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March 2002

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From:
Nancy Trumbull <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 12 Mar 2002 11:05:50 -0500
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First I want to say thanks for all the help everyone has offered.
I have to admit that I don't know that I understand all of it but things have greatly improved.

Sorry this make be a dump dump question.
But right now I can not understand your statement.
"Because of this reason any measurement of a Youngs modulus with a tensile testing machine is fruitless since any stress strain diagram found depends on how fast the crosshead moves.

Reason
I don't know what a Youngs modulus is or the term crosshead . I think crosshead is the point of change. But I need to be sure.
Sorry if this is dump. 
I tried looking them up,  

I hope everyone will continue with the subject.

So everyone will know.

I have found out that the original parts places in the chamber for thermal shock testing were  various degrees of defects.  So the evaluation was not valid.  The previous testing done with good parts  have all passed. I think some one just make a mistake. 

Oh and Warren if you read this 
What is HASS.
With temp change >30C/min with vibration.
Thanks again everyone I've got to go for now my head hurts from all this information.
And it's time for my meeting.
Nancy T.

>>> [log in to unmask] 03/12/02 04:42AM >>>
Jim

The question about shock testing of solder joints is somewhat complex and can not be reduced to a single temperature change rate. I'll try to explain our point of view as short as possible:

1) Deformation. The main deformation mechanism occurring in solder joint is creep. This means if a constant load is applied the material deforms continuously. There is no direct connection between load and deformation as in elastic or instantaneous plastic deformation. How much a material deforms is time dependent. Because of this reason any measurement of a Youngs modulus with a tensile testing machine is fruitless since any stress strain diagram found depends on how fast the crosshead moves. This means the Youngs modulus is not a material dependent constant but a testing dependent measure.

2) Degradation. Tin- based solder deforms in creep with two deformation regimes:
a) Grain Boundary Sliding. In GBS the crystals of a material move above each other along the grain boundaries. However, since the grain boundaries are not straight planes the grains move on complex three dimensional traces that also involve rotation. It is clear, that in cyclic deformation the grains will on their way back not sit in the place they where before the cycle started. Due to the uneven surface of the grain boundaries small openings occur in the material. Most of there openings are filled up by diffusion from the places where compression occurs to the places where tensile stress is present. It is important in this context that diffusion is time and temperature dependent. The faster the deformation and the lower the temperature the more openings remain only partially filled.
b) Dislocation Climb. If the stress is high enough it is possible to overcome the forces that hold the atoms in the crystal lattice and the grain itself deforms. This deformation is only possible if there are failures in the lattice called dislocations. If DC is activated not only the present dislocations move through the crystal causing one crystal layer to move above each other but also new dislocations are produced. If a dislocation reaches the grain boundary also a small opening is formed that might be filled by diffusion if there is enough time and temperature.
This means that each deformation regime has its own degradation behaviour.

Looking at point 2, one could say that fast cycling is more destructive than slow cycling. And looking at the degradation mechanisms this is true. 
However, point 1 plays an important role in accelerated testing. If you run a test with steep temperature ramps you build up stress  since the PCB and the component deform elastically. This stress has to be transformed into plastic deformation since slow cycle fatigue is strain driven. At high temperature this is no problem. Solder creeps so fast that above 40 deg.C it is virtually impossible to apply a temperature gradient that builds up any stress. The problem occurs at low temperature. Below -20 deg.C solder creeps very slow. If you apply a slow temperature change a lot of strain is already induced in the ramp during the time where your specimen is above -20 deg.C. If you apply a steep ramp you have to wait for hours until a considerable portion of the possible strain is induced. 

As you see it is a complex game between deformation and degradation. However, since the amount of cyclic plastic strain a solder joint sees in a thermal cycle is so crucial slow cycling is more damaging since more strain is induced into a solder joint than in a fast cycle. On the other side it is quite questionable whether it makes sense to activate a degradation mechanism that does not occur in reality just to have a faster test. 

Hmmm, I hope I made myself clear enough.

Best regards

Guenter

.

EMPA
Swiss Federal Institute for Materials Testing and Research
Centre for Reliability
Dipl. Eng. Guenter Grossmann

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1823 4054
mail:     [log in to unmask] 

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