DFM Review Items Before PCB Release

A design for manufacture review asks a short set of questions about a design before it is built, and the questions are the same for every board. They are asked at the point where the answers are cheap, which is before the artwork is released rather than after the first panel is scrap.

The Questions That Always Earn Their Place

What is the smallest hole and its aspect ratio, what is the smallest annular ring, what is the smallest line width and spacing, how many layers does the stack need, and what is the finished thickness and its tolerance. Those five describe most of what the shop has to achieve.

Each has a process limit behind it, and a design that is inside the limit on all five is a design that can be quoted with confidence. Our design release notes describe the review that closes it out.

Hole Size and Aspect Ratio

The hole sizes decide the drill, the plating and the aspect ratio. A small hole in a thick board is the combination that pushes the plating to its limit, and it is also the combination that fails in thermal cycling.

The question to ask is whether the smallest hole is necessary or whether it is smaller than the component requires. A drill table rounded up where the lead allows it is a free improvement. Our aspect ratio notes describe the limit in numbers.

The annular ring is what absorbs registration and drill tolerance, and it is the item most often reduced for routing density. The question is how much remains at the worst case rather than at the nominal.

Asking the question as a minimum ring rather than as a pad dimension makes the answer checkable. A pad size without a ring figure does not say what will remain after the hole has moved.

The minimum line width is a function of the copper weight, because the etch removes copper sideways in proportion to the thickness. A design that needs a fine line and a heavy plane uses two layers rather than asking one layer to do both.

The spacing matters as much as the width, and the two are usually limited together. A design that is inside the width limit and outside the spacing limit will still be quotable and will yield less. Our fabrication notes describe how the limits are stated.

The number of layers follows from the routing and the impedance rather than from a preference, and the finished thickness follows from the stack. A thickness that is quoted without the stack behind it is a wish.

The tolerance on the thickness comes from the press and from the material, and it is often the item that a mechanical interface depends on. Where the interface is tight, the tolerance is part of the requirement rather than a general note.

The surface finish and its thickness, the solder mask dam width, the array and the singulation method, the tooling holes and the panel keep-outs. Each of those is answered by a sentence and each of them causes a delay when it is missing.

A readme of half a page removes most of them. Our board quality notes describe how the answers are verified on the finished board.

Process Control and Verification

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

Is a DFM review a substitute for a stack-up? No. The review asks whether the design can be built and the stack-up says what it will be built from.

How long does a review take? A short one for a design that answers the questions, and the time spent is recovered at the first article.

What does gopcb provide for DFM? We provide a review covering hole size and aspect ratio, annular ring at the worst case, line width against copper weight, stack and thickness tolerances, and the items that are usually missing, with the answers recorded against the design that was quoted.

Checks Before Release

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 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 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 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.

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.