An HDI qualification report is useful only when you can show which production interconnects its test structures represent. Build that connection before ordering coupons: identify every relevant via sequence, map each to a test net, and retain the construction and lot identity with the results.
The test-coupon overview introduces representativeness. This article provides a traceability worksheet for reviewing a proposed coupon set. It is an engineering planning method, not a substitute for an applicable acceptance specification or an assertion that a particular test qualifies every HDI board.
Start with the question the test must answer
Separate construction qualification from routine lot acceptance and from investigation of a failure. These activities can use related evidence, but their decisions are different. A development vehicle may explore several structures; a production acceptance coupon needs a defined relationship to the supplied boards. State the decision, responsible reviewer and required evidence before choosing a convenient coupon format.
Record the assembly and service exposures that matter to the product. Identify planned reflow and permitted rework, then agree the test method, monitoring, sample plan and acceptance criteria with the responsible quality and engineering teams. Do not import a temperature or cycle count from an unrelated product merely because its report is available.
Why a room-temperature result can be incomplete
IPC’s 2019 microvia reliability warning described failures that could remain hidden at room temperature and discussed performance-based coupon testing with resistance measurements. That historical warning supports asking how the test observes the relevant failure mechanism. It does not mean every microvia fails, nor does it define the current acceptance requirements for your product.
Ask what is measured during the agreed exposure and what is measured only afterward. Keep these two categories distinct in the report. A final continuity result cannot be presented as continuous monitoring if no intermediate measurements exist. Likewise, a cross-section documents the inspected location and condition; it should not silently stand in for evidence that the complete electrical path performed throughout a test.
Inventory the production structures
Mark the interconnect sequences in the actual released stack. Include isolated microvias, successive microvia transitions, and combinations involving buried or through connections wherever they exist. Record the start and stop layers and the interfaces included in each sequence. Use neutral identifiers such as V1 and V2 so the same labels can appear on the board drawing, coupon drawing and report.
For each sequence, record the material construction, relevant dielectric and copper geometry, filling requirements and fabrication revision. The list should describe what is being evaluated without claiming that geometry alone determines reliability. A process change may matter even when the nominal drawing dimensions remain unchanged.
| Production item | Coupon evidence to request | Unresolved condition |
|---|---|---|
| V1: one adjacent-layer transition | Named chain containing that span and construction | No chain or drawing reference supplied |
| V2: successive transitions | Complete sequence, including intermediate interfaces | Only isolated transitions represented |
| V3: transition connected to a buried structure | Drawing showing the combined path | Buried portion omitted or different |
| Material or process variant | Explicit applicability statement and revision | Report belongs to another construction |
Check coverage without inventing equivalence
Consider an illustrative board with three named structures. The offered coupon includes V1 and V2, while V3 combines one of those transitions with an additional buried connection. The presence of V1 does not by itself establish coverage of V3. Mark V3 as unresolved and ask for a representative structure or a documented engineering justification for the proposed evidence.
This is a coverage check, not a claim that every geometrical variant always requires a separate test. The responsible team may have a justified grouping method. The useful requirement is that the grouping be explicit, reviewed and tied to the relevant risk, rather than assumed by a purchasing spreadsheet or inferred from a similar-looking photograph.
For each coupon net, retain its endpoints and the sequence between them. Check whether the measurement combines several different features in one chain. A detected change in that chain may require further work to locate the affected feature. Plan that investigation before treating a pass/fail label as a complete explanation.
Connect the coupon to the manufacturing lot
Ask how coupon identity follows the fabrication panel and delivered boards. Preserve panel identifiers, coupon locations and the relevant lot records. Document whether the coupon traveled with the intended manufacturing sequence and whether any processing or handling differed. A physically nearby specimen is not automatically representative if it followed a different history.
Agree retention of the coupon drawing and test data, not just a certificate summary. A report should identify the tested revision, method and deviations sufficiently clearly that a later engineer can determine its applicability. Keep any exclusions prominent: an omitted chain or interrupted acquisition is not equivalent to a completed passing result.
Make the report reviewable
Define how initial resistance, changes during exposure and final observations will be recorded under the chosen method. Identify equipment and fixture information needed to interpret the result, including relevant calibration or correction procedures required by the agreed test. These details belong to the test plan; this worksheet does not supply universal numerical limits.
When a result is unexpected, retain the original data and the disposition. Distinguish a confirmed specimen failure from a measurement problem only when the investigation supports that distinction. Do not delete an inconvenient result and substitute a retest without preserving why the retest was performed and what decision it supports.
Reopen applicability after a change
A new material construction, revised via sequence or changed assembly exposure can require the evidence to be reviewed again. The thermal-exposure guide helps frame that review. Keep a change log that identifies which coverage rows remain applicable, which need justification and which require new evidence.
- Name the decision and applicable acceptance requirements.
- Inventory complete production interconnect sequences.
- Map those sequences to identified coupon nets.
- Document construction, process history and lot traceability.
- Retain test conditions, data, deviations and disposition.
- Reassess the mapping when the released design changes.
Use the HDI design checklist to track open items and the reliability library for related reviews. For a construction review, include the coverage matrix with the stack drawing and proposed acceptance plan. That package makes the evidence question concrete before testing begins.
Source reviewed 9 October 2026. Examples and the coverage worksheet are illustrative; no qualification, certification or universal acceptance threshold is claimed.