Saturday, 20 August 2011

Advice for PCB assembly

At work I've been developing two new PCBs. These are both eurocard size boards with a high component count and some tricksy BGA and QFN parts. For prototype builds we typically use an external company to assemble the boards. I've been through the process of getting boards assembled several times before, but this current project is by far the most complex and the assembly process has been far from straight forward.

A three working day quote turned into a six day turn-around. Consequently, I've learned a few things about how to manage this process and I want to share what I have learned.

Lesson #1 Make the assembler's life as easy as possible. If you don't, they will make your life as hard as possible. An extra couple of hours of your time spent preparing the data files will speed up the build, reduce the number of component placement mistakes and curry favour with the assembler.

Lesson #2 Supply good assembly drawings. CAD programs are really good at auto-generating this kind of output, but left unchecked they will produce accurate but unhelpful drawings. A good assembly drawing will be a PDF showing each component layer (separately) and all the component designators for that layer. If the board is very compact, enlarge the print so it is more than just a 1:1 print. Use layer colours which are easy to read and which have high contrast.

For prototype builds, it is very unlikely that an assembler will use a pick and place machine to populate the board. It just isn't worth their time. Unless you are supplying parts on reels, the time taken to re-reel components and load the machine is crazy. This means your boards will be placed by hand and so providing clear, human readable drawings is essential.

Lesson #3 Carefully prepare the components This is mainly common sense. If you are getting multiple boards assembled, keep the parts for both in separate boxes. Order spares of inexpensive components. An extra few pounds spent on passive components is preferable over having to leave them off the board or delaying the build to buy more. If you order your components from Digikey, during the checkout process you can enter the component designators corresponding to each part. These will be printed (space permitting) on the component bag label when your order is prepared. This makes it much quicker to find components.

Lesson #4 Check the BOM carefully Again, this is pretty much common sense. Make sure that any no-fit components are removed from the BOM. Failing to do this can mean the assembly is delayed while they look for the missing parts.

Lesson #5 Keep track of your changes This one really caught me out. It is common to purchase components as soon as you have the first draft of the schematic ready. This means you have time to sort out long lead time components while you are revising the design. Inevitably, changes will need to be made to the BOM after this initial order has been placed as issues are discovered during the review/layout stages. If you don't keep track of these changes and order the new parts you will have problems during assembly. One method is to compare the first BOM with the final BOM and spot the differences, but this is tedious and prone to error. It is better to generate an Engineering Change Order (ECO) for each of the changes as you make them. This can be a simple spreadsheet listing the designators affected and the nature of the change. This may sound time consuming but it will save you a lot of lost time later on.

Lesson #6 Get progress updates It is worth checking in on the assembly process every couple of days. A short phone call to check everything is going O.K. may help you spot and resolve parts queries/shortages before they add half a day's delay to your project.

Lesson #7 Remember its a prototype I think most assembly companies acknowledge that during prototype builds there will be problems with the BOM and the odd component shortage. From my experience, they are happy to put the build aside for a day while you fire off a Farnell order to get hold of the missing parts.

This may all seem pretty obvious, but in the hurry to get your prototypes made and meet your next milestone, it is easy to let some of these points slip.


Gavin said...

Very good points Chris - if you are making a board yourself, it might also be a good idea to write a list of instructions of in what order you solder stuff on - it may sound a bit OTT, but you only need to make it a high-level thing, but it allows you to check you have all the components, and guesstimate the time taken for assembly - and makes sure that you solder all the small things on first etc.

Incidentally, most of the issues you have raised apply just as much to manufacturing - the more information you can supply any third party, the less confusion there will be. One thing which is common on drawings (and something I put on mine) is "IF IN DOUBT, ASK" - which may sound obvious, but it encourages people to ask you before making assumptions, which are often incorrect.

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