Libraries are the backbone of the PCB design – measure twice and design once

pcb librariesLibraries are the backbone of the PCB design

Firstly my apology for the analogy of wallpapering and pcb Libraries. It is only because as we approach the festive period I find myself having a couple of last-minute interior decorating tasks I need to complete before the big day. As I got the pasting table out, I remember back to when my father-in-law was showing me how to paper a wall properly, and his main lesson was ‘measure twice and cut once’. Even in this electronic design world the same applies to properly building PCB libraries!

It has been said many times that PCB libraries are the backbone of the PCB design process, without them there just is no design. Libraries are generally seen a barrier to starting the PCB design right NOW, when the plan generally is to ‘just get on with it ASAP’. This situation is exacerbated when a new, hot off the silicon press, PCB component has to be incorporated into the product’s design, to get the necessary new feature / extra performance, or formfactor that is required. Before and pcb component can be included in the design project, the symbol and footprint has to be made available to the engineering team.

Creating PCB libraries takes time and patience – like wallpapering

Let’s consider just the printed circuit board footprint for a moment, there are a number of approaches to its generation; From manually placing the elements that make up a component such as PCB pins, PCB body outline, PCB courtyard and 3D representation of the component, to assembly rules driven approaches using IPC 7351 calculators. More recently the ability to download the pcb component directly from the PCB vendor’s website has become a new option; it should be noted that for some CAD solutions this capability is embedded in their software offering as a default. Downloading the data certainly speeds up the creation process, but we should take for granted that it is all there, or correct?

Less haste – more speed in professional PCB design through better library management

Anyone who has read a PCB data sheet to create a board footprint knows how inconsistent they can be; different measurement systems such as nominal values with a unilateral tolerance or min and max dimensions or sometimes, a mixture of both is employed across the same vendor’s documentation. With the advent of the data supplied electronically, the landscape is pretty much the same as before.

“There currently is no real standard for the supply of electronic data for vendors of pcb components to follow so pcb libraries can be full of inconsistencies”.

As such each IC vendor has had to adopt their own approach to solving the issue, and put into the mix that the vendors having to supply the data in multiple software package formats, you can see why this may not be the most consistent solution at this point in time; but this development is much better than no component data at all, and provides a great starting point! All this aside, the big question is, and the only one that matters, ‘are the footprints correct?’ We all hope so, and expect it to be as it was created by the vendor; there is a way to find out, and it involves ‘measuring twice – design once!

So, in this case measuring twice really means double checking. This may not seem like the most enthralling part of the glamourous life of a PCB Design engineer, but it does help reduce the chances of having a design respin due to incorrect PCB libraries issues. As we all know design respins not only will have a financial impact, but also a major effect on project timescales so the implications can have big hidden impacts.

At TBS we speak from a collective of 60+ years’ experience – we check all our components, whether we created them in house or from downloaded, they all go through the same process. Believe us for we have found that a range of issues come from downloaded components. These have included soldermask missing, or no paste mask defined, erroneous keepouts. As a minimum check, the following areas should be always be considered.

1. The copper land itself – are the pads the correct size, numbered correctly, in the correct locations, and do they have the correct attributes for fabrication?
2. Are there footprints constructed? such that they support the companies design process, are the layers correct with the correct information on them that will support the design process?

The bottom line is that time spent getting the PCB board right at the beginning, and having an effective component checking procedure in place, can save a lot of time during the PCB layout and documentation steps, or as mentioned above, a printed circuit board that needs to be salvaged with a redesign.

The vast majority of the downloaded components will be good to go, and will not result in a bad PCB, but the reason for checking is that it can only take one to scrap a great PCB design. Conversely, measuring twice will not ensure 100% that the component is correct, there is always the possibility of a human error, but it will increase the chances greatly and be right first time!

Happy wall papering, and great PCB designing in the New Year!

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