FUNDAMENTALS OF DESIGN FOR RELIABILITY

1. FUNDAMENTALS OF DESIGN FOR RELIABILITY

2. Chapter Objectives • Introduce the need for design for reliability • List the main causes of reliability failures • How do failures relate to their mechanisms • Describe each failure • Propose design guidelines against the failure Microsystems Packaging

3. Introduction Microsystems Packaging

4. Reliability • Often not designed up-front. • Tested during the product qualification or after the product is manufactured. • Expensive and time-consuming approach. • Design for RELIABILITY as well !!! Microsystems Packaging

5. 5.1 What is Design for Reliability • Product performs the functions – reliable product • “Long-term” reliability (i.e. Automobile, Personal Computer) • Economically not viable to test “long-term” reliable products for several years before they are sold out. • To ensure over an extended period of time, two approaches can be taken: • Design the systems packaging up-front for reliability. • Conduct an accelerated test on the systems packaging for reliability after the system is designed, fabricated & assembled. Microsystems Packaging

6. 1. Design the systems packaging up-front for reliability • Predetermine various potential failure mechanisms • Create and select materials and processes – minimize/eliminate the chances for the failures • “up-front” design • Design for reliability Microsystems Packaging

7. 2. Conduct an accelerated test on the systems packaging for reliability after the system is designed, fabricated & assembled • After a system is built and assembled, system accelerated to test conditions. • Temperature ,humidity ,voltage ,pressure • Testing for reliability – Chapter 22 Microsystems Packaging

8. Comparison and usage • Industrial practice uses Testing for Reliability • If {problems = TRUE} Then (IC & system-level packages): RE[designed, fabricated, assembled, tested] • Expensive and time consuming • Design for Reliability = Solution Microsystems Packaging

9. 5.2 Microsystems Failures and Failure Mechanisms • High-level symptoms (i.e. computer, TV) • Underlying cause (i.e. chip, corrosion, moisture, electrostatic discharge) – PRODUCT NOT RELIABLE • Design for Reliability understands, identifies, and prevents such failures • Overstress Mechanisms – stress exceeds the strength or capacity of the component and causes the system failure. (single event) • Wearout Mechanisms – gradual and occurs even at lower stress level. (repeated event) Microsystems Packaging

10. Failure mechanisms is microelectronic system packages Microsystems Packaging

11. 5.3 Fundamentals of Design for Reliability • Important to understand the failure (why, where, how long, application, etc.) • Two methods for design against failure: • By reducing the stress that cause the failure. • By increasing the strength of the component. • Either one can be achieved by: • Selecting materials • Changing the package geometry • Changing the dimensions • Protection Microsystems Packaging

12. 5.4.1 What are Thermomechanically-induced Failures ? - Caused by stresses and strains generated within electrical package due to thermal loading. - Due to CTE (coefficient of thermal expansion), thermally-induced stresses are generated in various parts of system. • Figure - Illustration of thermo mechanical deformation in solder joints • αb BOARD • αc COMPONENT Microsystems Packaging

13. Tmax chip carrier αc(Tmax – T0) per unit length board αb(Tmax – T0) per unit length - Difference between the two expansions = net shearing displacement: L(αb - αc)(Tmax – T0) where L – distance (of the solder joint) from the neutral point (DNP) • Tmin chip carrier αc (Tmin – T0) per unit length board αb(Tmin – T0) per unit length - Net shearing displacement: L(αb - αc)(Tmin – T0) - Difference in the displacement at Tmax and T min: Δ =L(αb - αc)(Tmax – T0) - Shear strain: γ =Δ / h = (L / h)(αb - αc)(Tmax – Tmin) where h – height of solder joint Microsystems Packaging

14. 5.4.2 What is Fatigue? • Fatigue is the most common mechanism of failure and responsible for 90% of all structural and electrical failures. • Occurs in metals, polymers, and ceramics. • Metal paper clip example • Bend in both directions • Repeat the process • Breaks at lower load Microsystems Packaging

15. 5.4.3 Definitions Relating to Fatigue Fracture • Two approaches in determining the number of cycles to fatigue failure: • High-cycle fatigue – based on stress reversals to determine the number of cycles to fatigue failure. • Low-cycle fatigue – based on strain reversals and is used for situations where the material has plastic deformation. Microsystems Packaging

