Yes, and believe it or not, this actually has to do with much more than processing temperatures and even ties into a separate issue on the forum today, component and wire staking.
"Now how in the world are these two issues related?" you wonder.
The Tg of a certain PWB material slash type (as listed in IPC 4101) also relates to the particular laminate's coefficient of thermal expansion.
So if you are bonding many components or wires to a CCA during assembly, the adhesive you choose must have a similar CTE to the PWB. As an example, if you were to choose a hard Shore D type of material such as Loctite 3621, during subsequent thermal cycles such as rework, or curing conformal coating etc., the board can get hot enough to expand, enough to pop the component staking, cornerbonding, or wire staking right off the board.

So to kill two birds with a single posting,  consider the laminate slash type not only for processing, but also for other issues that could "pop up", so to speak. Certain bonding adhesives do not stick as well to a polyimide PWB material with a high CTE, compared to an FR-4 material with a much lower CTE.
One good wirebonding material that works well for both component staking and wire spot bonding (my favorite) is Scotchweld 2216. However, this may not be suitable for space use because it is not exactly a very low outgassing adhesive. But it is compatible for most conformal coatings, is relatively hard, but has a modulus of elasticity that can accommodate a significant PWB CTE, and has excellent resistance to chemicals and is extremely tough when cured (like the author of this post :-).

dean

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Hillman
Sent: Wednesday, April 13, 2016 10:21 AM
To: [log in to unmask]
Subject: Re: [TN] Material Tg for RoHS

Hi Jack - you are getting some excellent details for the herd so I'll just add one additional comment. In terms of lead-free assembly processing, understanding the laminate Td (temperature of decomposition) is a critical parameter because it is a measure of robustness with regards to the higher process temperatures used in lead-free soldering. In terms of solder joint integrity, understanding the laminate Tg (glass transition temperature) is a critical parameter because it allows you to understand the impact of CTE global mismatch on your solder joint integrity.  Both laminate characteristics are important and need matched to your product use environment.

Dave Hillman
Rockwell Collins
[log in to unmask]


On Wed, Apr 13, 2016 at 9:08 AM, Jack Olson <[log in to unmask]> wrote:

> Way back when we were making changes to meet RoHS requirements, we 
> were advised to use bare board materials with 170 Tg or higher. So we 
> changed our Fabrication Drawing Notes and everything has been fine.
>
> But recently I've noticed that the default material for several 
> different suppliers is around 155 Tg. Has the thinking "relaxed" in 
> this regard? Or should I stick with my 170 ?
>
> Jack
>
>
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