To Werner,
Xie xie ni.
As I know, many vertical insertion daughter boards which have big heatsinks glued to the chipsets, the stress will last, does that means their long term reliability is limited? I think there should be a resonable load for the solder joints. For you see, zero stress is unattachable in the real design for this reason or that. What we could do is avoid the worst case and choose the less worse one....
To George,
Thanks for your valuable advise and document.
I've got it : A null stress solder joint is the more reliable one, and our design is moving to a new one which will avoid this problem.
Regards
Li Yi
----- Original Message -----
From: "Werner engelmaier" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, March 06, 2009 10:11 PM
Subject: Re: [TN] About PTH solder joint strength
Ni hao Li Yi,
No, there is no "reasonable load" solder joints can sustain for any length of time [the exception is at temperatures below -40C], because the operating temperatures of electronic products is way above a homologous temperature of 0.5, and thus significant creep is inevitable.
Creep will continue until either the load is removed, or creep rupture—as you have experienced with your tests, occurs.
Thus, in order for your design to work, you have some options—and i do not know which would best work in mass production:
1) crimp the pins, taking the load off the solder,
2) make the hole diameter almost the same as the pins, this leaves essentially nothing but intermetallic compounds (IMCs) that do not creep, however, having the hole diameter properly toleranced is difficult and pin insertion is difficult as well;
3) use press-fit pins—may not be practical.
It looks like you need a re-design.
Werner
-----Original Message-----
From: 李义 <[log in to unmask]>
To: [log in to unmask]
Sent: Fri, 6 Mar 2009 12:54 am
Subject: [TN] About PTH solder joint strength
Hi Tech netters,
In our design process, We need a heatsink be assembled by a spring which is held
by two U-shape pin headers(each has two PTH pins),the spring keeps the heatsink
and the chipset a stable contact and the U-shape pin headers are soldered on the
PWB.
To be sure the heatsink is assembled good, the spring should supply a 40N load
which means each pin of the pin=2
0header should sustain about 10N.
Think of the solder's creep properties, we could not use the solder's mechanical
properties such as tensile strength or shear shength to get the PTH solder
joint's reasonable load.
The PTH hole diameter is 36mil and the solder pad is 90mil, the brass pin
diameter is 20mil,the PWB thickness is 79mil and the pin headers are soldered by
hand soldering.
In our test, the U-shape pin headers were loaded with 50N, one faild in 6 hours,
one failed in 8 hours, the other two failed in 5 hours.
We lose our faith in this design.
Does this design work in the long run? Any one could give me the way to
calculate the soldered PTH pins' reasonable load? Does a 50N test is to harsh
for the pin header, which means each pin should sustain 25N? Are there any test
methods about this?
Thanks and Regards
Li Yi
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