In volume production, we try to automate as much as we can, right down to placement and soldering of components. But not all boards will be assembled in an automated manner. In some cases, boards will still be hand assembled, such as when doing a quick prototype run and in-house assembly. SMD rework also demands hand soldering and assembly by an experienced technician. Whenever we have to deal with an assembly process, we probably have to think about DFA, or design for assembly.
Hand soldering and hand rework can both demand some simple DFA considerations that will aid any kind of manual work on the board, which especially makes life easier when building a proof of concept. Once a concept is proven and a first run prototype has been qualified, you can abandon the hand soldering DFA principles, but until then consider some ways to make your testing, debug, and rework much easier.
What's Needed For Hand Soldering?
Hand soldering does not always involve the use of a soldering iron, it can involve the use of solder paste and a heat gun, or even a desktop reflow oven. Regardless of the process, hand soldering could involve manual application of solder paste and placement of parts. The three biggest factors that lead to defects are
Part case and package sizes
Spacing between parts
Pad size margins in PCB footprints
No matter how you plan to work, hand soldering can be challenging once parts get very small. Part density also makes soldering difficult, as placing parts closer together makes parts more difficult to access by hand. Small footprints can make soldering with wire almost impossible, so the density level of footprints should be selected appropriately. These factors make hand assembly and rework much more difficult.
Therefore, in order to overcome some of these challenges when working manually, here are some tips to follow.
Use Larger Case Sizes
If you're doing hand assembly, probably the smallest case most designers will choose is 0603. More experienced designers and assembly technicians can be successful with 0402 case sizes. Once you get down to 0201 and 01005 case sizes, the components are simply too small for most people to use by hand, and certainly they can't be used without a visual aid.
To grab onto smaller case sizes, a good set of tweezers will be mandatory. When you're kitting out your lab, make sure to pick up a precision tweezer set that includes a pair of fine point tweezers. These will be needed to grasp and place the smallest case sizes on your pcb.
Use Lower Density Footprints
The IPC uses a Density Level designation as defined in the IPC-7351 standard, which are numbered density levels A (lowest), B, and C (highest). Lower density footprints have larger copper margin around the pad, which
Larger pads (Density Level A) footprints are typically the default pad sizes used in most component models. But if you find some libraries online or you use a third-party data provider, pay attention to the density level if you plan to hand solder or hand rework. These larger pad sizes force you to space things out, which could allow you to more easily place a solder wire, a flying lead, or a multimeter/probe.
Fine Point Probe Tips
By default, multimeters tend to come packaged with very large probe tips. These probe tips can be difficult to get into tight places so that an accurate electrical measurement can be gathered. Thankfully, there are companies that sell very fine-tipped probes that can be used to get into tight spaces.
These probes usually plug into banana clips, so they can be used with a handheld multimeter or with other sources/slash loads. These are a bit more expensive than replacement probes for a multimeter, but they are worth the additional cost as they enable probing in tight spaces. This is an important part of rework in hand soldering as it will be necessary to check the soldered connection for shorts to nearby components.
Solder Paste Applicator and Solder Wire Size
The size of your solder wire and your solder paste applicator can make life difficult when it comes to hand soldering. If you are using solder paste in a syringe, make sure the syringe includes a fine tip for application on small pads. This reduces the volume of solder paste and decreases the chances of bridging between pads.
Solder wire can come in a range of diameters, including very thick solder wire. For hand soldering with SMD components, thinner solder wire diameter is preferable. It will be much easier to get thin solder wire directly onto small SMD leads and connectors. In addition, if some molten solder from the wire drips onto the board, there will be less cleanup needed with a solder wick. Smaller solder wire should also generally be used with a smaller solder wick that can get into tighter spaces.
Invest in a Microscope
Hand soldering demands a certain type of microscope that allows manual work on the PCB. It is possible to complete an assembly without a soldering microscope, but it's rarely possible to fully inspect a board without one.
Microscopes used for hand soldering tend to have long focal length and lower magnification. They may use ring lighting with a camera on one ocular in order to capture an image on a computer. The benefit of soldering microscopes is that you can get your hands underneath to manually probe, clean, and solder a PCB. Once soldering and clean up are completed, the board should be inspected for any leftover wire slivers, solder blobs, bridging, and excess flux residue.
When you’re ready to design a custom PCB for hand soldering, make sure your company uses the best PCB design features in OrCAD from Cadence. If you’re ready to take even more control over net logic and board layout, you can graduate to Allegro PCB Designer for a more advanced toolset and additional simulation options for systems analysis. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.