PCB Underheaters and Hot Plates

PCB underheaters and PCB hot plates have had a place in rework and soldering for a long time, this is not new technology, but with the challenges of lead free soldering, a PCB underheater makes this process much easier and improves quality.

Before PCB underheaters were introduced the PCB would not be properly preheated before rework or soldering. Even today with this technology available the pre-heat process is often ignored, resulting in soldering difficulties and quality problems, many which are hidden. Poor Rework and Repair can be expensive, with PCB harbouring more problems after repair which if hidden from inspection, could cause premature failure later in the field.

Elevating the PCB for extended periods at temperatures such as 315°C – 425°C risks causing many problems. Substrate delamination, lifted pads, discolouration, board warping and burning to name a few, but all these should be found during PCB inspection. The hidden faults can cause high failure rate which would be difficult to attribute to one area of production, reflow profiling is typically blamed.

It’s hardly a surprise that damage is caused during rework or soldering on a PCB which has not been pre-heated. The PCB at ambient or room temperature of about 20°C is suddenly subjected to a localised heat 370°C from either a soldering iron, desoldering tool, or a hot air gun. The delta temperature change of 330°C to the board and its components.
“Pop-corning”, internal damage can be caused to moisture sensitive devices when heated too quickly. Pre-heating the PCB is recognised as required in the production process for reflow and wave soldering, this pre-heating stage is just as important during rework or soldering.

The pre-heat stage or ‘soak’ in the soldering profile allows the whole PCB to be heated avoiding thermal shock and reduces the ramp rate when the rework or soldering begins by the operator, the ‘spike’ in the soldering profile. Flux is activated improving the wetting process and resulting quality. PCB’s with large ground planes makes rework and soldering difficult, the pre-heat gets these area’s up to temperature and maintains the heat.

For simple repairs and for hand soldering production tasks, a stand alone PCB underheater is a useful tool, making the process easier, improving quality and reducing failures.

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The PCB underheater is the preferred choice as this method has more control, a PCB hot plate remains hot for longer and it is best to cool the PCB as quickly as possible after processing. Double sided PCB cannot be used on a PCB hot plate for obvious reasons.

These PCB preheaters have options to create profiles for particular jobs, see more information on their product pages.

Bench mounted PCB underheater kits are also available.

Flush mounting the PCB preheater into the bench lowers the working height which provides better access for the operator and also frees up bench space when the PCB preheater is not in use. 

For more complex rework and soldering, for critical assemblies and sensitive components a rework station is more suitable.

The rework machine has the PCB underheater built in, but also assists with the removal of components and has the capability of placing component accurately, performing the soldering with a more controlled and repeatable method.