Electronics Assembly

1. Electronics Assembly by Brent Staley

2. Materials & Design, Volume 30, Issue 10, December 2009, Pages 4502-4506 S. Mallik a,*, N.N. Ekere a, R. Durairaj b, A.E. Marks a, A. Seman a Wall-slip effects in SnAgCu solder pastes used in electronics assembly applications

3. Solder paste plays the crucial role in electronics assembly Provides electrical and mechanical bond between the electronic components and the substrate (wafer) Manufacturers of solder paste are continuing to striving to enhance its properties for miniaturization and the demand for “green” products This paper discusses: - The problems solder paste faces with slip conditions at the boundary layers - Improving the Rheological measuring methods of lead free solder paste - The experimentation with serrated and roughened surfaces to reduce wall-slipping Introduction

4. Originates from steric, hydrodynamic, viscoelastic, chemical and gravitational forces acting on the along the wall This causes a low viscosity depleted layer to form between the wall and the bulk fluid This layer acts as a lubricant to produce slip effects Conditions that cause slip effects: Large dispersed (or flocculated) particles Concentrated solutions Low flow rate Electrically charged particles Walls with smooth geometries Wall Slippage

5. Study of the flow of matter Non-Newtonian fluids-viscosity changes Rheometer used rather than a viscometer (Shear strain is applied to the fluid by the motor the resulting torque from the fluid is measured by the transducer) Requires more parameters to be set and measured Rheology

6. Shear-thinning: decrease in viscosity with an increasing shear rate or stress. Responsible for the effective transfer of solder paste through the aperture during the stencil printing process Thixotropic: Time-dependent decrease in viscosity when sheared at a constant rate, until an equilibrium is reached Yield-point: Minimum shear stress/shear rate required to initiate paste flow. A yield point value above zero is considered helpful to avoid slumping after the paste printing Visco-elasticity: Characteristics of having both viscous and elastic behaviour Desirable Rheological Characteristics of Solder Paste

8. Rheometers with smooth measuring geometries (e.g. smooth parallel plate and smooth cone-and-plate measuring geometries) are more vulnerable to wall-slip effects To correct the effects, roughened surfaces can be used such as grooved or corrugated geometries Sandblasting with a coarse grit or gluing a sand paper can also be used to roughen a metallic surface

9. Three lead-free solder pastes were used for the investigation (named as LF1, LF2 and LF3) In all three pastes, the powder alloy was Tin–silver–copper near eutectic solder (Sn96.5 Ag3.0 Cu0.5) All the constituents crystallize simultaneously at this temperature from molten liquid solution The material was suspended in a no clean flux medium The two varying factors between the pastes were particle size distribution (PSD) and composition of the flux system Experiment

10. Particle Sizes for Each Paste • An equal gap width of 0.5 mm was used for both smooth and serrated parallel plate measuring systems used in this study • The compositions of the flux system (F1, F2 and F3) are not known (because of the proprietary nature of the information) • Solder paste with different flux system are expected to show different deformation behaviors and flow characteristics

11. All rheological measurements were conducted using a Bholin Gemini-150 rheometer (Malvern Instruments Ltd., Worcestershire, UK) Three parallel plate measuring systems with different surface roughness values were used The roughness values of the plates were measured using a Surtronic 3+ (Rank Taylor Hobson Ltd., UK) portable roughness measurement device A shear rate range from a very low value (0.0005 s1) to a moderate value (0.1 s1) was used for the flow test study (low shear rate) Steady-state flow curves were obtained by progressively increasing the shear rate applied to the sample

13. Steady Flow Curve LF1 Paste

14. Steady Flow Curve of LF2 Paste

15. Steady Flow Curve of LF3 Paste

16. All flow curves measured using the serrated plate geometries exhibit similar behavior: an initial increase in viscosity and then shear thinning behavior after the critical shear rate The smooth parallel plate on the other hand showed the discontinuous behavior at low shear rates As the shear rate increases though, the viscosities measured from all three geometries converged This showed that the serrations on the surfaces prevented the formation of a depleted layer between the solder paste and the solid boundary The serrations cause the plates it to have good grip onto the solder paste sample during the application of shear Results

