PCB Design for Manufacturability

It is important for our customers to understand that PCB manufacturability is not a primary consideration for the developers of electronics design software. They are charged with creating a package that will allow the user to develop a project, the underlying circuit board layout being only a small part. As such the layout part of the design package is usually created in such a way to allow engineers not familiar with PCB manufacture, to quickly and easily create a PCB layout that suits the requirements of their project, without confining them to follow specific protocols. This is fantastic for allowing the designer to design anything you can possibly think of.

But guess what? Anything possible does not necessarily mean anything actual or probable and unfortunately, this means that the board designer can spend time laying out a circuit board that is not actually manufacturable if unrealistic design rules are used to guide the creation of a pcb layout. For example, you can specify design rules using minimum track/space of 2 thou (0.05mm) and pass a DRC with flying colours - but can your preferred board shop actually make it?

Another thing, there are plenty of important PCB manufacture instructions that a pcb layout, on it’s own, is silent on. That’s why the inclusion of an accurate fabrication or mechanical drawing is so important. This drawing completes the rest of the manufacturing instructions to the board shop, such as board thickness, copper finish, colour of mask (if required), colour of legend (if required), pad finish, special requirements for routing, the list goes on. As a PCB manufacturer, we can assume most of these ( “standard” or “usual” requirements) if a drawing is not supplied, but what happens when a drawing is supplied and the instructions are wrong or conflicting? We are not mind readers, and to make matters worse, we see many different designs in any given day from many different customers who have different levels of skills and use different design rules, techniques & software. What you might think is obvious, is really not actually that clear to us.

An example below we see unfortunately too often:

You have finished the layout - finally! You’ve just made your deadline and you have been thinking & working non-stop on all aspects of the project for days, and have had enough. “I’ll just send the board off now for quote” you think and quickly attach the pcb file to an email asking for a quote before disappearing for some much needed sleep. You also included a fabrication drawing with the pcb so you are confidant that the quote will be straightforward and will be waiting for you in the morning for you to approve and order in time to meet the PCB manufacturer’s job cutoff time. To save time, you used a fabrication drawing from an old project and amended the bits you needed to for this particular project.

But guess what? In your sleep deprived state, you forgot to change the required board thickness from 1.6mm to 1.2mm and also forgot to change the pad finish from HASL to Immersion Silver because of the fine pitch ICs on this board. The PCB manufacturer duly quotes the board based on the file supplied and the incorrect instructions as to pad finish and board thickness. What happens if you don’t notice the differences and order the job?