Design for Manual Prototyping and Throw Off Your Restraints
When I started driving, there weren’t many automatic transmission cars on the road. After driving for a decade, I got my first taste of an automatic transmission car. I adapted pretty quickly. However, if you’ve driven automatic transmission cars throughout your life, switching to manual can be a nightmare.
As hardware designers, you’ll experience the same disjunction when transitioning from luxuriously developing simple designs via machine prototyping to manual placement and soldering. Choosing components and customizing the footprint will be one of many challenges. Like driving a manual transmission car, manual prototyping requires far more considerations than machine prototyping generally does.
Why Do You Need Manual Prototyping?
If you’ve been designing on a relatively unlimited budget, you might find it nearly impossible to comprehend the need for manual prototyping. Why risk potential human errors when you can trust on intelligent pick and place machines and reflow technology? However, many small and medium-sized electronics companies work on a tight budget and can’t afford the benefits of machine prototyping.
Machine assembly requires a setup fee with a cost-per-unit that is only viable if the production volume is significant. Most companies would rather manually assemble five prototypes before proceeding to produce in bulk. Mistakes in the design are identified when the number of prototypes is at a minimum.
Useful Tips When Designing for Manual Prototyping
If you’ve been relying on machines to assemble a handful of prototypes, these tips might be new to you. It’s a lot like ditching your automatic transmission car for a manual one. And if a technician is soldering for you, these tips could help you make the technician’s job easier.
1. Use the right Surface Mount Device (SMD) component size.
SMD components are great for saving space and machine assembly. Naturally, the smaller the form factor of the SMD components, the smaller your PCB. Machines have minimal issues while handling the smallest ; your hand-eye coordination, however, will be tested to the limit when dealing with the smallest form factor for SMD components.
For example, SMD chip resistors are available in multiple sizes, with the smallest being 0201 and 2512 being the largest. The smallest size measures at 0.02 x 0.01 inches and is almost impossible to solder manually without causing solder bridge between the pads. For manual assembly, 0603 is the smallest that a skilled technician could comfortably solder.
Don’t go too small in size.
2. Make Quad Flat Package (QFP) component pads longer.
High-end microcontrollers are usually available in QFP package. The same applies to special function integrated circuits like an Ethernet chip. The standard recommended footprints in the datasheets work well when you’re using pick and place machines. But when you’re soldering with bare hands, the short length of the pad can make it hard to solder cleanly.
Some of the solder lead tends to attach itself between component pins and can be hard to draw out with the soldering iron. A useful method that I’ve picked up from my electronics mentor is to make the QFP pad longer. With a longer pad, you’ll be able to use a soldering iron to easily drag any solder lead to the end of the pad.
3. Avoid Ball Grid Array (BGA) packages, if possible.
Soldering tiny pitch components like QFP or Thin Soft Outline Package (TSOP) is difficult but possible. All you need is great eyesight and a steady hand. With enough practice, you’ll have impressive results. This does not apply to BGA package, however, where all the pads are located beneath the package itself.
You may eventually be able to master manually soldering BGA but it may not be a risk worth taking unless the only package available for the IC is in BGA.
BGA package - not the friendliest for manual prototyping.
4. Use Thermal Relief
Sometimes, it’s not the soldering that’s the problem. Mistakes happen when you’re prototyping manually and you may need to desolder a few components. If you’re using through-hole components where some of the pads are connected to a plane, be sure to apply thermal reliefs on the pads.
In case you’re unsure what a thermal relief is, it’s a method of connecting a pad to the copper plane using two or four narrow tracks. It prevents solder lead from cooling down too quickly when you’re trying to remove the components. If you’re using Altium Designer®, thermal reliefs are automatically applied after you set them up in the design rules.
Need more design tips for manual prototyping? Talk to an expert at Altium.