Soldering PCB above the Tg temparature - Page 1
Forum section: electronics / general

author: c64
author_rank: Frequent Contributor
timestamp: June 06, 2020, 02:07:53 am
content: Cheap FR4 has Tg (glass transition temperature) around 130 °C, more expensive 155 °C.But when you solder it, you obviously heat it up way above the Tg. Why it doesn't do any damage to PCB or make it warp?

author: jmelson
author_rank: Super Contributor
timestamp: June 06, 2020, 02:12:33 am
content: Quote from: c64 on June 06, 2020, 02:07:53 amFiberglass/resin has really LOW thermal conductivity. So, the heat quickly heats the copper, but takes quite a while to diffuse into the substrate.Jon

author: Siwastaja
author_rank: Super Contributor
timestamp: June 06, 2020, 07:15:31 am
content: The temperature rating is for long-term operation. Short operation above the rating is OK, obviously, otherwise soldering would be impossible.

author: Rerouter
author_rank: Super Contributor
timestamp: June 06, 2020, 08:40:28 am
content: That is the temperature that it begins softening, this is the temperature you want to stay away from long term if you can, its like how a metal starts loosing strength well below the melting point, if you get it hot enough, It will "melt" remaining mainly constrained by the friction of the weaving, but if you keep it hot for long enough, or cycle into this area, that weave will start to part, and then it will loose its shapeIf you heat a PCB to about 350C and let it cool down while bent at a radius it will remain slightly bent, this is part of why people balance copper on a PCB, if 1 side cools faster than the other, it will draw the FR4 into a radius as it cools.

author: c64
author_rank: Frequent Contributor
timestamp: June 08, 2020, 01:11:49 am
content: I have long PCB (10*50 cm) which I plan to pre-heat on hot plate and solder with hot air. And hot plate is not big enough, so I do one half first and then another. Do you guys think it can warp and how much should I pre-heat it?

author: Siwastaja
author_rank: Super Contributor
timestamp: June 08, 2020, 07:10:38 am
content: It can warp but such long PCB is kind of flexible anyway so I guess you are going to mount it somehow to straighten it, so does it matter?If this is the case, be careful of cracking ceramic capacitors. Avoid large sizes, and rotate them so that the direction of warp/flex can not bend the capacitor body. Very careful with 1206 because it has a long body.

author: GerardG
author_rank: Regular Contributor
timestamp: June 08, 2020, 08:58:09 am
content: Hello,The Tg doesn't say anything about soldering temperature.Soldering for a short time at 250C will give no problem on the Tg 130 material.The Td (Decomposition Temperature) of most materials is above 325C.Use a bottom heater set to 80C. This will assist the soldering on the top side.

author: JLCPCB Official
author_rank: Regular Contributor
timestamp: June 09, 2020, 03:10:48 pm
content: While soldering FR4 board temperature will heat up the copper areas before heating up the resin and fiberglass so the longer you expose the PCB to heat the more you could heat up the epoxy, resin and fiber glass part. normally heating up the solder past takes lower time to molten than the limit expose time of the PCB layers.Do not forget also that there is another parameter in this formula which is the electronic components, the longer you expose them to high temperature the more risk of damage you can make.

author: Cerebus
author_rank: Super Contributor
timestamp: June 09, 2020, 08:33:33 pm
content: Quote from: Siwastaja on June 06, 2020, 07:15:31 amNo, Tgis not a rating, it's theGlass Transition Temperatureof a material. See the linked Wikipedia article for an introduction.It's a peculiar concept in a thermosetting polymer like epoxy. In a proper glass like soda glass or a thermoplastic like acrylic it's obvious that Tgrepresents the point where the material stops being hard, brittle glass and becomes malleable before it melts at Tm. In something like epoxy that can't melt it doesn't have quite the same obvious transition to a plastic state but it does lose many of its desirable physical properties (hardness, stiffness) above Tg.