As I am a little slow catching up with emails, let me respond to this
question regarding a cost effective solution for base materials for lead
free temperatures.
It is very important to understand the differences between Tg and thermal
stability. Several years ago, availabile resin materials which had
increased Tg's were accompanied by increased thermal stability. However,
with the introduction of new resin chemistries, this relationship is now no
longer true. The significance of Tg is the temperature at which the thermal
expansion *rate* changes. Therefore, a higher Tg is beneficial as it allows
the board to maintain a lower thermal expansion rate for a longer time as
the processing temperature rises. The significance of thermal stability is
the time it takes at an isothermal temperature before an catastrophic
failure of delamination. These readings are listed as T260 and nowadays as
T288 or T300 as we look at higher processing temperatures. At a constant
temperature, the board does not continue to expand as it does during a
temperature rise, but instead fails all at once.
It is my belief that the higher reflow temps required for the lead free
solders should lead to the need for good thermal stability at this
isothermal temperature, not necessarily the Tg at this temperature. I do
recognize that a high Tg is still critical as the board has to rise to this
temperature, but I challenge everyone to take a look at the high thermal
stability products with a <200°C Tg and see how they perform. Perhaps this
can be a cost effective solution
-----Original Message-----
From: Periannan, Arulvanan [mailto:[log in to unmask]]
Sent: Monday, August 21, 2000 1:07 AM
To: [log in to unmask]
Subject: Re: [LF] Cost of base materials & Lead-Free
Hi,
I am replying this thread with another question, as the question is
indirectly related to the cost.
Understand that we have different options as below
FR4 Difunctional Tg 125°C
FR4 Tetrafunctional Tg 135°C
FR4 Multifunctional Tg 180°C
FR5 150-180°C
BT resins and other materials having Tg upto 280°C.
As we increase the Tg requirement, the cost also increases. The cost
difference can be upto 3 times higher(When compared to Tg 125°C to Tg
280°C). But which is the most cost effective solution?
In otherwords would like to know whether there is any data available which
gives the maximum permissible processing temperatures in the PCB assembly
for a given Tg ?
Appreciate your input.
Regards,
Arul
-----Original Message-----
From: Steve Collins [mailto:[log in to unmask]]
Sent: Tuesday, August 01, 2000 10:58 PM
To: [log in to unmask]
Subject: [LF] Cost of base materials & Lead-Free
Thanks to everyone who commented on my question yesterday "LGX file
extension". Today I have another one.
For the past 10 -12 months since incorporating the IPC-4101 Specification
for Base Materials for Rigid and Multilayer Printed Boards and implementing
Immersion White Tin as our surface finish and looking toward using
99.3Sn/0.7Cu for our wave soldering & 95.5Sn/4.0Ag/0.5Cu or 96.5Sn/3.5Ag
for our Reflow soldering I have been specifying the slash sheet 24 (Tg
150°C - 200°C) as our base laminate. The reason was to get a higher Tg
material in order to withstand the higher manufacturing process
temperatures (wave & reflow soldering). This is now in danger due to cost
reduction programs being looked at internally. Typically in the past what
we called out was a generic form of FR-4 and what we got was similar to
slash sheet 21 (tg 110°C Minimum).
I'm in the process of calculating just what the material cost differences
from our vendors is but wanted to put this out to the other industry
experts also.
Now for my questions, are any of those people looking at the "Lead-Free"
going through similar experiences. Was my concern for the higher temps in
manufacturing realistic. Was my approach realistic.
Sorry if this sounds pleading but with the changes around here I'm
concerned whether my ideas hold credibility or not.
Thanks in advance for any comments.
Steve
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To subscribe: SUBSCRIBE Leadfree <your full name>
To unsubscribe: SIGNOFF Leadfree
###############################################################
Please visit IPC's Center for Lead-Free Electronics Assembly
(http://www.leadfree.org ) for additional information.
For technical support contact Keach Sasamori [log in to unmask] or 847-790-5315.
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