What should happen after PCB design?

from the Circuit Mechanxi magazine:

Circuit Mechanix Mar 2016 - Cover

What should happen after PCB design?

The PCB design is complete, all the DRC checks are done. Hours of pouring over every part of the design by you and your colleagues have been made to make sure the latest electronic baby is as good as it’s going to be. What comes next?
There are many, many articles about different aspect of the printed circuit design process, or the bare board
fabrication do’s and don’ts. Very little is written about post design, this could be because it’s the boring bit that no-one wants to talk about because it’s as
much fun as being poked in the eye. But capturing design intent at this point is key, or the you could be doing it again sooner than you think.

Let’s assume for the purpose of making this a little easier to swallow that the basics like silk colour, solder resist and stack up etc etc are all documented on one of the design layers as they should be and the library components are tried and tested. All the gerber files have
been generated and are sitting there. What other documentation is needed?
1) Netlist
2) Component positions data
3) Assembly drawings
4) Build instructions
5) Important component information
6) Other special requirements
Designers and engineer’s know there are times when things have gone wrong because some key information is missing. It’s unreasonable to expect the companies making our boards to notice the unusual or to mind read our intentions. The good companies
sometimes pick up on those things that seem strange and ask for clarification, but relying on this is dangerous. What’s the simplest way of finding out what’s needed? Do what most engineer’s hate – pick up the phone and talk to them, if that’s too painful there’s good old email. The key thing is to ask them and find out.
Circuit Mechanix ©2016

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Circuit Mechanix March 2016 – First Issue!

Circuit Mechanix March 2016 – First Issue!

Circuit Mechanix Mar 2016 - Cover

 

Welcome to this first issue of Circuit Mechanix, a magazine for the Printed Circuit industry in the UK. This is a project that I’m going to be working on this year as a trial, after this we’ll see where it goes. Any and all support is appreciated as I think Circuit Mechanix could go places with enough readers.

 

 

 

 

Download the PDF for the magazine here:

Circuit Mechanix Mar 2016

There is a flip book planned, but at this time I’m not able to publish it. This may follow after if I work out the technical details.

Knowing your PCB assembly process

From the Circuit Mechanix magazine:

Circuit Mechanix Mar 2016 - Cover

Knowing your PCB assembly process

It’s always a time of mixed emotions when a new batch of PCB’s arrives on your desk, fresh from the subcontractor.
Excitement of testing the design that’s had time, blood, sweat and tears put into it. Hoping they work after all the
time and money that’s been spent on them. Knowing the assembly process can give a designer or engineer greater
confidence in the finished boards. Different aspects that can be considered are:

  • Paste Screens – How the paste apertures are defined in a design can be important to the assembler. Find out
    how the paste apertures are used in the solder screens. What often happens is the paste aperture in the gerber files for a PCB will be a 1:1 representation of the pad and this is used by a separate stencil fabricator to make the paste screens. Check what a subcontractor does though, an incorrect assumption
    could lead to the incorrect quantity of paste being applied.
  • Handling – How well setup is your  subcontractor to handle ESD (Electro Static Discharge) and moisture sensitive components? Are there equipment and controls in place to handle them? Controlling static is often regarded as unnecessary, however it is a real risk to  components. The same applies to moisture sensitivity, which components are rated as moisture sensitive in the design? What impact will this have?
  • Reflow – What reflow process does your subcontractor use as standard? Normally this doesn’t matter, however some components can be damaged by vapour phase reflow processes. Not only is a good knowledge of the assembly process required but an intimate search of component datasheets is sometimes necessary. Checking individual assembly
    requirements of the components is often hard as the information that’s important is buried deep in the datasheet on page 12 in small text which isn’t highlighted.
  • Cleaning – What kind of cleaning process does your subcontractor use as standard? How a PCB is cleaned can again affect components, check your datasheets. It’s also not a great idea to clean components like transformers as drying them can be hard and will have a
    negative effect on safety clearances (if it’s a safety / isolating transformer). With the cleaning process there should be a drying process as well, making sure it’s sufficient to completely dry out the board this is especially needed if using Rogers materials.
  • Inspection – AOI (Automatic Optical Inspection) is employed by most medium to large sub assembly companies. Check and make sure this is the case, do they inspect a sample or each PCB? Finding and curing a short or dry joint you can’t see is difficult to say the least
    and many component packages have connections underneath the component body. Your subcontractor should be able to check this with some kind of xray or specialist visual equipment. Depending on the product type and the PCB being made there could be more
    that needs to be known.

