that the structural methods cannot see: a wrong component value that still passes in-circuit, a firmware incompatibility or a timing problem.

The functional test requires the design to be brought to a state where it can run, which means a programming step, a power supply and the interfaces it needs.

Its coverage depends on the test cases written, and its diagnostic value is lower than the structural methods: a functional failure says the unit does not work but not which joint is open.

Coverage and the Fault Spectrum

The useful analysis is a comparison of the defects the process actually produces against the faults each method can detect. A defect that no method detects is one that reaches the customer.

The process defect spectrum is measured from the repair records, which is why the repair loop and its data matter. Once the spectrum is known, the test strategy can be aimed at it.

A common finding is that most defects are placement and soldering faults, which the structural methods catch, and that a small number are component or firmware related, which only the functional test catches. Our quality notes describe how the repair records are captured.

Order of Operations

The methods are usually applied in order of cost, from the cheapest and fastest to the most expensive. Optical inspection first, then in-circuit test, then functional test.

The order matters because a defect caught early is cheaper to repair. A short circuit found by optical inspection is repaired before the board is powered, while the same short found by functional test may have damaged a component.

The order also matters for the repair loop, since a board that fails the functional test after passing the in-circuit test points at a different set of causes than one that fails both.

Test Point Access

The in-circuit test needs access to every net that must be measured, and the access is designed into the layout. A design without test points forces either a flying probe test or no structural test at all.

The access must be available from the side the fixture probes and must not be obstructed by components. The design rules for the test point size and spacing are set by the fixture technology. Our inspection notes describe how the structural and optical methods complement each other.

Where the design is dense, the trade is between test coverage and routing space. The decision should be made with the cost of the undetected defect in view rather than for convenience.

Cost of the Strategy

The cost has a fixed component, which is the fixture and the program, and a variable component, which is the test time per board and the cost of the false calls.

A strategy with high coverage and a high false call rate can cost more than one with lower coverage, because every false call consumes an operator and a repair slot.

The strategy should be reviewed as the volume changes. A fixture that is justified at ten thousand units is not justified at two hundred, and the reverse is also true.

Documenting the Strategy

The strategy should be written down as a plan that states which methods are applied, in what order, with what acceptance criteria and what happens to a unit that fails each one.

The plan should also state what is not tested, which is the information that matters when a defect escapes. A strategy that claims full coverage will be believed until it fails.

Where a customer requires a specific test coverage, the requirement should be recorded and the strategy designed to meet it. Our inspection notes describe the coverage of the methods that are available for hidden joints.

Process Control and Verification

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Verification and Records

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Points to Confirm at First Article

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Documented Process Control

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

FAQ

Is functional test enough on its own? It detects that a unit does not work and it does not localise the fault, which makes the repair loop expensive and the defect spectrum invisible.

Can a flying probe replace in-circuit test? For low volumes, yes. Its test time per board makes it unsuitable for volume production.

What does gopcb provide for test strategy? We provide a plan based on the fault spectrum from the actual process, test point design that supports the chosen method, fixture or probe programs, functional test development, and repair records that feed back into the strategy as the product matures.