Design Review: Checks Before a PCB Layout Is Released

The last stage before a layout is released is a design review that compares the board against the documents it was built from. It is the cheapest point at which an error can be found, and the checks are the same ones every time, which is exactly why they are written down.

This article works through the checks in the order the work is done, from the completeness of the input documents to the verification of the assembly requirements. A written checklist does not depend on what the reviewer happens to remember on the day.

Document Inputs: What Has to Be Complete

Nothing should be laid out from partial information. The review begins by confirming that the schematic, the board template, the bill of materials, the design specification and any change request are all present and current, and that the fabrication and assembly requirements are stated rather than assumed.

The version of the template matters as much as the schematic. A template that has been superseded carries an outline, hole positions or keep-outs that no longer apply, and a board built on it inherits the error silently. Where the mechanical data comes from a third party, the same check applies to it.

Outline, Tolerances and Keep-Outs

The outline, the dimensions and the tolerances are compared against the mechanical drawing, and the two classes of hole are confirmed: plated holes that carry a connection and unplated holes that do not. The distinction is a fabrication instruction and an error in it produces a shorted mounting hole or an unconnected via.

Restricted areas belong in the layout database rather than in a note. The area around a mounting hole, the region reserved for a battery or a display module, and any keep-out required by the enclosure are drawn as constraints, so that the routing cannot pass through them by accident. Once the mechanical structure is confirmed, it is locked so that it cannot be moved by a stray selection later.

Component Placement Checks

Every component is confirmed against the company library, and any part that differs is updated before the layout is examined further. The transfer report from the schematic is read rather than dismissed: it lists the parts that were placed and the ones that could not be, and a part that failed to arrive is invisible on the screen until the final check catches it.

The placement is then checked as a whole: whether every part is on the board, whether the footprints that overlap are allowed to overlap, and whether the fiducials that the assembly machine will use are present and in the right places. Heavy components are confirmed to be near a support point, and the placement is locked once the mechanical relationships are correct.

Connectors, Press-Fit Parts and Mating

Connectors are checked against their mating parts. A press-fit connector has an area around it where nothing tall may be placed on the component side and where nothing at all may be placed on the reverse, because the insertion force has to be applied to the board itself.

Where a board plugs into another, the two are compared directly: the signals have to correspond, the positions have to align, the orientation of the connectors has to match and the silkscreen has to identify them unambiguously. The keys and the offsets that prevent a board from being inserted the wrong way round are part of the same check, and so is the clearance between components on the two assemblies.

Assembly Process Checks

The assembly route decides several placement details. Parts that will see the wave need the package variant intended for it, and a chip component that is to be soldered in a wave rather than in reflow has to be reviewed for shadowing. The number of hand-soldered joints is also worth counting, because a large number of manual operations raises cost and variability.

Devices with a high pin count, fine pitch or a package that needs a specific thermal profile are checked for clearance around them, so that the nozzle or the iron can reach the joint. Metal-bodied components are checked for contact with their neighbours, and tall parts are kept clear of the path that the placement head will follow. The requirements that placement has to satisfy are the same list seen from the design side.

Functional Checks: Domains, Clocks and Interfaces

On a mixed board, the digital and analogue areas are checked to confirm that they were kept apart and that the signal path through them makes sense. A converter that straddles the boundary is placed so that its analogue side faces the analogue domain, and the clock devices are kept clear of the sensitive analogue circuits and of the connectors that leave the board.

The clock distribution itself is worth reviewing at this stage, because a clock that has to reach several devices is a clock whose delay differences matter. The reasoning used to decide the arrangement is the same as on any mixed design, as the mixed-signal guidelines describe.

Layout, Drill and Via Checks

Once the routing is complete, the layout is checked against the netlist and against the rules, and the drill table is reviewed as a document in its own right: the number of distinct hole sizes, the finishing of each, and whether the smallest drill in the design is one the fabricator will produce without a premium.

The vias are checked for the same reason. An unused size adds a tool to the drilling programme, and a via that is larger than it needs to be consumes routing space that another net may need. The via design rules apply to the pad as well as to the hole, and the check is against the fabrication capability rather than against a default.

Sign-Off

The review closes with a list of the checks that were performed, the person who performed them and the date. That record is what makes the review repeatable, and it is also what allows a defect found later to be traced back to the stage that should have caught it.

The output package is confirmed at the same point: the layer files, the drill data, the stack-up drawing, the fabrication notes and the assembly data all have to describe the same board. Where the process is documented this way, the same sequence from rules to DRC applies to the next revision as well as to this one.

FAQ

Who should perform the review? Someone other than the designer where the size of the team allows it. A second reader catches assumptions that the author cannot see, and the check takes a fraction of the time that the error would cost. Where the team is too small for that, the review is made against the documents rather than against the memory of the author.

Which check is most often skipped? The one that compares the placement against the mechanical drawing. It is tedious, and it is the check that catches the errors that stop a board from fitting.

Should the checklist be the same for every project? The core of it should be, because the mistakes it guards against are not project specific. Project-specific items are added to it rather than replacing it.