Intrusive Soldering, also known as "pin in paste," is a lesser-known technique that uses a reflow oven to solder through-hole components and can offers advantages over traditional soldering approaches.

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Benefits of Intrusive Soldering

The primary appeal of intrusive soldering is that it integrates with the surface mount process. This means that parts with through-hole leads are placed on the board during the surface mount phase, thus eliminating the need for a separate selective soldering process.

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Additionally, because it is going through the reflow oven, the entire board is heated above the solder's liquidus point, resulting in solder joints with excellent inter-metallic connections. Such soldering yields arguably superior, more reliable, and long-lasting solder joints compared to other methods.

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Applications for Intrusive Soldering

Intrusive soldering becomes particularly relevant when dealing with hybrid components, meaning components that are a combination of both surface mount and through hole. Given their dual nature, ensuring proper solder fill in through-hole leads while also catering to the surface mount leads can be challenging. Techniques like overprinting or using preforms can aid in achieving the desired solder fill.

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Various USB types fall under the category of hybrid components. While mini USB ports were once prevalent, their use has declined over time. USB-A ports, often used for computer connections, can either be through-hole or hybrid. USB-B ports, the chunkier variants commonly found in printers, are mostly through-hole. The most common hybrid components in contemporary electronics are USB-C ports.

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How Intrusive Soldering is Achieved

Overprinting - The most common method used to make intrusive soldering successful is overprinting. What this means that the stencil aperture is up to 8 millimeters beyond the through hole. This is done so that when the board goes through reflow, the solder paste is sucked into the barrel, which helps to achieve a 100% volume fill.

Double printing - In cases where overprinting isn’t possible double printing is an alternative option. This means that when going through the stencil printing process the blades are drawn over the board twice, helping to completely pack the hole with solder.

Solder preforms - Solder preforms are precisely shaped pieces of solder that are designed to fit around the through-hole component leads. In cases where you can overprint just a little bit, these preforms can be placed near the holes so that when the board goes through the reflow oven it provides the additional metal needed to fill the hole barrel.

Solder dispensing - Solder dispensing can be done either manually by someone pulling the board off of the conveyer and adding the additional needed solder with a syringe or with dedicated automated dispensing machines.

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Design Considerations

Leave space between components -   Ensure that the PCB isn't overcrowded, especially near through-hole components. Overprinting can cause solder bridging between close pads, so adequate spacing is essential.

Component choice - Choose components that can withstand the thermal stresses of the reflow oven. For example, if you can choose a high temp plastic for your through hole components and we maybe be able to intrusively solder them, providing potential cost-savings and a more robust solder joint.

Board Thickness - Pay attention to the manufacturer's board thickness recommendations, especially like USB-C. Adhering to these guidelines can ensure proper hole fill during the soldering process. For example, not all USB-C ports are designed for 1.6 millimeter boards. If you still design your board to be 1.6mm thickness, although it will be soldered according to the IPC standards for intrusive soldering, you likely will not see 100% hole fill.

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If you’re interested in learning more about how we may be able to use intrusive soldering on your projects in order to achieve super strong, reliable, solder joints, we have a whole podcast episode dedicated to it!

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