Fabrication Capability Matrix and Its Use

A fabrication capability matrix is a short table of the limits a shop can hold, and it exists so that a design can be checked against them before it is quoted. Without it, the check is made by whoever answers the enquiry on that day.

What the Matrix Contains

The matrix lists the smallest hole and the largest aspect ratio, the smallest line width and spacing for each copper weight, the minimum annular ring, the layer count range, the maximum panel size and the finished thickness range with its tolerance.

Each entry is a figure with a tolerance rather than a range alone, because the achievable depends on the combination. Our fabrication notes describe where the design is checked against it.

Why Combinations Are the Real Limit

A small hole is achievable and a thick board is achievable, and a small hole in a thick board is where the aspect ratio limit applies. The matrix is therefore read as a set of interacting figures rather than as a list.

The interactions are where a quotation changes, which is why the matrix is used at the enquiry stage rather than at the first article. Our plating thickness notes describe the process limit behind the aspect ratio figure.

Yield and the Position in the Matrix

A design inside the matrix is buildable and its yield depends on where inside it sits. A design at the limit is buildable at a yield that is lower than one near the middle, and the quotation reflects that.

Stating the yield expectation alongside the capability is what makes the matrix useful commercially. A capability that is quoted without a yield is a capability that is discovered on the first order.

The enquiry is checked against the matrix, and anything outside it is either declined or quoted with a development. Anything inside it is quoted from the standard price rather than from a judgement.

That removes the variability between quotations, which is the main commercial benefit. Two enquiries that are identical receive the same answer.

The matrix changes when equipment or chemistry changes, and it is reviewed on a schedule rather than after a problem. A matrix that has not been reviewed since a new line was installed describes the old line.

The review is a measurement rather than a discussion: a test panel that exercises every figure in the matrix is run and measured, and the results update the table. Our board quality notes describe how the test panel is judged.

The customer uses the matrix at the design stage to avoid a revision later. A design that is checked against the limits before the layout is fixed avoids the situation where the density is chosen and the fabrication is then negotiated.

The matrix is also useful for a comparison between suppliers, because a list of figures is easier to compare than a set of assurances. The figures are the part that can be verified on a sample.

Acceptance and Its Evidence

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold. A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

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. Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process. 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 an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

The sequence of operations is part of the specification, because a different order produces a different result from the same steps. 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.

Checks Before Release

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 cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected.

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. Where the supplier and the user both measure the same property, they should agree on the method before the first delivery.

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first.

Verification and Records

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts.

Does a design inside the matrix always yield? It yields at the rate that position implies, which is why the matrix is read with a yield expectation.

Is the matrix the same for every shop? No. It describes the equipment and the chemistry of one shop and it changes when either changes.

What does gopcba provide for capability? We provide a published matrix of interacting limits with tolerances, the yield expectation at each position, enquiries checked against it before quotation, a test panel that exercises every figure, and a scheduled review so the table describes the line that is running.

Points to Confirm at First Article

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 record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

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.