The point is – know your subcontractor, know what they’re doing and as much as possible
tell them what is needed.
Circuit Mechanix © 2016

Shining the light on what your PCB fabricator does with your data

Article from Circuit Mechanix Magazine:

Circuit Mechanix Mar 2016 - Cover

Shining the light on what your PCB fabricator does with your data

After the design process is complete and the fabrication data, is generated and checked (I’m sure everyone reading this checks their data before giving it to the fabricator) it’s sent to the fabricator and after a few days some PCB’s arrive back, just like the PCB’s that were designed. It’s simple right?
But what has the fabricator done to / with your data?
Have you asked for your fabricator to check the gerbers against a netlist?
Is the panel data reviewed before production commences?
Why is it even important?

Like all these things, it’s impossible to know until the question is asked! The next time PCB data is sent to your fabrication company why not ask them a few things:

  1. Ask them to make a list of the changes they make. With the best will in the world the design rules the design is checked against are often not set up to the manufacturers constraints. This kind of information will help you understand what they need better.
  2. Most of the time PCB’s are supplied in panels, how they are aligned can cause issues with your product (where the breakouts are for example) or possibly the manufacturing process. Having a review on the panel data and possibly having the assembler look at it, (if the design complexity demands it) can help smooth out the assembly process.
  3. If you’ve provided your fabricator with a netlist to check the gerbers against, have you checked to make sure they can read it? Is your fabricator carrying out this check?
  4. There is normally some kind or test report generated when this process. Getting the results of this ensures that not only can they read the netlist and you’re generating it in the right format, but that it’s being carried out. Asking the fabricator to do this is often
    needed as sometimes the principle of ‘if it’s not asked for it’s not needed’ is used.
  5. The netlist will also help give the fabricator the data they need to test the bare boards. But of course, is this carried out? If it’s not, faults in bare board test will not show up until the
    board had been populated and testing is being carried out. If it’s being asked for, make sure that a copy of the test report is included with the delivered PCB’s.
  6. Time and money may be saved if bare board test is deemed unnecessary, but imagine the lost time and money is there’s an error on some or all of the populated boards. Really it’s a false economy.
  7. The next thing that’s interesting to look at is plated through holes. The hole size given in your design – is it treated as the finished hole size or the drill size? Surely the difference doesn’t matter? In a hole for a large transistor this is certainly not going to make much difference. But a small hole for a fine wire or lead may end up being smaller that expected as the plating can reduce the hole size by 0.1mm. Sometimes this counts.

Knowing this kind of information about how your fabricator works will help a designer enhance their designs by enabling them to work with the fabricator more effectively. Also going through the post design manufacturing review shouldn’t take as long as it sounds, but it could save a mess up and they cost far more time and money.
Circuit Mechanix © 2016

Circuit Mechanix – March 2016

Circuit Mechanix March 2016 – First Issue!

Circuit Mechanix Mar 2016 - Cover

 

Welcome to this first issue of Circuit Mechanix, a magazine for the Printed Circuit industry in the UK. This is a project that I’m going to be working on this year as a trial, after this we’ll see where it goes. Any and all support is appreciated as I think Circuit Mechanix could go places with enough readers.

 

 

 

 

Download the PDF for the magazine here:

Circuit Mechanix Mar 2016

There is a flip book planned, but at this time I’m not able to publish it. This may follow after if I work out the technical details.