Using Hotplates for SMD Soldering

Using Hotplates for SMD Soldering#

Today I was testing out a couple of cheap hotplates I picked up from Amazon.

One is a 20 cm × 20 cm large hotplate, and the other is a much smaller 5.6 cm × 5.6 cm mini hotplate. Both turned out to be surprisingly useful.

The large hotplate cost around £50, while the smaller one was about $20 (65 W, USB-powered). Both appear to be generic white-label units — if you search for “electronics hotplate” on Amazon, you’ll likely find several listings that look identical.

I also picked up some generic lead-free solder paste with an Sn42Bi58 composition, marketed as Type 4 solder paste.

The idea behind the larger hotplate was to solder multiple components — or even multiple boards — in one go, while the smaller unit seemed better suited for targeted heating, such as smaller boards or connectors. It also helps that I don’t need to clear my desk to use the smaller one.

The solder paste I used was from Flintronic. It seemed like a reasonable addition to the order, and to be fair, it worked well in my initial tests. That said, I’ll want to try a few more syringes before forming a proper opinion — consistency between batches can vary with this kind of product.

Mini Hotplate#

The mini hotplate is a neat little device, with a bright OLED display on the front and two buttons on the back for adjusting the temperature setpoint and for showing the current temperature of the hotplate.

I initially tried powering it from a standard wall adapter, just to see what would happen. It did heat up — slowly — but wouldn’t go beyond 100 °C. There’s a voltage readout on the front, which makes it clear what’s happening: at 5 V, you’re simply not going to get much power out of it.

Switching to a USB-C laptop power supply (capable of delivering 20 V at 3.25 A) made a noticeable difference. The unit negotiated the higher voltage properly and heated up without issue.

I used:

• 100 °C for a preheat stage
• 145 °C to begin reflow and then a bump to
• 175 °C briefly for reflow before removing the heat

At those temperatures, it performed well.

After removing the board and allowing it to cool naturally, the results looked good. Only a couple of minor touch-ups were needed on an SOIC package.

Overall, it’s a genuinely useful little tool. I’ll likely keep using it, particularly for small boards and quick jobs.

That said, I’d be cautious about pushing it anywhere near its claimed maximum temperature (350 °C). For now, I’ll be keeping it below 200 °C, where it feels much more predictable and controlled.

Large Hotplate#

The large hotplate is a fairly sturdy device and feels well built with temperature controls at the front that allow you to set and monitor the temperature in degree increments. It also heats up very quickly — fast enough that I ended up stepping the temperature up gradually to reach the 100 °C preheat stage, just to avoid thermal shock.

It then handled the jump to 145 °C and then to 175 °C without any issues. As before, I removed the board from the plate to cool naturally.

The results were good overall, with no obvious issues in the solder joints.

This device, having a large surface area, can also be used for other purposes such as (in my case) phone screen repair, preheating boards before using hot-air or heating liquids in an appropriate glass container, so again I will be keeping this in my workshop. I also might think about using it for making candles, but that’s another project 🙂

Observations on Low-Temperature Paste#

Using Sn42Bi58 paste makes this process much more forgiving.

Because it melts at around 138 °C, you can:

• avoid stressing components
• use lower hotplate temperatures
• reduce the risk of damaging the board

However, it’s worth noting that low-temperature solder is more brittle than traditional lead-based alloys, so it’s best suited to prototypes and low-stress applications. You also don’t get that shiny reassuring surface you would with leaded solder.

You can also touch up your soldering with a soldering iron set to 250 °C without too much issue, just be sure to clean the tip of the soldering iron first.

I run Protoworks (NI) in Northern Ireland, helping small businesses design prototypes and build custom software. If you have a hardware or software idea you’d like to sketch out, feel free to get in touch.