PCB Assembly Cost Model and Drivers

Assembly cost is driven by the number of placements, the number of process steps, the test method and the rework rate, and it is usually dominated by the first two. Understanding the model allows a design to be assessed before it is built.

The model is simple arithmetic applied to estimates, and its value is in showing which decision has the largest effect rather than in producing an exact figure.

Placement Cost

Every component consumes machine time, and the time depends on the package, the feeder arrangement and the number of nozzles required. A board with a thousand small parts costs more to place than one with two hundred larger ones.

The number of unique parts matters as much as the total, because each new feeder occupies a position and may require a changeover. Reducing the variety is often cheaper than reducing the count.

Process Steps

Each additional step adds a setup, a pass through a machine and a handling operation. A second reflow, a selective soldering step, a coating and a cleaning step each add cost and each adds a source of defects.

The cost of a step is not only its cycle time, since the handling and the work in progress between steps consume labour and floor space.

Test and Inspection Cost

Test time is a direct cost, and the method determines it. A fixture test is fast once the tooling exists, while a flying probe is slower but needs no tooling. The comparison is described for test methods.

Inspection adds time without adding value when the process is capable, which is why process control reduces cost as well as defects.

Rework and Yield

Rework consumes labour and risks further damage, and its cost is often underestimated because it is distributed across shifts. A board that is reworked twice may cost more than the material it contains.

Yield loss should be priced at the full value of the board at the point of loss, including the components already placed. This is the point that the scrap rules described for rework decisions address.

Volume Effects

Fixed costs such as tooling, programming and setup are spread over the volume, so the cost per board falls as the quantity rises. The model should separate the fixed and variable parts so that the effect of a quantity change can be seen.

A small order placed without amortising the fixed costs is frequently priced below their real level, which is why the second order is the one that reveals the true cost.

Panel and Format Effects

The number of boards per panel changes the machine cycles per board and the handling. A panel that is well filled reduces the cost per board for the same process.

The panel also affects the yield, since one placement error on a filled panel affects several boards at once. That trade is described for panelisation.

Labour and Skill

Labour cost depends on the number of operators the line requires and on the skill level. A process with more automation needs fewer people and more engineering support, and the balance shifts with the volume.

Hand operations such as wire assembly, connector fitting and rework scale with the volume rather than with the machine, and they are usually the first place where cost rises unexpectedly.

Using the Model

The model should be built early enough to influence the design, since the largest decisions are the placement count, the step count and the test method. A model produced after the design is complete only explains the cost.

Comparing two design options on the same model is the practical use, and it should include the yield difference rather than only the direct cost.

Records

The model should be reviewed when the process or the volume changes, and the assumptions recorded so that they can be challenged. An estimate that is never revised becomes a belief.

The measurement that validates the model is the actual yield and cycle time, from the data described for line performance.

Process Control and Verification

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.

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.

FAQ

What dominates assembly cost? Usually the placement count and the number of process steps, with the test method third.

Is a lower component count always cheaper? It is if the function is unchanged, and reducing variety often saves more than reducing count.

How should yield be included? As a multiplier on the total cost of the board, priced at the value at the point of loss.

When should the model be built? Early enough to compare two design options, which means during the concept rather than after the layout.