Thermal Processing. Moving from a bad residue to a good residue.

In the world of electronics, reliability and cleanliness go hand-in-hand.

They ensure long-term success. They set the foundation for circuit boards to do exactly as they’re supposed to - make electrical and mechanical connections that last.

But many people forget cleanliness isn’t always a simple, straightforward process. Cleanliness is an activity of managing variables and methodically reducing risk. And while maintaining a good board is clearly the goal—it’s a moving target.

One way problems arise on boards is through the growth of dendrites. Once the dendrites have grown and the residue has turned from good to bad—reliability is at stake. How do we right this wrong? How do we mitigate this risk and move toward consistent reliability?

That’s where thermal processing comes in. But this isn’t always straightforward either. Thermal processing is a balancing act. It’s a tightrope walker toeing a thin line to ensure success.

A few key parts of this balancing act:

• Soldering

• Flux

• Heat

• Time

Soldering needs very specific heat and time amounts. Too little or too much heat, soldering fails. Too little or too much time, same failed results.

The range of heat in the soldering process is the first of the tricky parts. The flux applied before soldering becomes benign with levels above 150 degrees celsius. Alloys melt at 224 degrees celsius though keeping the temperature balanced between these 150-224 is key.

Another important step in the process—time. Soldering only takes 2-3 seconds. But thermal management of the flux takes 3-5 minutes. The problem at this stage is patience. Many focus on solderability and do that part correctly, but immediately start the cool-down process. This doesn’t give the flux enough time to finish heat activating for the required 3-5 minutes. Then you’re back to square one: dendrite growth, a lack of cleanliness, and a bad residue.

Every board configuration is different. Thus, going from a bad residue to a good residue requires deep analysis and understanding of interactions in the process. As you start with a bare board, you add the surface mounts. Then as you continue to add layers to both sides, the thermal challenges compound. As you add solder you could compound flux problems or even trap flux beneath the component.

The variables in this process cloud the path to reliability. You have to look beyond the surface and know what you are looking for. The root cause rarely presents itself in an obvious fashion.

Foresite can help guide your path from a bad residue to a good residue and set those standards.