16. Typical Fatigue Load Cycle • Stress vs. time, max & min, ΔS,Sa • Fatigue cycle – successive maxima/minima in load or stress • The number of fatigue cycles to failure designed by Nf • The number of fatigue cycles per second – cyclic frequency • The average of the max and min stress – mean stress, Smean Microsystems Packaging

17. 5.4.4 Predictive Fatigue Models • Used fatigue models for solder joints fall into following categories: • Coffin-Manson-type fatigue model • Strain-energy-based fatigue model • Fracture-mechanics-based fatigue model • Continuum damage mechanics-based model Microsystems Packaging

18. Coffin-Manson Low-Cycle fatigue model • Predict low-cycle fatigue life, Nf, of metallic materials in terms of the plastic strain range: Where m and C are constants and is 1/2 of the plastic strain accumulated over one fatigue cycle. • Solder joint fatigue applications, the fatigue can be expressed with respect to inelastic shear strain range: Where Nf - cycles to failure (fatigue life) - fatigue ductility coefficient c - fatigue ductility exponent Microsystems Packaging

19. Solomon’s Model • Determined low-cycle fatigue expressions for Pb-Sn (Lead-Tin) solder joints for temperatures at [-50, 35, 125, 150] degree C. • Average values: θ = 1.14 and α = 0.51 • In the table are given constants for θ and α Microsystems Packaging

20. Engelmaier’s Model • Based on Coffin-Manson model • The frequency-modified low-cycle fatigue model Where Microsystems Packaging

21. Design Guidelines to Reduce Early Fatigue Failure • The strain increases with the CTE mismatch between the chip carrier and the substrate. Use CTE close to the effective CTE of the chip carrier. • The strain increases with distance from the neutral point. Design distance from the neutral point as small as possible. • The strain in the solder interconnects increases with temperature. Design thermal paths such that the heat is easily dissipated, so that high thermal gradients do not exist. Microsystems Packaging

22. 5.4.6 Design Against Brittle Fracture • Brittle fracture is an overstress failure mechanism that occurs rapidly with little or no warning when the induced stress in the component exceeds the fraction strength of the material. • Occurs in brittle materials (ceramics, glasses and silicon). • Applied stress and work could break the atomic bonds. Where is the fracture strength and E is the modulus of elasticity of the material. • Flaw Modeled as an Edge Crack Microsystems Packaging

23. 5.4.7 Design Guidelines to Reduce Brittle Fracture • Designs with materials and processing conditions that would produce the least stress in brittle materials should be created. • The brittle material should be polished to remove surface flaws to enhance reliability.

24. 5.4.8 Design Against Creep-Induced Failure • What is Creep? • A time-dependent deformation process under load. • Thermally-activated process: the rate of deformation for a given stress level increases significantly with temperature. • Deformation depends on both • The applied load. • The duration through which the load is applied. Microsystems Packaging

25. 5.4.8 Design Against Creep-Induced Failure • Creep can occur at any stress level. • Creep is most important at elevated temperatures. • Homologous temperature: • The ratio of the operating temperature to the melting point of the material in absolute scale. • If homologous temperature is above 0.5, creep will be a problem. Microsystems Packaging

26. Creep Example Creep fatigue failure in a lead/tin solder circuit board connection Microsystems Packaging

27. 5.4.8 Design Against Creep-Induced Failure • Creep Strain Curve • Arrhenius creep equation: • Creep Strain Rate = A(σn)e-(Q/RT) Microsystems Packaging

28. Design Guidelines to Reduce Creep-Induced Failure. • Use materials with high melting point if the application calls for harsh temperature conditions. • Reduction of mechanical stress will reduce creep deformation. • Creep is a time controlled phenomenon. Microsystems Packaging

29. 5.4.9 Design Against Delamination-Induced Failure • What is delamination? • The debonding or the separation of adjacent material layers which were bonded before. • Two Categories • Embedded: delamination occurs in the interior of the package. • Free Edge: delamination occurs at an edge of the package. Microsystems Packaging

30. Delamination Example Delamination in the circuit board assembly Microsystems Packaging

31. 5.4.9 Design Against Delamination-Induced Failure • Causes of Delamination • Processing Issues • Inadequate surface preparation, presence of contaminants, moisture, inadequate baking, inadequate material dispensing. • High Interfacial Stresses Microsystems Packaging

