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High TG?

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aj

#45013

High TG? | 13 November, 2006

Hi all,

Should we need to order the Higher TG to run boards safely thru a RoHS Process (reflow only)? We had an issue with the previous batch delaminating and were advised by supplier that we should have stated the higher TG rating even though the previous boards were marked as RoHS Compliant.

Also, what is the TG rating for specifically ? is it the FR4 ?

Should higher TG prevent delamination?

aj...

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CK the Flip

#45022

High TG? | 13 November, 2006

Tg, by definition is the glass transition temperature for materials. It's the temperature where your board goes from rigid to "rubbery".

Not sure if a higher Tg would help prevent delam. One of the PCB experts of the forum might know this.

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#45028

High TG? | 13 November, 2006

Higher Tg values are only one thing to look out for. Other things that are very important to watch are: * Td or Decomposition Temperature (the temperature that the epoxy actualy starts to break down). * CTE for the material.

Some argue that Td is more critical than Tg. Search the fine SMTnet Archives for more, while you're waiting for others.

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Loco

#45034

High TG? | 14 November, 2006

To make it even more compicated, you have CTE before Tg and after Tg, altough the after Tg is way more important, because the expantion ratio will be much bigger, a pcb supplier will usually give the before Tg value.

To come back to the delamination issue, there was a tread a while ago where several peoples had the same problem. We now also have a couple of boards of a product we have run in thousands leadfree in the past, never had any problems, FR4 spec hasnt changed, neither has profile... I find it rather strange that all of a sudden this issue shows up at the same time at different companies, could be coincidence of course, but on the other hand, could be a huge world wide conspiracy against process engineers.

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Board House

#45057

High TG? | 15 November, 2006

Hi AJ, The biggest mistake in the industry today is documentation for lead free assembly required product. We are still seeing prints coming in saying Lead Free / High TG FR4. When in reality standard FR4 170 Tg. material is considered High Tg. and Lead free but is Not Made to with stand lead free assembly Temps. For Lead free assembly product - it should be calling out High Tg Material capable of with standing assembly temps of 340 C. This would let the board Manufacture know to switch his material to an Isola IS410, Nelco 4000-29 or equivalent Material. These Materials have the Higher TG of over 170 plus the Td - which is the Decomp of the material in the 350 c. range. It�s a more Ceramic looking material. For Lead free assembly, the Td or Decomp is the most important factor, then Tg. Then all the other stuff, Dk etc... We have Customers that still use FR406 170 tg. / td. 288 c. for lead free assembly, but it is at their own risk, we will not Warrantee this product due to chance of Delam.

Regards,

Board House

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#45083

High TG? | 15 November, 2006

W Engelmaier [a respected industry graybeard] is pushing for a Soldering Temperature Impact Index [STII] to address this. Where: STII = (Tg+Td)/2 + [% thermal expansion 50 to 260*C]x10. He would specify a STII>215.

For more, look here: http://www.rohsusa.com/papers/PTV2.pdf

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