Pin Count for a PCB Design Quote: Where the Numbers Come From

Layout services are usually priced by the pin, because the pin count is the closest thing to a measure of the work. It correlates with the number of connections to be routed, the number of pads to be placed and the volume of data that has to be checked, and it is a number that both sides can agree on before the work starts. The difficulty is that the number has to be counted from something, and the something is not always a finished layout.

Why the Pin Count Is the Unit

A pin count is objective, it is visible in the design data, and it scales with the effort in a way that board area does not. A small board with two large ball grid arrays can be far more work than a large board with a few hundred discrete parts, and pricing by area would get both of them wrong. Pricing by pin captures the routing effort, the placement effort and the amount of verification that the design will need.

The convention that has to be stated in any design quote is what is being counted. The usual definition is the number of component pins or pads that belong to a net, which means that the pins of a connector, the balls of a BGA and the pads of a discrete part all count, while the features that are not component terminations do not.

What Counts and What Does Not

The list is worth writing into the quotation, because the difference between two interpretations can be large.

A BGA with 484 balls contributes 484 pins, whether or not every ball is connected, because the balls are pads that have to be placed, routed or deliberately left unconnected and checked. A connector with a hundred positions contributes a hundred. A thermal pad under a package counts, because it is a pad that has to be dealt with in the layout and in the stencil.

Vias do not count as pins, even though a report may list them alongside pads, because a via is created by the designer rather than by the netlist. Mounting holes, fiducials, test points and tooling features are usually excluded as well, and it is worth saying so explicitly, because the same item can be counted either way and both are defensible.

Mechanical parts and shields are the awkward category. A shield frame has pads that must be placed and soldered, but it has no electrical function. The practical rule is to count the pads, because the layout work is the same, and to note the decision in the quotation so that a later build does not produce a surprise.

Counting from a Finished Layout

When the layout exists, the count is a report rather than an estimate. The layout tool can list every component with its pad count, and the sum of that list is the pin count. The report should be run on the same database that will be used for fabrication, so that the number matches the design data rather than a copy of it.

Two checks are worth doing on the result. Compare the number of components in the report with the component count in the bill of materials, because a component that appears in the layout and not in the parts list is a design error as well as a counting error. And compare the pin count with the number of connected pins in the netlist, because the difference between them is the set of deliberately unused pins, which should be a small and explainable number.

Counting from a Schematic

The harder case is a design that is quoted from a schematic, before the footprints exist or before the library is complete. The count then comes from the symbols, and a symbol does not always map cleanly onto a component.

The reference designator is the key. A symbol carries a designator and a list of pins, and the report is generated by walking the schematic and accumulating those lists. The pitfalls are all in the mapping.

A multi-part component appears as several symbols sharing one designator. Counting each symbol as a device doubles or quadruples the part in the count, and the correct treatment is to count the pins of all the units and the component once.

Supply pins are sometimes hidden. A symbol that models its supply connections as a property rather than as drawn pins produces a count that omits them, which matters on a device with many supply balls. The fix is to check the symbol against the device pin list, which is a task that has to be done anyway before the part can be laid out.

Alternate and deferred symbols can leave pins out of the schematic that will exist on the board. A pin that is marked as not used still has a pad, and a pad that has no net still has to be placed and covered by the stencil.

Connectors are counted position by position, and a connector symbol that is drawn as one block may not carry a pin list that matches the physical part. The physical drawing is the reference here, not the symbol.

Making the Number Reliable

Three practical measures reduce the argument later.

Count from the same source both times. If the quotation is based on a schematic count and the invoice is based on a layout report, the two numbers will differ, and the difference will be attributed to scope creep rather than to the counting method. The way to avoid that is to state which source the quote uses and to re-count from that source if the two disagree.

Publish the per-component list with the quote rather than only the total. A list of components with their pin counts takes a page, and it turns a disagreement about a total into a discussion about one item. The parts list behind the number is generated from the same database that feeds our component procurement review, so the two always agree.

State what happens when the design changes after the quote. Adding a ball grid array in a later revision adds hundreds of pins and a routing problem, and a quotation that covers the measured work is fairer than one that has to absorb it. Our design and layout quotations are itemised for that reason, and the pin count is re-checked against the released data when the layout is complete.

Finally, remember what the number is a proxy for. Pins on a dense board with fine pitch parts, impedance-controlled routing and a stack-up requirement are not the same work as pins on a two-layer board with discrete parts, and a quotation that reflects only the count will be unfair to one of the two. The pin count is the starting point; the assessment of the design is what makes the price right. Our PCB capabilities page describes the constructions we regularly build, and it is usually the first thing reviewed when a design is quoted.

FAQ

Do unconnected balls on a BGA count as pins? They are pads on the board and they have to be placed and covered, so they are normally counted. The quotation should state the convention.

Should test points be counted? Usually not, but the point is worth agreeing in writing, because a design with several hundred test points is a different amount of work from one with none.

What if the schematic and the layout give different totals? Re-count both from the same source. The difference is usually a multi-part symbol, a hidden supply pin or a component that exists in one database and not the other.