Simplified Thermal Stress Analysis

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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.

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TCE

• Problem when one material is bonded to another
  with a much smaller temperature coefficient of
  expansion (TCE)
• ETCExDT
• Estrain (length/length)
• DTtemperature differential across sample
• SEY
• Sstress (psi/in or Pa/m)
• Ymodulus of elasticity (lb/in2 or Pa)
• When total stress (Smax dimension of sample)
  exceeds tensile strength, cracks will form
• Note that this analysis is simplified (Dr. Yee
  might not approve.)

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Types of cracks from thermal stress
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Other thermal stress properties

• The stress can cause displacement in the
  tangential direction.
• Poissons ratio nstrain in tangential direction
  /strain in normal direction eT/ eN
• Shear modulus GE/2/(1n)

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Die-Die Attach-Substrate

• Two types of problems caused by TCE dieltTCE
  substrate
• 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.
• Temperature differential exists, causing stress
  may be caused by large thermal resistance of die
  attach

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Total strain when both cases occur

• E(TCED-TCES)(TD-TA)TCES(TD-TS)
• where Ddie, Ssubstrate, Aambient 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.

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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.

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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

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Force required to cause breakage
KICplain strain fracture toughness in psi-in1/2
or MPa-m1/2 Zdimensionless constant, usually
1.2 amicrocrack length/2
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To Minimize Stress

1. Match TCE of component and substrate as much as
   possible
2. 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)
3. 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
4. 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.
5. 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.
6. Reduce temperature fluctuations due to better
   thermal management.

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To Minimize Stress, cont.

• Increase bond thickness greater ability to flex
  when force applied often used with solder joints

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Helpful properties to use with examples