Engineering guides / PROJECT FIELD GUIDE

Testing & Inspection

Match each inspection method to the failure modes you need to detect.

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ENGINEERING → PROCUREMENT → PRODUCTION

THE PRACTICAL ANSWER

What to know before you decide

PCB inspection and test methods answer different questions. Optical checks assess visible conditions, X-ray can reveal selected hidden structures, and electrical or functional tests examine connectivity or behavior within their coverage. No one method proves every aspect of a product. Build a plan around likely defects and required functions, then define the access, fixtures, limits and records that make results repeatable. Separate engineering validation from routine production acceptance.

01 / DECISION GUIDE

Make the trade-offs explicit

Map defects to detection methods

List faults that matter, such as wrong parts, hidden-joint defects, missing firmware or failed interfaces. Assign each to a plausible detection method and identify gaps. This is more useful than requesting every available machine. Define whether a check applies to all units, a stated sample or an engineering build, and ensure the evidence matches that scope.

Design electrical access early

Test points, connectors and programming access compete for space, but inaccessible nodes can make diagnosis costly. Discuss probe contact, board support and powered-test protection while layout remains editable. Boundary scan can help where devices and chain design support it, but coverage is design-dependent. Document these assumptions instead of treating a technology name as complete coverage.

Write measurable functional criteria

Specify input power, loads, firmware, conditions and acceptable outputs. Replace instructions such as check that it works with repeatable steps and limits. Include failure handling and a retest rule so repeated attempts cannot erase intermittent behavior. Version the procedure with the product configuration and define who approves changes to limits or fixtures.

Request useful records

One project may need a lot pass statement; another may need serial-level measurements and repair history. Agree format and identifiers before the build. Keep exploratory validation results separate from production acceptance data. If process changes affect coverage, review them before applying a revised test to released units and retain the original failure information. For example, a unit may draw the expected startup current but fail a communication interface. The current check should remain described as a current check, not as a complete functional pass. Add a defined interface stimulus and expected response if that behavior must be accepted in production. Identify the firmware and fixture used, and decide how a failed first attempt followed by a passing retest will be recorded and reviewed.

02 / SIDE BY SIDE

Compare the choices

PCB assembly testing inspection methods — planning considerations
Choice or scope	Useful when	Review before release
Visual/AOI	Visible conditions	Program limitations and disposition
X-ray	Selected hidden structures	Features and acceptance rules
In-circuit test	Electrical/component faults	Probe access and fixtures
Boundary scan	Supported digital interconnects	Device and chain coverage
Functional test	Specified behavior	Inputs, limits and firmware

03 / REVIEW BEFORE RELEASE

Questions that prevent rework

Naming equipment instead of coverage

State the defects or functions to assess.

Using a prototype script without limits

Control inputs and acceptance thresholds before production.

Retesting until success

Preserve failures and define authorized retest conditions.

04 / COMMON QUESTIONS

Get a clear answer

Can AOI replace electrical test?

No. The methods observe different aspects of an assembly. Select complementary checks for the expected faults, and record what each cannot detect. Equipment availability does not establish the coverage of a particular test program.

When is a functional fixture useful?

When manual connections or measurements are too variable, slow or difficult to reproduce. Define the test first, then evaluate fixture needs against volume, safety, access and diagnostic requirements rather than designing a fixture without a clear acceptance task.

What should a test report identify?

Product and revision, procedure version, lot or serial number, result and agreed measurements. Specify how failed, repaired and retested units are represented. A bare pass label may be insufficient when traceable evidence is part of the purchase requirement.

Technical references

Check each source’s scope and revision against your project requirements. Confirm the proposed construction with your manufacturing partner.

YOUR PROJECT / YOUR NEXT STEP

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Prepare a test-planning brief. No prediction of coverage or reliability is implied.

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