Hand soldering may not have the same throughput as automated assembly processes like reflow soldering, but it is still used in many situations. And if your product will go through a round of hand PCB assembly, it will need to be inspected for certain defects that are not as common in automated assembly. To help ensure your product quality and yield remain high, keep yourself aware of these common defects that arise in hand PCB soldering.
Common Hand PCB Soldering Defects
Hand soldering can proceed with solder paste or solder wire, as well as the use of flux to aid wetting to metal surfaces. When soldering with these materials, there are some common defects that can result from hand soldering, some of which may not go noticed until there is a problem with PCB.
Leftover Flux Residue
This is probably the most common hand soldering defect. The use of flux aids wetting of solder to metal surfaces, and the flux removes oxides from metal surfaces. After solder is applied and a weld is formed during soldering, there can be a lot of leftover flux on the surface of the board. This leftover flux residue can be seen with the naked eye as a shiny substance covering the soldered area.
Leftover solder flux residue.
Manual application of flux, as well as solder paste, can leave leftover flux residues. This leftover residue requires cleaning to properly remove from the board, despite claims that it does not cause reliability problems. What typically happens is:
- The leftover flux is insufficiently cleaned, leaving behind a film;
- The leftover flux goes unnoticed and remains on the completed assembly
In both cases, because the remaining flux needs to be fully removed, hand soldering requires additional cleaning than you would normally apply to a reflow soldered PCB.
Leftover Solder Balls
Solder balls result from incomplete melting of solder paste, leaving behind small deposits of metal particles. In some cases, these solder balls are harmless, but when components get dense they create the potential for short circuits. Sometimes, solder balls simply become trapped in flux and are later removed through cleaning. In other cases, they need to be removed by melting solder balls into a solder wick.
Solder ball on the leads of a DPAK package.
Copper Wire Whiskers
When a component needs to be reworked or have copper leads attached, hand soldering will involve bonding stranded copper to a hole or receptacle on the PCB. Leftover copper whiskers, even if they are tinned copper whiskers, are notorious for creating short circuits. They are also quite difficult to identify by eye. To help prevent this, wires can be manually pre-tinned before being soldered onto the board.
You can usually tell when a component has been hand soldered with a large wire. These component leads tend to have very large welds on them, partially due to the physical size of the wire. These large welds on leads could also be harmless depending on where they are located in the PCB. If you need to prevent large welds, a smaller iron and wire may need to be used. For very small components, solder paste and a hot air gun are preferred over large wire and a soldering iron.
Thinner solder wire will help prevent fat welds.
Another problem that can result from hand soldering with a large iron at high temperature is a lifted pad. Small SMD pads can be used for soldering with an iron, even when solder paste is used. The problem comes when the iron temperature is too hot: when small copper pads reach high temperature, the copper film can delaminate from the PCB. The result is that the pad lifts up and becomes detached from the pcb. This leaves a very weak connection or no connection. This is solved by operating the iron in the appropriate temperature range.
Excessive Solder on Opposite Surface Layer
Sometimes, too much solder might be applied during hand assembly, particularly on through-holes or pads near untented vias. When this occurs, some of that solder could flow through an open hole to the back side of the PCB, where it then solidifies. Depending on where this occurs, it could be harmless and it would not need to be removed. If it does need to be removed, you will need to use a soldering wick and an iron to pull up the excess flux from the back side of the board.
To make sure you implement the best layout choices to ensure successful hand assembly, read these guidelines on DFA practices for manual soldering.
When you’re ready to implement DFA practices that help prevent hand soldering defects, use 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.