circuit board assembly time!

You can have the most beautifully crafted and electrically perfect circuit board ever produced by man, but if it has the wrong parts installed it won't be worth a penny to you!  Obviously, it's your circuit board assembly provider's job to nail this operation perfectly for you.  So what can you do as a customer to insure your provider can do the best job possible?

To be clear, it is the customer's main job to provide the most accurate information for the entire circuit board fabrication and assembly process.  One of the area's customer's fail the most is in providing a clean Bill of Material or BOM.  Like previously stated, if the assembly is wrong the printed circuit board will never function as intended.  A clean and accurate BOM will help with not only creating the fastest and most accurate quote possible, but more importantly, the fastest and most accurate assembly.  This includes both customer supplied part kits and part acquirement services, so don't assume that if you are supplying the parts the BOM doesn't need to be on point.  Your BOM should be generated from your CAD software tool along with your other fabrication files.  Let's dig into how your BOM should look and what information is most important to include.

Typically, too much information is never a bad thing when it comes to your Bill of Material for your PCB assembly.  Most BOMs are generated with columns of information with each column supplying information and rows of line items of each unique component.  Here are the things that should be included in your BOM as a MINIMUM:

• ALL reference designators (if they are Do Not Populate or DNP it's still best to include)
• Reference designators utilizing the same part number should be grouped together as as single line item.
• A manufacturing (MFG) part number for each unique part being placed. (Certain exceptions can apply)
• Exact quantity for each part.
• A brief part description (most critical for passive components)

If your supplied BOM is missing ANY of this information, you need to STOP, go back to your CAD software or vendor, and get this information added at a MINIMUM!  

The most common issue we find in PCB assembly, even when the above minimum information has been properly provided, is out date or obsolete manufacturing (MFG) part numbers.  These are generally seen in older designs, but some professionals use CAD libraries that have been developed over the coarse of years, this can lead directly to the use of obsolete parts in new designs.  It's extensively seen in passive components, but many times active components can and do become obsolete, as well.  This is why supplying accurate descriptions is important for passives especially.  These descriptions should include some or all of the following data: 

•  value
•  tolerance
•  case size
•  voltage requirements
•  current requirements
•  wattage

This information will help allow us to make quick suggestions for drop-in equivalent alternate MFG parts for you circuit board assembly.  This will help in saving time and typically a lot of costs as drop-ins will be cheaper to acquire than original parts that are now obsolete and no longer manufactured.  

Providing a good list of approved equivalent drop-in alternates for as many parts as possible is highly recommended.  This will help save time and avoid unnecessary holds on the quote or assembly while waiting for customer approval.  Of course, alternates are not always a possibility.  In cases where original parts cannot be purchased, substituted, or have long lead times it is up to the customer on how to proceed.  

Besides a clean BOM the second most important document for a circuit board assembly to have on hand is a good centroid file.  This file provides a list of X, Y coordinates for each component on the board as well as a theta orientation value.  This file is essential for a fast and accurate assembly.  Some very simple assemblies don't always require a centroid as we can manually program, but when there are dozens, hundreds, or even thousands of components in a single board assembly you start to understand the vital necessity of such a file.  

Centroid location files are generated inside your CAD software along with the rest of your data files for fabrication.  It is typically in a .CSV file type and should have the following minimum data:

• Reference designator for each part
• X and Y coordinates for each part
• Theta value for rotation
• Top or Bottom assembly designator
• MFG part number

IMPORTANT:  Before generating location data, please verify the reference point (0,0) location is physically located at a board corner edge or some other hard point physically within the PCB layout.  A hard point can be a mounting hole or other TH.  We will need to match your reference point when we load your data into our programming software.  If you reference point is floating somewhere off in space away from the actual PCB assembly outline in your CAD it makes it very difficult to import that data correctly.

We are happy to accept customer supplied part kits, but we do have some strict requirements that must be met for us to even start your job.  

