What to Bring to the PCB Design Review
Every job eventually gets to tape-out day. But before that day comes, a lot of moving parts are wrangled into place. Even the simplest layout will require deliverables for assembly including custom paste stencil and a bill of materials to associate the correct component for each location on the board. Along the way, a set of physical and electrical properties will be used to gauge line width and length among other parameters. Getting the responsible parties to give their guidance on the many assumptions made during layout is the point of the design review.
Image Credit: Business Matters
Placement Spacing and OrientationTwo types of assembly data are used in the factory. The kind we can see or touch and the kind that has meaning only to the machines. Robotic assembly is programmed from an X-Y coordinate file. An assembly drawing provides a visual representation including reference designators, component outlines and importantly, the orientation with a pin-one mark. A hard copy of the assembly drawing is an item for the review.
“Every MCAD to ECAD flow has the potential for missed requirements. To close this loop, invite the Physical Design Engineer to the review.”
If you’re lucky and well supported by physical design (MCAD) then you might also see an outline drawing for the PCB and perhaps an interface control drawing (ICD) that presents all of the electrical details pertaining to any connectors to the outside world. You don’t usually expect these types of drawings until everything is said and done and then only because the customer requested the drawings. Every MCAD to ECAD flow has the potential for missed requirements. To close this loop, invite the Physical Design Engineer to the review.
Routing Tricks, Workarounds, and Neck-DownsRules were made to be bent, folded, spindled and mutilated in order to complete the job in the real world. Ok, in this case, the real world is a virtual representation of a PCB that does not yet exist, but the point is that we can’t always meet every spacing and line width requirement. Zones set up to facilitate fine-pitch connectors or BGA packages will allow the rules to be relaxed.
The trade-offs have to be carefully managed. Signal integrity and power integrity (SI/PI) are battling for space. It’s a tug of war for an air-gap around the clock or metal for the VCC. Let the border wars begin. When everyone is equally uneasy, it’s time to ship. Every major link in the design has a champion. Bring all of them together at least once, especially when the pickings are slim.
The Document PackageOf course, there will be a screen to show off the layout details. As you go through the highlights, comments can be captured in “red ink” on the printed docs. Speaking for myself, the act of turning on each layer of the design and plotting it out has revealed at least as many mistakes as the design reviews have. Fix and replot.
Image credit: Velodyne Lidar
The CAD screen can show details as you pan around. The hard copy makes errors like that one upside-down reference designator stand out in black and white. The plots are most useful for the folks who show up for the review on time. They can dig into any part of it that is of interest while you set up the A/V equipment. They’re interested in something or they wouldn’t be there.
Down Rev Artwork For ComparisonWe can always count on some nostalgia for the old revision if the design under consideration is based on an existing rev. Reasons for not wanting change if it isn’t broken are valid, especially for analog where there is an element of FCC regulatory requirements. Having to go through certification again works against the time-to-market drive.
The same holds for a hairy DDR maze or the power tree for a mobile computing device. Setting up slides where, for instance, you have the old work in blue, the new in red and the common elements in the overlapping purple helps people visualize the upgrades to each layer. If it becomes a Jackson Polock sort of image, then they know you’ve been busy.
Image Credit: 1st Dibbs - before and after editing.
List of Major Upgrades, Design Rules, and Exceptions
The schematic is normally the warm-up act for the actual layout. This is more to share the interesting parts than to hash out schematic requirements. That was (or should have been) given its own review slot. One famous company I worked at had the EE locked in for 5 hours while people came and went reviewing their sections.
Still, the co-development doesn’t halt just because your 11th hour is nigh. The motto seems to be that if it wasn’t for the 11th hour nothing would get done. Design reviews can easily go off on a tangent, so make sure you’re the one sharing your screen for the most part. The spotlight is on placement and routing as far as we’re concerned. A chip by chip approach is often more useful than a layer by layer presentation.
“Spelling out the risks makes them shared risks”
A slide deck is useful to an extent since you can capture screen-shots and add some captions for everyone to “take-home”. It allows us to pace the review and ensure that we at least acknowledged the data. Spelling out the risks makes them shared risks. When it breaks down is when questions come up. That will bring the board file back to the screen while you take measurements or whatever. I liked slide shows as teaching aids and the fact that they were more portable than the PCB database.
Beyond hard and soft copies of everything, a willingness to accept new information is key to surviving a design review. Action items are expected. “OK2FAB” is phrased as a question. Asking that question leads to answers. Until the situation becomes “OK”, keep closing loops and closing deals and on 2 FAB!