Close

Combining high-temp and low-temp solder on the same board

A project log for Assembling dense SMD PCBs - lessons learned

I've assembled many dense(-ish) PCBs with small components (down to 0201) during the last three years. A summary.

christophChristoph 10/16/2022 at 21:080 Comments

Here's a mixed alloy board with a switch in the top right corner, a little BGA, and a u.Fl connector (not visible, but it's there! I promise). Those have high temp solder, the rest is low temp and not even completely done. I'll save that for later so that I can take some appropriate pictures.

Remember that when building a prototype, we don't have to assemble everything in one go. We are allowed to

These two points are related via the use of low temp solder. It's awesome in the right place. When I want an easy and relaxing assembly experience, i try to use low temp solder in as many places as possible. That's a hard to write list, so let's flip it around:

Use high temp solder where you have to, and low temp everywhere else.

Where to use high temp solder:

I usually try to limit the number of high temp reflow cycles for two reasons:

So the first two reflow cycles are for a limited set of parts as outlined above; one for each side of the board. A stencil doesn't really help, because only a limited set of pads will get high temp paste. Of course you can design a stencil just for high temp parts though, but it's probably not worth the effort.

BGA tips

Example for a weighed down BGA (STM32WLE5):

This was one of my earlier boards and I forgot to add some temperature indicator. Like a blob of solder paste (no available space for that) or one or two passives. This is better (a voltage regulator with caps, to the bottom left in the picture at the top of this log):

Here I had applied high temp paste for two capacitors (0201 and 0402) right next to the 4-BGA to tell me when the paste reflowed. If the caps are done, the BGA is done, too. I didn't have an extra weight for this one, but it worked nonetheless (again on a hot plate). You can also see that the BGA's silkscreen outline is completely useless.

Spoiler: You can also add extra weight to LGAs:

But more on that later when we get to the low-temp tips. LGAs don't come with the right amount of solder in the right place, so we have to do that ourselves - but at least we get to choose.

Discussions