Hot Plate Soldering Using Low Temperature Solder

This is a guide on how to assemble SMT devices on a flexible low-cost PET PCB. We use a hot plate to solder SMT devices to the PET substrates using low temperature Tin Bismuth solder.

Soldering components onto PET substrates must be done using low temperature solder, normal lead free solder is not suitable. Tin Bismuth Sn42Bi58 low temperature solder paste melts at just over 150°C (lead free solder melts at around 220°C).

This method does not require a soldering iron.

The video above shows the whole process start to finish.

You will need...

• Your bare printed PET flex circuit (this one was printed by CIT)
• Sn42Bi58 Low Temperature Solder Paste for deposition – 10 g syringe (or equivalent) Spec sheet attached
• A well ventilated area in which to work
• Hot plate – ideally temperature controlled
• Steel plate ~1 mm thick – larger than the circuit and easy to take on and off hot plate
• Heat resistant gloves to handle steel plate when hot
• Safety glasses
• A heatproof surface on which to place the steel plate

Setting up the hot plate

In summary: The PET is placed on a stainless steel sheet which assembled and placed on the hot plate to reflow.

The whole circuit is heated so the solder melts and reflows, low temperature Tin Bismuth solder reflows between 150°C and 175°C. The aim is to minimise the time the PET is exposed to high temperature, therefore we heat the steel plate to just over 150°C promptly but without exceeding that temperature for too long. This may take some trial and error. For our 1 mm thick stainless steel plate we found that setting the hot plate to around 260°C achieved the reflow temperature in just over a minute and all the solder reflowed in about 30 seconds. The top surface reaches about 160°C.

Before we did our first run we played around with the temperature settings and connected a thermocouple to the top of the steel plate to track how quickly the plate warmed up and what temperature it topped out at.

Smooth the bare PET circuit onto the steel plate ensuring it is as flat as possible to make good thermal contact with the metal. TIP: works better if the steel plate is warmed a little first.

Deposit small blobs of solder on the pads, we do this directly from the syringe, but could be done by transferring the solder to the pads on a toothpick for example. The solder will de-wet from the PET on reflow so don’t worry about being 100% accurate.

Place the components onto the solder paste, pressing in gently. The solder paste is tacky enough to keep the components in place.

Pre-heat the hot plate to your selected temperature, (see preparation note), we are aiming to have the top surface of the steel plate reach about 160°C.

Place the steel plate on the hot plate and watch for the solder to start reflowing. You will probably find this starts at one point and moves across the circuit. It is easy to see as the matt paste turns shiny. The solder will also tend to centre any slightly crooked components on their pads.

Once the reflow starts ensure that it happens across the whole circuit. Then wearing heat proof gloves immediately remove the steel plate and place on a heat proof surface to cool.

Carefully remove the PET circuit from the steel plate. Note: the plate will retain heat for some time so take care!

Visually check your circuit to ensure all the solder has reflowed then test your circuit.

There you have it - all your components soldered in under two minutes and not a soldering iron in sight.