Layer Stack: Manufacturable Design: Decisions That Belong Up Front

Casting a design in the language of a manufacturing process forces a level of precision that a schematic review does not, and the exercise usually finds something. The items below are the ones that most often turn out to be decided by habit rather than by requirement.

The Layer Stack Is a Design Decision

Many designs inherit a stack from a previous project and adjust it only when the routing fails. The stack determines the impedance of every line, the coupling between adjacent layers and the mechanical behaviour of the board, and it is worth designing rather than inheriting.

The exercise is short. Name each layer, its purpose, the dielectric beside it and the copper weight, and check that the result is symmetric and that the impedance targets are achievable. Our layer assignment notes describe the method.

Ground Is Not a Dumping Ground

A ground plane is a conductor that carries the return current of every signal referenced to it, and treating it as a place to put whatever copper is left over produces a plane with slots, islands and narrow necks. Each of those forces the return current to take a longer path.

The plane should be drawn deliberately, with the slots placed where they serve a purpose and the stitching placed where the current actually crosses. A plane that is continuous beneath every fast net is the goal.

Decoupling Is a Loop Problem

A decoupling capacitor works by closing a loop with a small area, and its value is secondary to the area of that loop. Two capacitors placed close to the pins they serve outperform one larger capacitor placed at the edge of the device.

The loop includes the via, the plane and the pad, not only the capacitor. Short, wide connections with vias placed directly at the pad are what make the difference. Our capacitor placement notes describe the arrangement.

Thermal Relief Is a Solderability Decision

A pad connected directly to a plane conducts heat away during soldering, and the consequence is a joint that does not reach temperature. Thermal relief spokes are the standard answer, and the trade is a small increase in impedance against a joint that forms reliably.

The trade should be made explicitly rather than by applying a global rule, because a high-current connection may need to be direct while a signal pad does not.

Test Access Belongs in the Layout

A node with no test access is a node that cannot be verified, and the decision to leave it inaccessible should be made deliberately. Bringing a node out to a pad costs area; discovering the problem after the fixture is drilled costs a fixture.

The same reasoning applies to power rails, which should be separable so that the current drawn by each can be measured during bring-up. Our design for testability notes describe the provisions.

Tolerances Should Follow Function

A tolerance is a cost, and it should be specified only where the function requires it. An impedance tolerance tighter than the system needs, or a hole tolerance tighter than the component lead requires, adds cost at every panel without adding value.

The useful discipline is to state the requirement in terms of the function: the impedance the line must achieve, the force the connector must withstand, the temperature the joint must survive. Each of those translates into a tolerance, and the translation should be visible.

The Bill of Materials Is Part of the Design

The choice of a part is as much a design decision as the choice of a value, because the package determines the footprint, the tolerance determines the yield and the lifecycle determines whether the product can be built in three years.

Recording the reason for a choice, and the parameters that are critical, allows a substitute to be evaluated later without repeating the analysis. Our bill of materials notes describe the fields that make that possible.

Manufacturability Is a Requirement

A design that cannot be built at the quoted cost is not a design that meets its requirements, however elegant the schematic. The capability of the shop is a constraint in the same sense as the supply voltage.

Confirming the design against a capability list before the release is cheaper than adjusting it afterwards, and the confirmation is a few minutes of comparison rather than a study.

Verification Should Match the Risk

Not every net needs a simulation, and not every parameter needs a measurement. The verification effort should follow the risk, which means the items that could fail and that cannot be corrected cheaply get the attention.

Writing the risk list is part of the design work, and it is what makes the review efficient rather than exhaustive. Our design review notes describe the order in which the items are checked.

Process Control and Verification

On a design of this kind, thermal relief is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Process Control and Verification

On a design of this kind, thermal relief is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Process Control and Verification

On a design of this kind, thermal relief is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

FAQ

Is this a checklist or a mindset? It is a mindset expressed as a checklist. The items change with the product, but the habit of asking what the decision costs and who it constrains is what produces the benefit.

Which item is most often neglected? The stack-up, because it looks like a manufacturing detail rather than a design decision. It constrains the impedance, the coupling and the mechanical behaviour at the same time, and it is the most expensive item to change later.

What does gopcb review before release? We review the stack-up, the material, the minimum features, the panelisation, the finish and the tolerance set against our capability and against the quoted price. Where a design could be built more cheaply without loss of function we say so.