pcb temperatureratings - Page 1
Forum section: electronics / projects

author: Amper
author_rank: Frequent Contributor
timestamp: November 12, 2021, 05:50:34 am
content: Hi,im planing on building a component tester with an integrated heater. Since the heater will not last forever no matter how it will be designed in this case i want to include the resistive element into the layout either as traces or resistors. The goal temperature is 125°C, the DUT will be soldered to the backside of the same pcb and tehre will be a temperature sensor in some sort of surface mount package.My question now is, will TG150 be sufficient for a few thousand houres of operation? What should be the safety margin to the glass transition temperature in continuous operation?Hopefully someone has some experience. Its essentially a very small (2cm²) 3d printer bed heater.

author: jonpaul
author_rank: Super Contributor
timestamp: November 12, 2021, 07:33:29 am
content: Hello: Use of a PCB copper trace as a heater has many issues: Very low R, temperature coefficient of resistance of Cu, safety.Good practice is to use a material designed for heater like Nichrome resistance wire.TG150 is a general spec.FR4 Epoxy Fiberglass is UL rated and self-extinguishing flammability characteristics.Many small heaters are avail eg for precision voltage refs or TCXO.I would explore the heaters/enclosures and control.Bon Chance,Jon

author: Siwastaja
author_rank: Super Contributor
timestamp: November 12, 2021, 11:21:01 am
content: The problem is, localized temperature right at the trace is always higher than on what you are trying to heat. Add that to uneven track width (due to manufacturing tolerances) and there will be hotspots.So even though 125degC is fine for long-term use, you won't be able to get 125degC where you need it. I would aim at maybe 100degC max for the PCB surface itself, and then comes the limited thermal coupling to the DUT.Maybe you could make PCB the heater, but help the task a bit by using say four ceramic body power resistors forming a "cage" around the DUT. OTOH, if you make those resistor run too hot, they will scorch the PCB, too.

author: Amper
author_rank: Frequent Contributor
timestamp: November 13, 2021, 10:51:57 am
content: @jonpaulI know how to build a heater but the task is to integrate it into an autmated manufacturing process as faar as i can, there is no way i will hand wind >100 heating elements from nicrome. PCB heater do work, its not a new idea and the good conductivity of copper is not an issue considering how fine pcbs can be manufactured. Im not sure what you want to tell me besides that, yes, fr4 is fire resistance spec but how is it related to my problem? TXCOs are not run at >100°C.@SiwastajaThe localized temperatrue is an issue but i think its not a big factor as i dont plan on heating with a lot of power, the pcb will be encased in insulation so it will be more of a temperature keeper at 1-2W/cm² and no real hotplate. Thermal coupling to the dut should be no issue for that reason too. Also in case i use a dfifferent style of heater ill still have to mount the dut on a pcb and heat from the bottom for electrical insulation reasons. Same if i use power resistors (which is a backup plan, yes).To clarify a bit more, this entire setup is going to be a quality control setup, im going to mount the components on a more or less disposable pcb that includes heater and sensing device as well as dut, it will run for 1000-2000h and the device will be characterized again to find unreliable parts. So the pcb including heater will be used only a hand full of cycles before being thrown away anyways. Im concerned though if TG150 or 170 will be sufficient for even those several thousand hours or if i will have to go up to some fancy stuff like rogers or polimide wich will make things much much more expensive and the choice of pcb manufacturers much smaller.

author: wraper
author_rank: Unknown
timestamp: November 13, 2021, 11:05:36 am
content: If you use 4+ layer board, and leave solid copper on the layer underneath the heaters, I guess it should be fine as it will act as a decent enough heat spreader, and will prevent traces overheating.

author: Psi
author_rank: Super Contributor
timestamp: November 13, 2021, 11:50:29 am
content: Aluminum PCB maybe?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 14, 2021, 01:10:16 am
content: Why look any further then simple and cheap power resistors?The wire wound variant on a ceramic substrate (either the tube or the "square" outline) can get pretty hot without damage.Don't use the aluminium variant that's designed to be mounted on a heatsink. These are very sensitive to overloads and temperature.

author: Amper
author_rank: Frequent Contributor
timestamp: November 14, 2021, 07:46:59 am
content: As is said before.