I'm finding it's stackup dependent.  We run a lot of thinner uVia 2n2, 3n3 and ELIC fine through our qualification reliability protocol with 150C-ish Tg materials (e.g. Panasonic 1566WN, EM-355D, SY-1000).  

When they start to go north of 1.4 and approach 1.6mm I see a need for 170Tg material develop. The first ones to need the higher Tg are 4 lam 3n3 stackups that have a multilayer in the center sub and uVias stacked on that center sub's mechanically drilled via (VIPPO).   In that case the VIPPO fill material and supplier processes becomes a key factor in success too (it's a short list that fab an 18+ layer 3n3 that will pass 9x260C reflow sim followed by 1000 IST cycles to 150C).


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Peter G. Houwen
Sent: Thursday, April 14, 2016 6:36 AM
To: [log in to unmask]
Subject: Re: [TN] Material Tg for RoHS

170Tg - after how long?

What manufacturers are finding is that the laminate itself often does not reach Tg temperatures in the time it takes to reflow an assembly, so 155, even 140 is OK in some circumstances.  If you have an assembly that's going to need a lot of heat and/or time to reflow,  170 may be needed.  But be careful, after that first pass, Tg degrades, and some 170 materials can end up with a lower 2nd pass Tg than some 140Tg materials.  And even if you've used 170 for a hot board so you don't end up with a gooey blob of copper-laced glass, you may now have CAF waiting to happen.

Nope, Tg isn't the measuring stick for RoHS.  Td, CTE are far more critical now.  And some of of those 170 materials use dicyandiamide curing to reach 170, opening the door for CAF and reduced 2nd pass properties, I've been specifying non-dicy.  It's not much of an issue any more, manufacturers don't want their product failing, so most are non-dicy.  But with the pricing pressures leading to cheap at any cost mentalities, it's safer to specify exactly what's important to me.

As for "RoHS" notes, stating "use RoHS materials" also leaves loopholes.  Materials aren't the only issue, process chemicals matter, too.  And "RoHS Compliant" doesn't always mean "RoHS compatible" when you factor in temperatures, cleaning processes, etc. required for RoHS.  Again, better manufacturers don't want their product failing, but there's that cheap at all costs mentality still,

And it's not just RoHS.  What about conflict mineral, and whatever they come up with this week?  We've taken to referencing a separate Environmental Spec on our drawings, so when the rules change, we change one document, not all of them.

Pete

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