Hi GA, 

We measure Young's Modulus on FR4 PWBs with DMA. Our sense is that
repeated thermal excursions associated with lead-free changes Young's
modulus. There appears to be a hysteresis induced by thermal excursions
associated with assembly so I would expect elasticity to be changing
with each thermal excursion.

We do not test assembled boards but the bare circuit board is definitely
changed by thermal excursions associated with lead-free assembly. We see
plastic deformation due to those thermal excursions in our
microsections. We think the plastic deformation relieves internal PWB
stress to some degree, but may induce stress in surface mounted
components. We know that delamination reduces the stress within the
printed wire board. We assume this stress relieves surface mount
components to some degree but have no experience in that area.

A good study would be to associate PWB reliability to changes in Young's
modulus due to the thermal excursion associated with tin-lead and
lead-free assembly. We have not done these studies yet but I expect they
would be informative.  If the PWB is getting stiffer with each thermal
excursion, I expect more stress to be induced in the internal
interconnections and mounted components.

Sincerely,

Paul Reid

Program Coordinator

PWB Interconnect Solutions Inc.
235 Stafford Rd., West, Unit 103
Nepean, Ontario
Canada, K2H 9C1

613 596 4244 ext. 229
Skype paul_reid_pwb
[log in to unmask] <mailto:[log in to unmask]>
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tan Geok Ang
Sent: Wednesday, August 13, 2008 9:11 PM
To: [log in to unmask]
Subject: [TN] Elasticity

Hi All,
		Can the elasticity of FR4 rigid PCB and solder joints
(SnPb, SAC) be definite (with value)? Thanks in advance.

Best Regards
GA Tan

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