First and foremost, we will not start a job until ALL parts have been supplied and with proper quantity.  WE NEED MORE THAN EXACT QUANTITY!  This cannot be stated enough, but we also understand that costs need to be controlled.  We can help guide you on what you can supply close to or at exact quantity, mostly expensive IC's or other costly parts.  However, with cheap passives, the more the better.  Again, we can help give you some guidelines prior to starting a job.

Second, we require all SMT parts to be supplied in either reel, tube, or tray.  Be advised, long strips of cut tape (more than 50 quantity) will incur additional processing fees from Devil Circuit Boards as we will need to custom reel them prior to machine runs.  Long strips of hundreds of components not properly reeled will literally be hanging out of our part feeders and laying on the floor!  That is not a good assembly practice and something we cannot allow in our assembly shop.  

We can provide part acquirement services to customers.  This where we purchase all the parts for your job (certain restrictions apply).  Many times, we actually prefer this method as it makes sure we get what we know we need to get the PCB assembly done properly and on time.  

We will employ strategies that will balance what we need for a machine run of parts with what is cost effective by utilizing custom reel part purchases from Mouser and Digikey, our two main suppliers.  This does two things for us.  First, it gets all the parts on reels.  This makes it fast and easy to load into our machines for your circuit board assembly.  Second, while it does add some costs for a custom reel, it's MUCH cheaper than buying full reels of expensive components.  Bottom line is our customers sit back and relax while we do all the work!

At Devil Circuit Boards, we are able to accommodate almost any type of circuit board and, in some cases, PCB boards that other shops cannot support.  There is not much we cannot handle and we look forward to helping you on your next job!

Our machines have a hard minimum limit of 4.5"W x 4.5"L  overall dimension and .031" thickness for unassisted processing.  On the larger side of assembly, we can handle a total overall board size of 20"W x 25"L total outside dimension and up to .250" thick.  Our machines are specially modified to handle these larger boards and also give us the ability to move heavy boards that some other shops are unable to process.

If your supplied PCB board overall dimension is smaller than our specified minimum above then it will need to be fabricated in an array so that the overall array dimension is 4.5" square minimum.  Devil Circuit Boards will work with your PCB fabricator of choice to make sure the array meets our requirements.  All PCB fabricators understand this and it should not add any costs on your side of the fabrication price.  

However, if your PCB board or array is already manufactured and it's smaller than 4.5" square and/or is  .031" or thinner it will need special processing to run PCB assembly at Devil Circuit Boards.  

Special processing means that we will need to charge an NRE fee to fabricate a custom PCB carrier pallet for your circuit board assembly to process through our machine.  This helps it meet our minimum requirements for both overall size and thickness.  In most circumstances, this means the overall outer dimensions of the PCB board or the array is smaller than 4.5" square or is thinner than .031".  

It can also mean that the board is odd formed (not rectangular) or round.  If a board is round or too odd formed there is no way for us to move it through the machine and maintain a strict board orientation during machine assembly processing.  This means that the machine will be unable to accurately find fiducials and won't be able to place parts properly.  A custom pallet allows the board to sit in a perfectly pre-determined position so that after it indexes into the machine it is sitting in the exact same position as we have called out in the machine programming.  

Processing PCB boards for assembly that are .031" thick and thinner can sometimes be a challenge.  The main issue here is overall PCB board rigidity.  As our machine places components on to the PCB, it requires a certain amount of force to make sure the part is seated properly into the solder paste.  This is not a large amount of force, only a few grams for most applications.  However, if the PCB does not have enough support underneath the assembly, even with this light force, you can get a trampoline effect.  This where the PCB bends and springs back just like a trampoline.  This can cause parts already placed to literally fling off the PCB board.  Most automated equipment has tooling to nullify this effect, including ours, but we find a customized pallet is a more robust solution and leads to faster and more efficient circuit board assemblies in a majority of our process runs.