The PCB designer is our hero in this story and hero’s never get an easy ride. This story involves the evil manager of the PCB designer walking up to him one day, possibly with the even more evil mechanical engineer in tow. The PCB engineer’s next task is to re-design a board that they’ve put blood, sweat and tears into and make it even smaller, based on the mechanical engineer’s drawings.
Happy that this task has been offloaded the evil manager and mechanical engineer disappear, leaving our hero to work out how to do this.
But what does our PCB engineer do next? The chances are that this isn’t small reduction, they’ve not messed about and want it half the size. Because the mechanics have already been made there’s no chance of negotiation.
The first thing is to go back to the engineer – what can be cut out of the design. Every component counts and spare connectors can free up lots of room. Once the circuit has been agreed and all the necessary working out of what the circuit needs to do has been worked out there’s the next step.
Components: Most components come in more than one package type these days – selecting the one that’s going to be the right fit is easy. Making sure it’s available and still meets cost restrictions can be harder.
PCB Strategy: Before starting any design work having a design strategy is going to save messing about later in the process. Every detail that can give or take space needs considering.
Layers: Estimating how many more layers will be needed ensures the design can be completed, too many will add unnecessary cost.
Track and gap:
Reducing the track and gap can have a big impact on saving space in a board design. If the components are being reduced in size then it makes sense to reduce the track and gap in line with this. Check to see what the fabricator can achieve before committing and see what the cost / yield implications might be.
Via’s: Via’s can suddenly seem huge when using smaller components. See what the fabricator can make as a minimum, making sure the aspect ratio is achievable.
Annular ring: If there are any plated through holes in the PCB design, reducing the annular ring can help add some much needed space. 2 x the drill hole diameter is accepted as the standard, however 1.5 x the drill diameter is typically the minimum that should be used.
Silk Markings: So after all of this there still is nowhere near enough room to squeeze everything in. Component references on the silk screen are very useful, but they do take up a lot of space. A component reference for an 0402 component would need to be about twice as big as the component to be readable.
If the naming strategy hasn’t been created well the component reference can dwarf the component. So getting rid of them on the board will save a lot of space. Make sure there’s a well checked and marked assembly drawing for the PCB – fault finding will be pain as it is, don’t make it harder!
Now our hero the PCB designer has worked out how to work the impossible there are still issues that can foul things up. EMC, thermal considerations and creepage and clearance are just some of them. Assembly issues can arise from shrinking a design too and that’s before testability and service and rework problems.
When shrinking the size of electronics there are always compromises and they’re usually sent down the process chain. This is almost unavoidable.
© Circuit Mechanix 2016