PCBA Testing Services

Testing in the PCBA Production Flow

Testing is sometimes treated as a step that happens at the end, after the boards have been built and packed. In practice the useful testing is distributed through the process, because the earlier a defect is caught, the cheaper it is to correct. A solder paste problem found by SPI before placement costs a wiped board. The same problem found at functional test costs a rework cycle and, if it is not caught, a field return.

Inspection built into the process

• 3D SPI before placement. Solder paste volume and registration are measured on every board before components are placed. Most fine-pitch defects originate in the paste deposit rather than in the placement.
• AOI after reflow. Automated optical inspection checks solder joints, missing, skewed and reversed components, and placement offset.
• X-ray where joints are hidden. BGA, QFN and other area-array packages have their joints under the body, so they are verified by X-ray rather than assumed from the surface appearance.
• Visual inspection to the acceptance class. Workmanship is judged against IPC-A-610 at the class agreed at quotation, whether that is Class 2 for general industrial products or Class 3 for medical and automotive work.

In-circuit test (ICT) and functional test (FCT)

In-circuit test checks component presence, value and orientation against the design, using a fixture that contacts the board test points. Functional test powers the assembly and checks that it performs to the customer specification. Functional test needs a specification from the customer: the conditions, the measurements and the pass limits. Where no specification exists, we agree an inspection-only scope and state in the quotation what it covers, because the difference between verified function and inspected appearance is exactly the thing a customer needs to know.

What each test method does and does not catch: SPI vs AOI vs X-ray vs ICT vs FCT

Test methods are chosen for what they can find, not for how thorough the name sounds. Reading the limits honestly is what makes a test plan useful.

No single method is complete, which is why the useful test plan layers them. Fine-pitch and area-array work in particular benefits from combining SPI, AOI and X-ray — the approach described on the high-density BGA assembly page.

First article, pilot run and approval

Before a programme moves into volume, the first assembly is treated as an approval gate rather than as one more build. The first article is inspected and tested to the full scope agreed at quotation rather than sampled, and the results are recorded against the design. Nothing is released to volume until that result has been reviewed and approved in writing, which is what keeps a process error from being repeated across a whole order.

Three things come out of the gate. The FAI report (first article inspection report) records the measurements, the test results and any deviation found during the build, so the customer's own quality system has a document to file. The golden sample is the approved board retained as the reference that later builds are judged against. The approval is the written release, after which the process, the fixture and the material are frozen for the run.

A pilot run is the small build that follows, used to confirm that yield and process capability hold over more than one board. On prototype quantities the first article and the pilot run are usually the same build, which is why the prototype stage is inspected to the same acceptance class as volume production. Where the customer has an approval process of their own, the records are issued in the format that process needs, agreed at quotation rather than reconstructed afterwards.

Choosing the test scope for a programme

How much test a programme needs depends on what a missed defect costs downstream, not on a house preference. Three factors usually decide it. The first is volume: at low volume the economics favour functional test on every unit, while at high volume fixture-based in-circuit test spreads its cost across enough boards to pay for itself. The second is consequence: a failure in a consumer accessory and a failure in medical equipment or automotive electronics carry very different costs, and the acceptance class follows. The third is rework cost: assemblies that are expensive to rework justify earlier and denser inspection, because finding a fault before reflow is measurably cheaper than finding it after.

What to specify in the RFQ

Test scope is one of the few parts of a PCBA quotation that the customer's own document decides, so it is worth writing it down rather than leaving it open. These are the items that change the test line on a quotation:

• The test specification: what the assembly has to do, the conditions it is measured under, and the pass limits.
• The acceptance class, IPC-A-610 Class 2 or Class 3, which sets what the visual inspection is judged against.
• The test scope: in-circuit test, functional test or inspection only, and whether every unit or a sample is tested.
• Whether a fixture is needed, and how its cost is treated: charged on the first order or amortised across the programme.
• The test report format, where the results have to feed a quality system of your own.
• Any programming to be carried out, and the image version to be loaded.
• Whether burn-in or aging screening is required, and for how long.
• Any traceability requirement beyond lot-level records.

Where an item is not specified it is not silently assumed: the quotation states what the test scope covers, and anything outside it is listed as a separate line. The other inputs a quotation needs are set out under quotation requirements. A test specification and the expected quantity are enough to start; send them with the Gerber and BOM to alden@kilandu.com.

Programming and device configuration

Where a board carries programmable devices, programming is done in the production flow so that the assembly leaves the line configured rather than needing a separate step by the customer. Programming can run before or after placement depending on the device and the volume, is verified by reading the device back against the supplied image, and the image stays under the customer's control and version. Application software and firmware are the customer's to develop; what happens here is loading the supplied image onto the hardware and confirming it took.

Fixtures and programme cost

Where in-circuit or functional test requires a fixture, it is quoted as a separate line with its amortisation stated, so it is visible whether the cost sits with the first order or is spread across the programme. A fixture is retained for the product and reused across later releases rather than rebuilt each time.

Records

Testing produces data, and the data is held. Lot-level records tie the finished assembly back to the component lots, the reflow profile, the inspection results and the test outcome, so a question raised about a board months later can be traced back to the production conditions that produced it. Where a customer needs test data in a particular format for their own quality system, that is agreed at quotation so the records can be generated as part of the build rather than reconstructed afterwards. The quality assurance page describes how the records are structured.

How testing fits the turnkey flow

Testing is built into turnkey PCBA rather than sold as a separate step at the end, and the scope is fixed at quotation alongside the material plan and the DFM findings. Test coverage that is agreed early can also be matched to the inspection plan used on the floor, which is why the test scope and the acceptance class are usually decided together. To discuss a test plan for a specific board, send the design files and any existing test specification to alden@kilandu.com.