Ask a new designer what process would be used in through-hole assembly, and they will probably answer “wave soldering.” That was mostly true in the past when boards contained exclusively through-hole components. It is still used today, including in boards that contain some surface mount parts, but there is another option with through-holes: pin-in-paste soldering.
This method uses reflow on through-hole pins, but requires depositing solder onto pads and allowing the molten paste to flow into a via barrel during assembly. The process is an acceptable replacement for wave soldering in boards that mostly have SMD parts and only a few through-holes requiring assembly. But in other cases, the tried and true wave soldering approach will still work, even with SMD parts present on the PCB.
How Does Pin-in-Paste Work?
The use of pin-in-paste as a soldering process involves placement of through-holes with solder paste, where pins are placed directly into the solder paste and reflowed. In other words, it allows reflow soldering of through-hole parts by applying solder paste to open through-holes in the bare PCB. This process is simple enough that an assembly house can eliminate a wave soldering step in boards that have SMD parts and through-hole parts.
The process is quite simple:
Solder paste is dispensed on top of the open through-holes
Reflow soldering begins and molten solder bonds to the hole wall
The hardened solder leaves behind some flux residues to be cleaned
The cleaned board is passed to the next step in assembly
Pin-in-paste requires construction of a mask in the PCB footprint so that the paste can be applied around the through-hole. The size of the mask varies and could be applied by the fabrication/assembly team. As long as the mask opening is the right size, a reasonably large solder joint can form on through-hole leads without creating a molten solder pool on the opposite board side.
Is Wave Soldering Bad For Mixed Technology PCBs?
Although pin-in-paste soldering is an option for mixed technology PCBs, that does not mean wave soldering is not useful, including when producing at volume. For example, when a board has a certain configuration or placement of the SMD parts, wave soldering can still be used to assemble the through-hole parts. This involves some additional tooling, which is why it is most cost effective at higher production volumes.
Here are some use cases:
1. SMD only on one side
Wave soldering can be performed after reflow if all SMD parts are on one side of the board. After reflow, the board is placed into a preform with only the through-hole component pins exposed on the back side of the board. The board then passes through the wave solder bath and the solder fills in the through-holes. If you had SMD parts on both sides of the board, then the alternative might be selective soldering or hand soldering in order to avoid repeated reflow passes on the SMD parts.
For example, this board uses SMD parts only on one side, but it also has several through-hole parts. The through-hole parts could be soldered from the back by hand or with selective soldering. Another option is to place the design in a form that covers the SMD parts, and then use wave soldering. This would solder all the through-holes from the back side.
This board has many through-holes that can be soldered with wave soldering using a small form to cover the SMD parts.
2. All through-hole assembly
If everything in the PCB will be through-hole assembly, then it might make the most sense to just run it through wave soldering. This process gives high throughput for through-hole assembly without requiring deposition of solder paste and later cleanup. Entirely through-hole boards are less common today, but assemblers that offer wave soldering can offer an inexpensive option with wave soldering that will be much more competitive than pin-in-paste for all the components.
If you are unsure as to whether your PCB can be assembled with a pin-in-paste process, talk to your assembler. They can look at the mask data and placements, and they can explain whether the technique can be used or whether a form with wave soldering will be needed to complete assembly.
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