Transcript Simplified Thermal Stress Analysis

Simplified Thermal Stress
Analysis
Reference: Sergent, J., and Krum, A., Thermal Management
Handbook for Electronic Assemblies, McGraw-Hill, New York,
1998. Chapter 7
Another helpful source: Vaynman, S., Mavoori, H., Chin, J., Fine,
M.E., Moran, B, and Keer, L.M., Stress management and reliability
assessment in electronic packaging, National Electronic
Packaging and Production Conference--Proceedings of the
Technical Program (West and East), v 3, 1996, p 1711-1726.
TCE
Problem: when one material is bonded to another with a
much smaller temperature coefficient of expansion
(TCE)
E=TCExDT
E=strain (length/length)
DT=temperature differential across sample
S=EY
S=stress (psi/in or Pa/m)
Y=modulus of elasticity (lb/in2 or Pa)
When total stress (S*max dimension of sample) exceeds
tensile strength, cracks will form
Note that this analysis is simplified (Dr. Yee might not
approve.)
Types of cracks from thermal stress
Other thermal stress properties
The stress can cause displacement in the
tangential direction.
Poisson’s ratio n=strain in tangential direction /strain
in normal direction =eT/ eN
Shear modulus G=E/2/(1+n)
Die-Die Attach-Substrate
Two types of problems caused by TCE die<TCE substrate
1. When the temperature is at equilibrium (component
and die at same temp), stress greater than tensile
stress of the die can occur. This happens when there is
temperature cycling.
2. Temperature differential exists, causing stress; may be
caused by large thermal resistance of die attach
Total strain when both cases occur
E=(TCED-TCES)(TD-TA)+TCES(TD-TS)*
where D=die, S=substrate, A=ambient with power off
Experimental results will usually be somewhat less than
this. However, note that there are other causes of stress,
too, such as vibrations or material faults.
*Note again that this is simplified, so other sources may
have a somewhat different version of this equation.
Stress due to processing
Processing temps are usually higher than operating temps, so they
may cause the maximum stress. The stress maximum in this case
is at the corners.
Stress concentrations
During manufacturing, small stress concentrations often occur – small
cracks when a semiconductor die is sawed, small voids formed.
When external stress is applied, these concentrations amplify the
stress and may cause a fracture.
For an elliptical microcrack with major axis perpendicular to applied
stress, max stress at crack tip
Force required to cause breakage
K IC
Sc  Z
a
KIC=plain strain fracture toughness in psi-in1/2 or MPa-m1/2
Z=dimensionless constant, usually 1.2
a=microcrack length/2
To Minimize Stress
1.
2.
3.
4.
5.
6.
Match TCE of component and substrate as much as possible
Use an intermediate layer with a TCE in between that of the die
and substrate; molybdenum often used (TCE between that of
silicon and alumina)
Choose materials that need the lowest processing temperatures –
a large amount of stress is induced on the components as they
cool from the processing temp
Small voids in the bond distributed uniformly over the bond can
help reduce stress. However, these voids will increase thermal
resistance, increasing the junction temp, so this may not be a good
thing. Also, watch out for stress concentrations, such as those
caused by large voids.
Use compliant bonding materials, such as soft solders and soft
epoxies. Pb-Sn solder balls in BGA, or J-, gull-wing, and other
types of leads in surface mounted devices are good. Again, note
that a bonding material with a high thermal resistance will increase
Tj.
Reduce temperature fluctuations due to better thermal
management.
To Minimize Stress, cont.
7.
Increase bond thickness – greater ability to flex when force
applied; often used with solder joints
Helpful properties to use with examples