Tg is the glass transition temperature of the PCB material and very important parameter.
At this temperature the raw PCB material changes from stiff glass-like material to a elastic and bendable plastic-like material.
The biggest problems in the PCBs during the assembly is the Z-axes thermal expansion as above Tg temperature PCB expands rapidly and if kept long time about Tg temperature the small PCB vias may/will crack internally.
The most evil part is that when the PCB cool off the vias will shrink and may still give unreliable contact like touching together two copper wires, but later with temperature changes, vibration moisture these internal cracks in the vias can cause lost of electrical connections on random basis.
Tg 135 was commonly used and good enough for the old SnPb technology.
Since 2006 the new Lead Free technology require higher soldering temperatures and Tg 135 is not good choice.
PCBs assembled by ROHS lead free technology should be done with material with Tg 170 to be reliable. Especially this is valid for multilayer PCBs with small vias less 0.4 mm.
olimex 
Aug 18, 2016 @ 16:19:58
FUUUUUUUUUUUUUUUUUUUUUUUUUUU
WHERE ARE THOSE PRECIOUS NEWS ABOuuuttt your DIY laptoooooopp?77
Aug 22, 2016 @ 12:22:25
Vias are tricky and coud be evil. They are long known for being one of major causes of HW failures which are also diffucult to pinpoint and could pass testing but go loose later.
IMHO Its really nice Olimex ppl are cool enough to educate others about odds of electronic devices development. So, Lyra, don’t you mind open source and diy is all about learning the sfuff and ability to change the world? Olimex are rare kind of company who both learned how to get it right AND cool enough to learn others to do the same, rather than just stick to being dumbass and boring consumers. I think we should value their will to share expertise and experience and “FFFUUU” is just nowhere nearly appropriate. Not to mention doing things like that takes quite some time. If someone got different idea: show us how to do it better. But sorry, yet another proprietary thing from large company does not counts. They’ll never publish even schematic, not to mention proprietary BIOS, management engine backdoor and of course crappy support of opensource OSes like Linux, where vendor does not even cares to fix bios/uefu bugs or even unable to do so due to licensing terms.
Aug 23, 2016 @ 05:59:01
Welcome back, Olimex. ‘Hope you had a nice summer vacation. 🙂
And yes, I agree with Linux User, sharing information on technical topics like the Tg of PCB material is excellent to see, and usefully educational. Your audience comprises many engineers and technical enthusiasts of all kinds, not just passive consumers.
I found the article informative and interesting, and directly relevant to OSHW fans who design their own boards, whether or not they then manufacture them or send them off to be manufactured.