6 Traps to Avoid When Changing from Hand Assembly to Machine Assembly

Maker News Technology

In the past, it was usually pretty easy to find chips in both surface mount and thru-hole packages. Somewhere in the last decade or so, component manufacturers stopped introducing thru-hole versions of their newest chips as standard practice. In many cases, new components can only be found in tiny QFN (quad flatpack, no leads), or wafer scale BGA (ball grid array) packages.

The maker community — never shying away from a good hack — found ways to work with many of these parts while still hand building. There are very few components used in the pro-design world that are still unusable by a creative DIY maker.

But, what happens when a maker has a great design and wants to mass produce it?

Sometimes the techniques that make things work when hand soldering will completely break a machine assembly process. To cure that ailment, I’ve compiled five common traps to avoid when moving from hand to robotic assembly.

1. Consider moisture sensitivity

It may not seem logical, but plastic does absorb moisture, and, it doesn’t have to be dropped in the sink for it to happen. Just sitting around exposed to the air, plastic chips will absorb humidity. In a reflow oven, these parts can end up acting a bit like popcorn.

The moisture turns to steam, and if it can’t outgas fast enough, may split your chips open. Often, the damage isn’t visible to the naked eye, but will show up as an unreliable product in the field. When us DIY folks hand build boards, we tend to open the component packages and then just let the parts lie around without giving thought to proper storage.

If you are going to send your project off to be machine assembled, you can do two things with moisture-sensitive parts. First, you can order the parts when needed, not before, and keep the packages sealed. Alternately, you can send in parts that have been exposed to the air; if you inform your assembly house that the parts are moisture sensitive, and ask that they be baked prior to assembly. Pre-baking will remove the moisture safely.

2. Don’t skimp on solder mask

Some board fabrication houses offer reduced prices if you order your boards without soldermask or silkscreen. That’s not a problem when you’re hand building — you can regulate the amount of solder by eyeball.

Solder mask allows for much higher quality solder joints.
Solder mask allows for much higher quality solder joints.

However, when a stencil is used to apply solder paste and the board is run through a reflow oven, the solder will spread back on the exposed copper traces. This may leave your parts without enough solder on the pins to create a reliable connection. Solder mask may add a bit of cost up front, but will increase reliability and reduce cost in the long run. Creative choice of solder mask color can also add some personality to your boards.

3. Silkscreen is important too

Lack of sIlkscreen isn’t a reliability issue, but it can make accuracy of assembly more difficult to achieve. In a perfect word, the CAD files would tell the assembly machines exactly where each part is supposed to go and what angle and orientation it needs. Unfortunately, we don’t live in a perfect world — who knew?

It’s far too common to have footprints with errors in them, or components with ambiguous marking to depend on the CAD files alone. Clear silkscreen will help to ensure that any errors in the data are caught visually. If you don’t want to clutter your PC board with reference designators and polarity markings, you can put the designators and any other important markings in the document layer in your layout software. Then, tell your assembly house to look on that layer for the information.

4. No need to fear surface mount

One of the easiest ways to ensure that a board can be hand-built is to stick with thru-hole parts. But doing so puts many limits on a design, and rules out a lot of new technologies. Little breakout boards — a small surface mount chip pre-mounted on a PC board, with hand solderable headers — are available for a lot of new parts, but not all. That’s helpful, but they take up a lot of extra board real estate and cost more than the part alone.

 Machine assembly allowed the smaller board on the right to do the job of both boards on the left.
Machine assembly allowed the smaller board on the right to do the job of both boards on the left.

If you’re hand building a prototype or a small number of boards for your own use, go ahead and use a breakout board. But, when it’s time to get a thousand built up to sell, relayout your PC board to use the chip without the breakout board. Just don’t forget the bypass capacitors or any other required support components. As a bonus, many breakout boards are open source, so you may be able to use a proven schematic and layout for that part of your design.

5. No open vias in pads

QFNs and BGAs have pins/pads under the part, often completely inaccessible. That’s fine for a reflow oven, but what if you’re soldering it by hand? A common hand-soldering practice is to put large vias in the pad; fix the part onto the board with tape; then turn the board over and stick solder and a small tipped soldering iron through the via. By doing this, you can hand solder almost any leadless surface mount part.

You can probably guess that I’m going to tell you open vias in pads will not work with automated assembly. The solder will flow down the via and end up on the back side of the board. You may end up with shorts on the back side, and parts that fall off of the front side or just don’t connect with all of their pads. If you use the open via hand solder technique, you’ll need to relayout your PC board without any open vias in the pads before sending it our for manufacture.

6. Go for it

It wasn’t that many years ago when the tools and services necessary to get an electronic product manufactured were so complex and expensive that it pretty much made it impossible for DIY’ers to turn a hobby project into a small business. But times have changed, and with those changes, the hardware startup is back — and within just about anyone’s reach.


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Duane Benson

Chief Technology Champion at Screaming Circuits, a

prototype PCB assembly electronic manufacturing company in Canby, Oregon.

View more articles by Duane Benson