Process Control in Contract Electronics Manufacturing

I have always believed process control is the most significant part of any PCB manufacturing facility. Once you have an understanding of the capability, then decisions on the type of work that can be accepted profitably is a much easier. Process control is such a big topic I am going to cover it over the next two months.


To control a process, you must first understand the equipment and what you are trying to achieve.


I always say there are 5 influencing factors: Materials, Machinery, Method, Manpower and Maintenance.


Materials

…should be bought to the highest specification the product warrants. There is little point in putting a 3 track single sided board used in say a hair dryer onto anything more sophisticated than SRBP or FR4 but likewise, don’t try and downgrade the material on more complex boards, it will cause problems. E.G. Using coarse weave prepreg on the outside of a multilayer build to bulk up the thickness when you are putting 0.004’’ track and gap on the outer, trouble will arrive.


This goes for all the process materials, whether it is consumable or part of the build.


Machines

I always start by characterising the machines, what I mean by this is understanding their ability to carry out the process consistently and repeatably.


Some examples might be ovens. A box oven with a fan may claim to give controlled heat in every part of the box, but things like extraction, door seals, cold days, hot days all have an influence on the character. Once you understand that the back left corner or whatever doesn’t achieve temperature, adjustments or repairs can be made and work can be distributed to compensate.


Etching may be another example. The top of the board going through the etcher will always process differently to the bottom. This is mainly due to the puddling effect on the top side. This prevents fresh solution hitting the copper. Adjustments to the top and bottom etch pressures can significantly change this to a more balanced position but because you have characterised the process, the underside spray bank will always be used for tracks and the top for ground planes when possible.


Knowing the machines’ characters is a helpful start position.


Method.

Before you start to try and control a process, the method must be correct, documented and proven to be useable by anyone with training. It may sound obvious but I have witnessed thousands of pounds worth of scrap created by bad methodology. Don’t make the mistake of thinking it is so obvious that no one could get it wrong as they most certainly will.


Go through every possible aspect of failure, variables, adjustments etc and document everyting with pictures if necessary.


Manpower.

Ensure there is enough of the correct type of manpower to carry out the job. Again, it sounds simple but know the strengths and weaknesses of the staff is the most important part of any operation. I don’t think embellishment on this is needed, square pegs in round holes should suffice.


Maintenance.

Maintenance is such a major issue it is a category all by itself. There are arguments to leave well alone but my overriding experience says change the schedule but don’t leave it until it falls over.


I understand production don’t want to give up the machines for routine maintenance but all that happens is that the throughput flatters the output as more and more go in the bin. I have seen too much scrap created by ignoring maintenance to ignore it. If the machine needs maintenance monthly, don’t do it weekly as the support off the production will be negative if they see it as too much.


Process control is about feedback and responsibility for the process including scrap so next time, a delve into how to control the processes to give profit and on time delivery.


 

(Source: internal information)

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