Tg rating for PCB material - SAC305 reflow; - Page 1
Forum section: electronics / general

author: mrpackethead
author_rank: Super Contributor
timestamp: November 04, 2018, 07:17:20 pm
content: For double sided reflow, where the peak reflow gets to 260C for 30 seconds or so, what grade of PCB would you use so it does not delaminate or deform.I'm currently using Vapour Phase, and it peaks at 230 so, Tg140 has been fine, but it is a slow system, and i'm considering getting IR as well.

author: SMTech
author_rank: Frequent Contributor
timestamp: November 05, 2018, 08:40:33 am
content: IF the PCB is at 260C for 30 seconds, that would IMO be too long and pushing or breaking the spec of all sorts of parts. TAL (time above liquidus) should be in the range of 30-90 seconds and your peak temperature in that time should be under 260C as there may well be areas of your board you haven't profiled that get hotter than the ones you have.IR ovens are a waste of time these days. Not only do they not really exist outside of little Chinese things like the T961 but they are subject too many process limitations which include massively uneven heating, colour sensitivity, shadowing and are not exactly the best friend of things like plastic connectors. If you want higher throughput than you are getting with your VP setup, you either want an inline VP machine$$$, VP that takes larger batches or the more conventional Convection Reflow oven, the established names for reflow typically make these at 3.5metres+ in length so they are something of an undertaking in space (+extraction) however depending on your requirement some of the other brands from Europe&Asia do make smaller ones in the $10k-$20k range that are a little smaller. Like thesehttp://www.emstonline.com/ems_reflow_oven.php..although this smells like a wrird hybridhttps://www.alibaba.com/product-detail/reflow-soldering-machine-price-lead-free_60788978701.html?spm=a2700.details.maylikever.14.69e3540bABuRNt

author: mrpackethead
author_rank: Super Contributor
timestamp: November 05, 2018, 09:27:52 am
content: Sorry, SMTech, not sure why i wrote IR, Becuase i meant a convectionSo, lets just assume a sensible lead free profile, ( probaly getting to 250-255 ) what grade of PCB would you use.Part of me, just says get more VP. It does such a good job and its so easy, but its just slow. I think having a bigger VP bucket is not a bad idea.

author: SMTech
author_rank: Frequent Contributor
timestamp: November 05, 2018, 09:39:43 am
content: Fair enough, we do some double sided never had an issue just using bog std TG140, I have a some blanks that I abused running some basic profiling and sure after 10+ runs through the oven they stop looking so good. But then by my understanding the TG rating is more about how much prolonged heat the board can withstand, so the VRM circuits on modern motherboards and GPUs would be seriously pushing the limits of TG140 as would many LED boards.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: November 05, 2018, 11:16:45 am
content: From what I've been told it all depends on the life of the product, specifically if you expect it to be reworked later on (repairs). If yes then you want a higher TG (like in the 170's), if no then your standard will do just fine

author: jmelson
author_rank: Super Contributor
timestamp: November 05, 2018, 08:08:13 pm
content: Quote from: mrpackethead on November 04, 2018, 07:17:20 pmI've been doing FR4 boards for years, at about 238 C for the peak temp. The ONLY problem I see is the silk screen sometimes darkens a little.If the boards get to 250 C or so, then maybe the hotter spots will darken the laminate, too.I've never had deformation. The only time I've had delamination or blistering is when old boards that had absorbed water were reflowed. The fix for that is a 1 hour bake at 50 C, then 2 hours at 70 C. Then, even boards that had been stored in my basement for two years reflowed with no problem.This is all done with an IR system, so temperature is not perfectly uniform over the boards.Jon

author: jmelson
author_rank: Super Contributor
timestamp: November 05, 2018, 08:10:25 pm
content: Quote from: ANTALIFE on November 05, 2018, 11:16:45 amOK, I think I've seen this. Boards that were hand soldered were MUCH easier/safer to rework than boards that were reflowed, initially. On the reflowed boards, it was a lot easier to lift traces during rework.Jon

author: mrpackethead
author_rank: Super Contributor
timestamp: November 05, 2018, 09:06:04 pm
content: This is an itneresting observation.. and one that i've seen too, but had'nt considered it in terms of the Tg.I run pcbs through VP, ( so it peaks at 230 ), and yes, if things need rework, it is quite easy for a pad to lift on the board.Maybe time to get a higher grade.

author: jmelson
author_rank: Super Contributor
timestamp: November 05, 2018, 10:31:31 pm
content: Quote from: mrpackethead on November 05, 2018, 09:06:04 pmYes, but...I did do a few boards with higher Tg, and found little to no difference in the attach strength of the traces. I don't know if I got Tg of 170 C boards, though. Getting the Tg that high might be a REAL expense.Jon

author: asmi
author_rank: Super Contributor
timestamp: November 06, 2018, 02:18:44 am
content: All my multi-layer PCBs are Tg150 (which is default material for multi-layer boards in many pcb fabs), and I never had any issues with lifting pads, even though I run them through 2 reflow profiles as I do one side at a time.

author: jmelson
author_rank: Super Contributor
timestamp: November 07, 2018, 10:25:01 pm
content: Quote from: asmi on November 06, 2018, 02:18:44 amOK, the lifting pads issue is in MANUAL rework, AFTER a board was reflowed during initial assembly, such as when a whole chip needs to be replaced. I have definitely found that the "hot peel strength" is less on a board that has been reflowed than an identical board that was hand-assembled with a soldering iron. Still, with a good hot air desoldering tool, there is rarely a problem. But, I don't have a hot air tool at one place, and my crude hand methods of rework there do run into trouble.Jon