PCB Fabrication Inspection & Bare-Board Testing Capabilities

From Board Features to Manufacturing Records

The documentation associated with each fabrication check has value if it can relate the fabrication check to the fabricated product. An assessment of a copper pattern issue, hole position issue, plating condition, solder mask shift, surface finish issue, or flatness reading can be more thoroughly assessed if each condition can be related to the point in the process at which it developed.

The clearest indicator of fabrication check results is whether the specific result can be traced back to the supplier’s drawing, build requirements, measurement record, digital image, coupon test, or job traveler. This type of connection can also indicate whether a finding is isolated, relates to a specific fabrication step, or signifies a condition significant enough to warrant corrective action prior to assembly.

Job Travelers and Records That Make Decisions Traceable

Traceability is a key value in this stage. A job traveler is the production record that follows the lot throughout production; a measurement sheet can document a dimensional decision; a cross-section photo may document the plating or hole-wall review; and a finish production record or solderability result can clarify if the surface condition is correct for the next step or requires further discussion before moving to the next step.

SUGA uses records for shipment decisions. SUGA does not require the same record set for every order. Some records result from normal manufacturing controls and others depend on the information contained in the drawings, feature tolerances, material requirements, acceptance criteria, or specified inspection requirements. Clearly defined records assist the determination of what action should be taken, such as continuing the lot, retesting, remeasuring, or holding the lot until further clarification.

This record-based approach keeps the focus of the work on the quality of the fabricated circuit board. It also helps prevent a last-minute issue from being treated as a large testing problem when the root cause may be a fabrication issue in imaging, drilling, plating, solder mask, surface finish, or flatness.

Visible and Internal Features Need Different Checks

AOI is appropriate for reviewing surface-visible features. For example, AOI can be used to review copper patterns, solder mask alignment, legend condition, and other visually identifiable features. The primary limitation with AOI is that, while it addresses visually identified features, it does not address electrically identified features; the results of AOI will not provide evidence of continuity or isolation for circuit paths.

X-ray is appropriate for reviewing internal features or hidden structures in accordance with the design drawing or the requirements of an inspection plan. The use of X-ray should be limited to verification of presence, position, registration, and internal structure where an image will provide useful verification. Therefore, X-ray should not be used to replace cross-section inspection and should not be the primary method of confirming plating thickness on a finished board.

Cross-Section and Wetting Evidence Require the Correct Sample

Microsectioning is often a more suitable approach for obtaining evidence inside the board structure compared with other non-destructive inspection methods. For example, a microsection can be used to obtain verification of the hole wall condition, plating thickness, laminate condition, or cross-section acceptance. All of these require the use of a physical sample to confirm evidence of these items.

A review of the solderability of a product answers a different question than what microsectioning would answer. A solderability test provides verification of the wetting performance of a product when a specific surface finish is being used or is important; thus, the solderability result can confirm acceptance of the inspected sample, but cannot be used as an indication of total assembly yield performance.

The inspection method can be matched to the particular question being asked. For example, if there is visible surface evidence of a defect, an image-based inspection is appropriate. If the condition is associated with internal or structural issues, X-ray or cross-section inspection methods may be necessary. If the focus is related to wetting, a solderability inspection method will provide the necessary proof. Treating all inspection methods as interchangeable may lead to an inflated sense of confidence in the result when, in fact, the basis for the result resides below the surface or is based on a sample.

Drawing Notes That Point to the Required Fabrication Check

Notes on the drawing file help SUGA identify the fabricated board evaluation required to resolve a finding. A hole/slot tolerance gives instruction to check the dimension; a plating note or cross-section callout provides direction to use coupon work; surface finish issues require solderability assessments, and flatness limits require measurements for bow and twist.

Fabrication Files That Set the Acceptance Basis

Fabrication files communicate to SUGA what the bare board must meet before assembly can take place. The Gerber files, fabrication drawings, stack-up construction details, netlist information, controlled impedance targets, surface finish requirements, and IPC-D-356 are all examples of fabrication files that provide valuable insight into how the end product should be manufactured.

By using these fabrication files, SUGA can identify what features on a PCB are controlled by the fabrication drawing. SUGA can also determine which factors require greater scrutiny when evaluating a PCB. Tolerance for holes and slots may lead to dimensional checks. Controlled impedance targets may lead to coupon-level checks. Surface finish requirements may determine whether an additional solderability check is needed. The goal is not to provide SUGA with more fabrication files than necessary, but to eliminate ambiguity in acceptance criteria before the PCB moves to assembly.

Inspection Notes That Clarify Acceptance

Inspection notes assist when a drawing alone does not clearly define an inspection requirement. Some examples of inspection notes are requests for cross-sections, flatness limits, specific surface finish requests, solderability questions, or particular outgoing records, such as measurement sheets or images.

Inspection notes indicate the specific board feature to which they apply. A cross-section sample can verify plating when drawing notes require physical proof. A dimensional measurement can document outline-fit confirmation when the drawing defines a controlled feature. A solderability result can document wetting acceptance when the condition is tied to surface finish or storage exposure.

When SUGA has sufficient information about the project, SUGA will have a clear understanding of whether standard production records are adequate, additional measurements are necessary, or further clarification is required before assembly.

When standard production records do not adequately address an issue, SUGA will identify the affected feature and the additional check needed before assembly.