32. Design Guidelines to Reduce Delamination Failure • Careful selection of processing conditions. • Reduce the mismatch in engineering properties between adjacent materials. • Improve adhesion properties between different material layers. • The geometry of the package should minimize sharp corners. Microsystems Packaging

33. 5.4.10 Design Against Plastic Deformation • What is Plastic Deformation? • When the applied mechanical stress exceeds the elastic limit or yield point of a material. • It is permanent. • Excessive deformation and continued accumulation of plastic strain due to cyclic loading will eventually lead to cracking of the component and make it unusable. Microsystems Packaging

34. Design Guidelines Against Plastic Deformation • Limit the design stresses in the packaging structure below the yield strength of the materials used. If possible, use materials that have high yield strength. • Design and control the local plastic deformation at regions of stress concentrations. Microsystems Packaging

35. 5.5 Electrically Induced Failures • What are Electrically Induced Failures? • Failures caused as a result of electrical overstress. • Three Types • Electrostatic Discharge • Gate Oxide Breakdown • Electromigration Microsystems Packaging

36. 5.5.2 Design Against Electrostatic Discharge • What is ESD? • The transfer of electrostatic charge between bodies at different potentials caused by direct contact or induced by an electrostatic field. • Two Types of Failure • Immediate Failure • Delayed Failure Microsystems Packaging

37. Guidelines against ESD • Workstations can be provided with measures like conductive tablemats, wristbands, and conductive flooring. • Air ionizers neutralize static charges on nonconductive materials used in manufacture. • All test and soldering equipment should be provided with ground potential and should be checked periodically. • Antistatic foams can be used for protecting ESD sensitive devices for storage and transportation. • Monitoring devices such as field meters can be used to measure and control static charge on materials. Microsystems Packaging

38. 5.5.3 Design Against Gate Oxide Breakdown • What is Oxide Breakdown? • An electrical short between the metallization and the semiconductor disabling the functionality of a MOSFET. Microsystems Packaging

39. 5.5.3 Design Against Gate Oxide Breakdown • Causes of Oxide Breakdown • Process induced defects or particles. • Accidental discharge of voltage. • The risk of dielectric breakdown generally increases with the area of the oxide layer, since a larger area means the presence of more defects and greater exposure to contaminants. Microsystems Packaging

40. 5.5.4 Design Against Electromigration • What is Electromigration? • Atom flux induced in metal traces by high current densities. • Metal atoms (such as solders) experience a mechanical force and get dislodged from their position. • This results in the formation of metal voids in the conductor, which eventually result in electrical opens. Microsystems Packaging

41. Electromigration Example Before After Microsystems Packaging

42. Design Guidelines Against Electromigration • Electromigration has been mostly noticed in aluminum and silver metallization. Copper traces are more resistant. • Use shorter traces. Tradeoff is more routing layers and greater complexity during fabrication. • Tightly enforce current density design rules based on electromigration data. Microsystems Packaging

43. 5.6 Chemically Induced Failures • What are Chemically Induced Failures? • Chemical process such as electrochemical reactions can result in cracking of vias, traces, or interconnects leading to electrical failures. • Two Types • Corrosion • Intermetallic Diffusion Microsystems Packaging

44. 5.6.2 Design Against Corrosion-Induced Failure • What is Chemical Corrosion? • The chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties. Microsystems Packaging

45. Design Guidelines to Reduce Corrosion • Metals with a high oxidation potential tend to corrode faster. • Use hermetic packages to prevent moisture absorption. • Ensure there are no trapped moisture or contaminants during the processing an assembly of the packages. Microsystems Packaging

46. 5.6.3 Design Against Intermetallic Diffusion • What is Intermetallic Diffusion? • During wirebonding and solder reflow, the joining process generates intermetallic layers which are byproducts of the joining process. Microsystems Packaging

47. Design Guidelines Against Intermetallic Diffusion • Limit the process temperatures and control the time exposed to high temperatures during the joining process. • Control the temperature range and cycles of exposure at the high temperature period. • Application of nickel/gold coating on the bare copper pad surfaces. Microsystems Packaging