Getting boards assembled

Several people I know have been asking me for advice when it comes to getting boards assembled lately, so I wanted to write-up my experiences and some tips.

During the design process

At the design process, I get that you just want to churn out your boards, but now is the time to decide on your assembler. Several of the Asian PCB manufacturers have a turn-key solution where you can send them your board files and a BOM, pay them and you get back the boards ready assembled. For simpler things that may be a reasonable approach, but be aware that you have no control on the supply chain this way. Those 1% resistors making up a reference voltage may now be 5% or worse. That microcontroller or low noise Op-Amp may now be a fake.  I like to use a local assembler within driving distance, and to be there when they do a run. That way, I can catch problems during assembly without doing a whole run, if we need to rework a panel.  Often you can pick up great hints from the production personnel when assembling your boards.

Questions to ask your assembler when doing the first contact:

  • What is the preferred size of panels? Larger than this and they won’t fit the machine, smaller and you may get a fee for them having to change the size. Most machines don’t go below 51mm. I have done panels as small as 100x100mm, and as large as 450x600mm.  This is a excellent article about panelization.
  • What kind of frame do they prefer? some have minimum sizes, or want holes in specific locations. Frames are a good place to place fiducials.
  • What size fiducials do they prefer? most places uses a 1mm fiducial.
  • What is the general setup cost? What file format do they want? this can be a major part of the total assembly cost.
  • What is the setup cost pr. feeder?  Some times Tubes has one setup cost that covers several tubes, may be worth it if you can use as many of them as possible.
  • What is the smallest component they can handle?
  • Can they handle fine pitch BGA and QFN? Do they have inspection tools for it?
  • Can you visit when your boards are being run?
  • Do they have a rate for hole mounted components, test and assembly into enclosures?
  • What kind of stencil size and frame type do they use?
  • Do they usually run leaded or lead-free? There is often a fee if they need to clean down for lead-free.

When choosing an assembler, make sure you know how many boards you need to assemble (and sell) in order for it to break even. I know someone doing runs of 100 boards at a time, when the break even point was 122 boards. No wonder they didn’t make money on that run.

Observe that in high cost countries, finding a assembler willing to do a couple hundred boards can be difficult. If they complain about stopping their high volume work to do your few boards, find a different assembler. Experience says that the prices will be high, and the bickering constant.

Also make sure you have a plan for testing and assembling the boards into their final enclosure. Testing can be quite complicated! Time spent during the design process avoids time being spent reworking or scrapping boards.

Before you order your boards and parts

  • Make sure to build the panel of PCBs in your PCB CAD! There are several offerings that board manufacturers can assemble them for you for a fee, but you may end up without fiducials or with a V score separating the boards on the only visible side of the PCB.
  • The panels should have 2 sides square and parallel strips at least 12mm wide where they are fed into the board conveyor. This is often called technical rails.
  • Make sure that the panel, when going to the pick and place process is stiff enough that it won’t flex and make problems assembling. If in doubt, bond the boards together with some mousebites or V-Grooves.
  • Boards with uneven ground pour may scorch in the reflow oven.
  • Complex parts may need a local fiducial to improve the accuracy of the placement. This is something your assembler should be able to help you with.
  • Order your components as reels! If you come with un-reeled strips of tape, you will look like you do not know what you are doing. Most suppliers offers to Re-Reel your tape with the proper leaders and such for a minor fee. This is quite useful for your more expensive parts.  Add 5%-10% more components to the reel than what your board run needs. This is to account for rejected parts in the pick and place.
  • Re-Reels have a tendency to be more hassle than original tapes, due to glue on the leaders and such. If you want to annoy your assembler, supply only Re-Reeled components.
  • For cheaper passives, get a whole reel, and keep the leftovers untill the next board run. It may seem obvious, but I have seen people paying more to Re-Reel 5% resistors than a whole reel would have cost.
  • 7″ reels is often prefered, but there should be no difference if you come with a 13″ reel.
  • Order a couple extra panels. These are used for setup, and for you to touch and look at. I avoid assembling PCBs that have lots of fat from fingers.  If there is a problem with the PCB after assembling, its nice to have a panel that is not assembled to find the fault.  A good PCB manufacturer should deliver boards that are 100% tested.
  • Now is also the time to order the stencil.

At this point, you should try hard to reduce the number of lines in your BOM. Can that 20K resistor you need 1 off, be made up of 2 10K in series? Are you sure you need both 22µF and 10µF capacitors? or could just 22µF do? If you pay per feeder (you probably do), reducing the number of feeders needed helps to reduce the setup cost and lower your break even point.

IMG_1932

Local Fiducial in center of W5500 IC to improve placement location accuracy

 

Getting boards assembled

All your parts have arrived, and you are ready to get the board assembled. At your assembler, you are not the only customer so make sure to not film nor photograph anything other than your board. Ask first.

 

 

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About Thomas S. Knutsen

Electronics engineer with interests in RF and analog electronics.
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