17. The roughness on SPP-2 was higher the SPP-1; so why was it lower in viscosity? The high roughness value of the SPP-2 geometry might have actually helped the paste to break-down The roughness of the SPP-2 geometry was probably more than it required to avoid slip effects

18. Solder paste can also be classified as a visco-elastic fluid The solder paste has both solid and liquid properties Oscillatory tests are used to study the solid and liquid characteristics of solder pastes to know the viscous and elastic properties of the pastes This done by applying a sinusoidally varying stress (or strain) to the sample and measuring the induced sinusoidally varying strain (or stress) response Two types of oscillatory tests (stress-sweep test and frequency-sweep test) were conducted using both smooth and serrated plate geometries Visco-elastic Effects

19. Stress was varied from 0.5 to 500 Pa with a constant frequency of 1 Hz Identifies the linear visco-elastic region (LVR) and determines how strong or weak a sample is The maximum deformation that can be applied to the sample without destroying it A longer LVR enables the sample to ‘withstand’ a broader range of deformations before the structure breaks down This test also determines the crossing over point from liquid to solid or vice versa; usually the lower the value of stress at the crossing over point, the more liquid-like the sample Stress-sweep Test

20. For all the samples, serrated plate geometries produced higher values for both the LVR and crossing over point compared to the smooth plate geometry A higher value for the end point of the LVR (for serrated plate geometries) implies that the use of serrated plates has actually made the solder paste samples more stable and solid like Results of Stress Tests

21. The frequency was varied from 0.1 to 20 Hz with the applied stress constant Frequency-sweep test is capable of producing a unique fingerprint of the sample and is quite useful to demonstrate the relative process time behavior Again, the serrated plate geometries produced higher values for the crossing over point when compared to the smooth plates Frequency-stress Test

22. The viscosities measured by the serrated plates were much higher than the viscosities measured by the smooth plates The serrated plates also created a higher crossing over point value and helped the paste to maintain a more solid like behavior by preventing the formation of a thin depleted layer of continuous phase next to the walls of the measuring geometry As the shear rates increase, the slip effects seem to disappear with the viscosity values from both the serrated and smooth plate geometries were converging with each other Serrated plates help prevent wall –slippage, but must not be serrated too much so they don’t help break down the paste Conclusion

23. The experimental technique demonstrated in this study could be of great help to the research and development personnel involved in new paste formulation and for developing quality control procedures used in solder paste manufacturing and packaging It’s hard for me to draw conclusions to these results: -Don’t know the composition of the flux -I thought they needed to implement more disparity in their factors to obtain more accurate results from a compete DOE -The roughness of the surface obviously affects the wall- slipping, but you would have to perform these experiments again to optimize your system Application

24. W. R. Schowalter (1978) Mechanics of Non-Newtonian Fluids Pergamon ISBN 0-08021778-8 [1] Lapasin R, Sritori V, Casati D. Rheological characterisation of solder pastes. J Electron Mater 1994;23/6:525–32. [2] Anderson R, Maria FG, Kolli VG. Solder paste rheology and fine pitch slump. J SMT 1995:12–8. [3] Kolli VG, Gadala-Maria F, Anderson R. Rheological characterisation of solder paste for surface mount applications. IEEE Trans Components, Packaging and Manufact Technol, Part B 1997;20/4:416–23. [4] Bao X, Lee NC, Raj RB, Rangen KP, Maria A. Engineering solder paste performance through controlled stress rheology analysis. J Soldering Surf Mount Technol 1998;10/2:26–35. [5] Nguty TA, Ekere NN, Adebayo A. Correlating solder paste composition with stencil printing performance. In: IEEE/CPMT international electronics manufacturing technology symposium, Austin (TX); 1999. p. 304–9. [6] Durairaj R, Mallik S, Marks A, Winter M, Bauer R, Ekere NN. Rheological characterisation of new lead-free solder paste formulations for flip-chip assembly. In: 1st Electronics system integration technology conference, Dresden (Germany); 2006. p. 995–1000. References

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