Screaming Circuits - 5 things/hand to robotic assembly

 

 

A lot of factors go into the decision to hand build or
outsource circuit boards. I hand build my own sometimes, simply because I enjoy
the challenge. Of course most of the projects I design are for my own use, so
timeliness isn’t that important.   

When I do design something that will go out to a
customer, like my electronic business card holder, I will send the board
through our shop. In those cases, quality is important, as is delivery, and the
quantity is often too high to hand build. Machine building also allows me to
use smaller and more complex parts.  

That same decision — hand build or outsource — takes
place in the heads of designers all over the country. When the decision is to
outsource, there are a few important things to consider. Some things that work
fine when hand soldering may stand in the way of quality, repeatability, and
reliability when machine assembling.  

Here are five of the most important considerations when
changing from hand built to outsourced.   

1. Use Solder Mask & Silkscreen 

A good solder joint needs the right amount of solder in
the right place. Solder will tend to flow down bare copper, bleeding outside of
the area it belongs, and down exposed copper traces and vias.  

The main purpose of solder mask is to keep the solder where
it belongs. It also protects the traces, but that’s a longevity issue. Solder
bleeding is a manufacturing and reliability issue. This isn’t a problem when
hand soldering. In fact, it can even work to your advantage when hand soldering
really small parts. It gives you more room for your soldering iron to hit
metal. 

Not so with solder paste and machine assembly. Use solder
mask.   

2. Avoid the Pseudo Panel  

Keeping small boards in a panel is the recommended best
practice in the manufacturing industry. We appreciate it and, while not always
necessary, can reduce your costs. We sometimes see what we call a “pseudo
panel.” This is a board where multiples of the board are put in the same
PCB, like a panel, but unlike a panel, the boards don’t have routing or V-score
between them. Sometimes the designer will put a bunch of vias to outline the
board, or just ask that we use a bandsaw to separate them.  

That’s a time consuming, expensive, and potentially
damaging process. The vibration of the saw can crack solder joints, and, you’re
unlikely to get boards that are all the same size. Have small boards panelized
by your board house.   

3. Family Panel (Pseudo or Not) 

Similar to the pseudo panel is the family panel. A family
panel is a case where a project is made up of several different PC boards, and
they are all laid out together, as though they are one design. If the board
isn’t routed between to designs, you’ll have the pseudo panel problem described
above.   

The bigger problem, though, comes with reference
designators. We typically see family panels with duplicate reference
designators. Each design, for example, will have its own C1, R1, Q1, etc. We
use the reference designators as position identifiers/ If you have three
different parts labeled R5, our machine programmers will have a problem with
it. It’s even worse if the values differ; on one design, C1 is a 0.1uf
capacitor, while on another design, it’s a 22pf cap.  

If you’re making a family panel, give each and every
placement a different reference designator. One way would be to us extra
digits. For example on one design on the family panel could have C100, C101,
C102… The next would be C200, C201, C203, and so on.  

And, don’t forget the routing or V-score between the
designs.   

4. QFN — Hole in the Middle 

A common technique in the hand soldering world, for
soldering QFNs and other parts with thermal pads underneath is to put a big via
in the middle of the center pad. By doing so, you can stick a soldering iron
and some solder down through the hole and get a good solder connection on the
bottom pad.   

This doesn’t work with machine assembly. the solder paste
will flow down and out the hole in the reflow oven. You’ll end up with a poor
connection (or no connection) to the thermal pad, and solder slop on the back
side of the board.   

5. Parts and the Bill of Materials (BOM) When I build my
hobby projects, I often get a bit carefree with the bill of materials. It’s not
good practice, but I do. I’ll put a part in the BOM that I used before, and not
check to see if it’s still in stock. I’ll put parts in the BOM with just the
values and not any part numbers. Things of that sort require tribal knowledge,
which only the designer has.   

When building, sometimes I’ll just grab a part that’s
close. If I need an 0805 1uf, 10 volt capacitor, I can grab a 16 volt, 25 volt,
etc. I can even make an 0603 part work. You as the designer may know that
something close will work, but an outside house can’t know. You need to tell
them exactly what the part is.   

Before sending anything through our shop, I do clean up
the BOM. In order for us, or any manufacturer, to build the boards, the BOM
needs: 

A unique reference designator for each part placement 
• The
quantity of each part used on the board
• The manufacturer
• The manufacturer’s
part number
• Digikey part numbers can be used as well   

The transition from hand building to outsourced machine
building can be an intimidating one. But, with a few considerations, it can be
an easy and rewarding transition.  

Company: Screaming Circuits