The Spark Gap Podcast - Episode 26

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Show Synopsis


Karl and Corey are joined by Bob Coggeshall of Small Batch Assembly to talk about designing PCBs that will go through SMT at a professional assembly house.

Bob can be found all over the internet. Here are just a few of the places you will find him:

Picking and Organizing Your Parts

Managing your Bill of Materials (BOMs)

There are a ton of places to actually search for parts. Most of our listeners are probably fully aware of most of the big name distributors. I’ll list them just to make sure:

Ordering parts on Cut-Tape doesn’t work so well for Pick and Place machines. Therefore, many of the suppliers offer a reeling service where they add a leader to a strip of cut tape parts. Digikey calls this Digi-Reels. Mouser calls it MouseReels.

DigiReel

MouseReel

Bob also offers this service. He gets the leaders and tools from the same place almost everybody else does. From a company called TapeSplice.

Common Parts Library

Handling multiple parts vendors of the gerneic passives adds complexity to your BOM. The creation of the Common Parts Library was meant to simplify things.

I started looking into about the same time Bob did. Originally, it was SeeedStudio who was trying to create a catalog of common jelly bena components. They called it the Open Parts Library, which I guess is now a subset of the Common Parts Library.

BOM Managers

There are a ton of ways to manage a BOM, but I mostly see excel files spread all over the place. When I used Eagle more, I used user made plugins to help with my BOM management. Eagle does have some built in BOM tools but they are unsurprisingly linked to buying from www.newark.com.

Specifically, I used BOM-EX.

Sometimes though, you don’t actually want a part populated on your board. Bob answers how to specify which parts to not place in his FAQ. Look for the heading “I have an entry in the BOM I don’t want populated”

The Factory Floor

Also, I wanted to mention this great series of articles from Bunnie Huang. You really should read all 4 parts.

Placing Your Parts on the PCB Correctly

Centroid Files

Pick and place machines need to know where to put your part on the board.
They do this by reading a centroid file.
Screaming Circuits has a nice overview of what that is:

Tombstoning Capacitors

How you connect up your placed components also matters.
If you do it wrong you could have problems during reflow.
A classic example of connections to parts causing issues is the case of the tombstoning capacitors.
Don’t let your PCB turn into a component graveyard!

All about Solder Paste

Even applyng solder paste to your board before reflow requires a fair amount of consideration.

The popularity of QFN parts and other packages with pins under the package have increade the need to be careful about applying too little or too much solder paste to your board.

Here are some documents talking about the necessary manufactorability considerations when using QFN parts:

A lot of times vias are used to help transfer the heat generated by an IC to a large copper plane. This can be very helpful but you need to make sure you are not introducing other problems. Take a look at these before you ship out your next board:

Solder Paste Stencils

Your solder paste doesn’t get on the PCB by magic. It usulaly is squeezed through a stencil.
For small batchs and large pad spacing, Kapton stencils can be used.
If your pad spacing is smaller than 1mm or you need to run a lot of boards, Stainless Steel is your best bet.

Ohararp is the Stencil supplier Bob uses. If he likes them than they are good enough for me.

Just a couple of articles discussing the differences between stencils and how to use them:

Panelization and Fiducials

The AmpHour spoke with OSHPark awhile back. On their page is a great picture of a very complex panelization job:

Here is a great article. It should tell you everything you wanted to know about panelization.

This article shows a good illustration of a panelized board. Including the fiducial markings and an explanation of the V-score vs. routing.

Adafruit’s article regarding Fiducials is pretty good too.

Here is another that shows fiducials on the panel frames as well as the global fiducials:

Dave Jones also did some videos about panelization, but I’m tired of linking them in. Ha! Feel free to search eevblog. Always good stuff.

Pick and place machines

Bob wrote a nice article about “How to pick a pick and place”

Here is a more recent listing of commercially available machines:

There are some Hobbyist Level Pick and Place Machines coming out of China now. Dangerous Prototypes was the first place I learned about TM220A hobbyist level pick and place machine. Kinda neat but not appropriate for a commercial assembler.

Another Great Blog for DFM Tips

Just wanted to mention this blog. Some really impressive work went into these articles.

Nixie Tubes

Here is Bob’s “how to buy nixie tubes” article.
My advice, don’t buy any. More for me!