What is the Advantage of Intrusive Reflow Soldering over Wave Soldering, and How Does it Affect My Design?

If you have not heard of intrusive reflow soldering, let us briefly introduce it here and outline some of its advantages.

If any engineers out there have used this technology in their designs, please let us know what you think. Was this technology beneficial in your project? Let us know how it affected your designs and any benefits or snags that you found.

Intrusive reflow soldering is also known as "pin in paste". This is a soldering technology that enables the assembly of through-hole components without the need for hand soldering or wave soldering. If you are designing a product due for automated assembly in large batches, then it is worth considering this technology.

A key feature of "pin in paste" is that through-hole pins are soldered during the same production stage as the surface-mount components. This can be a significant saver for both production time and cost compared to solder-wave production or hand soldering. 

Surface paste is deposited on and around the pin holes of the PCB. A large "overprint" of solder paste can be used, which will attract into the hole during the temperature cycle, which melts or “reflows” the paste. Oblong shapes or other unique paste patterns can be used to avoid contact with adjacent pins while ensuring that a sufficient volume of paste is provided to fill these holes.

In the modern PCB world, the majority of components used are surface mounted. Components such as BGAs, QFNs, and very small-scale packages can only be soldered using the surface-paste technology. But often, we must consider just a minority of through-hole pins that need to be assembled after the reflow soldering is complete. These are typically connectors or other power components. This requires an entirely extra production stage, which is both costly and time-consuming. The "pin in paste" technology replaces the need for this additional stage.

But are the advantages of intrusive soldering reserved for production only? The answer is that there are benefits for electronics performance also due to intrusive reflow.

The close placement of decoupling capacitors or circuit protections nearer to connector pins is one such advantage. Wave-soldered connector pins require large component placement keepouts on the underside of the board to allow the solder wave to pass over all the connector pins. With intrusive reflow, this bottom-side distance is no longer needed. This enables designers to place these components closer.  

The density of the PCB layout improves in the region surrounding these pins. This leads to shorter lines and current loop lengths. The location of connectors, which is sometimes dictated by the product requirements, becomes less critical due to lower keepout spacing and wave-access concerns.   

Thermal reliefs are used in wave-soldered designs as a tool to prevent heat escaping too quickly from the pin hole during soldering. This ensures that the hole itself remains sufficiently hot for long enough to get a good solder joint at the pin. However, the reflow process for surface-mount soldering provides external heat using reflow ovens. This heat needs to progress quickly "into" the hole from the outside atmosphere. Therefore, thermal reliefs can be replaced with full contact to copper planes on the outer layers. This brings the heat “to” the pin during production. This technique has the additional benefit of providing a better electrical connection.

Caution needs to be taken when selecting components that are sufficiently rated to accept the higher temperatures of the reflow oven heat cycle.

If you have any comments, questions, or insights related to the above discussion, please share below. We will be happy to answer or open the topic for a group discussion. All comments are welcome!