Welcome to SMTA Brazil Chapter 2013 Presented by Authors Ivan Castellanos Edward Briggs Brook Sandy-Smith Dr. Ron Lasky Tim Jensen
Advantages / Concerns HP testing Mechanical properties New work Area ratio Experimental design Stencil printing results Reflow results Slide #2
Reduce thermal damage Delamination (PCB, BGAs and other components) Pop-corning (BGAs) Temperature sensitive components Rework Slide #3
Sn melts at 232 C Ga (Gallium) Liquid at room temperature Cd Toxic In Co$tly Bi Slide #4
Metal Source Price / Lb Date Bismuth Metal Bulletin $11.45 Avg. Copper Metal Bulletin $3.32 9/27/2013 Indium Metal Bulletin $343.00 Avg. Lead Metals Week $0.94 Avg. Silver NYMEX $324.00 9/27/2013 Tin Metals Week $10.66 9/27/2013 Slide #5
Near eutectic (137 C - 139 C) It s Pb-Free Similar mechanical properties to Sn63 under many conditions 250 200 150 100 50 0 Reflow Profile Peak Temperatures Sn Bi Ag Sn63 Pb-Free (SAC305) Slide #6
Known as a brittle material Expands on cooling Fillet lifting Low melting eutectic in the presence of Pb Slide #7
Wetting Surface tension Pad metallization Flux activity Wetting Angle Lower flux activation Slide #8
Solder joints experience shear stress (mechanical stress) caused by differences in CTE (coefficient thermal expansion) Slide #9 http://www.emeraldinsight.com/journals.htm?articleid=1509648&show=html http://www.ami.ac.uk/courses/topics/0123_mpm/º
However, HP found that in comparison to Sn63, Bi Sn had higher shear strength at 20 C, nearly equal at 65 C, and lower (but comparable) at 110 C. For more information please refer to 57Bi-42Sn-1Ag: A Lead Free, Low Temperature Solder for the Electronic Industry. Ferrer, Holder
Constant load applied at elevated temperature causing deformation or flow over time Performed using Bi Sn (no data available for the Bi Sn Ag) Strain vs. Stress Bi Sn exceeded Sn63 at 25 C and 65 C For more information see Low Temperature Solders Mei, Holder, and Plas Slide #11
Interesting finding during thermo-cycling (-20 C - 110 C) Cu OSP vs. Sn Pb HASL surface finish Sn Pb HASL / Bi Sn Ag failed ~ 500 cycles Low melting eutectic 96 C Later studies concluded < 0.3% wt. Pb required, 0.1% recommended Lead-free transition Slide #12 ASME Technical Paper 95-WA/EEP-4
Onset Temp, deg. C 250 225 200 175 150 125 100 75 0%Pb 3% Pb 6% Pb 50 Sn-3.8Ag- 0.7Cu Sn-2Ag-2Bi Sn-2Ag-4Bi Sn-2Ag-7.5Bi Sn-10.5Bi Sn-12Bi
Small additions of Ag or Au Increased thermal fatigue to 7000cycles (-25 C - 75 C) outperformed Sn63 Even at 0 C to 100 C (absence of Pb) was comparable to Sn63 Slide #14
3 PCB surface finishes: Cu OSP, ENIG and Imm Sn 256 PBGA (Plastic Ball Grid Array) 4 different sphere alloys: 96.5Sn/3.5Ag 95.8Sn/3.5Ag/0.7Cu 99.3Sn/0.7Cu Sn63 4 point bend test on as assembled boards after aging isothermally 90 C: 1, 3, 10 days Finding: paste volume critical when using high Sn solder spheres Slide #15
High Sn alloy solder spheres didn t collapse Low temperature reflow process for Sn Bi Ag solder paste All failures were in the Bi Sn Ag solder Adequate paste volume increased solder joint strength so that failure was by pulling out the Cu pad The example here 30mil sphere; Sn63 4000 4500 mil3 Sn Bi Ag 8000 9000 mil3 Square vs. circular aperture Slide #16 Strength and Fatigue Behavior of Joints Made With Bi-42Sn-1Ag Solder Paste: An Alternative to Sn-3.5Ag-0.7Cu For Low Cost Consumer Products Hua and Gleason
Lower surface tension Bi Sn Ag Backside components (May not hold bottom side components) Print and placement accuracy (No self alignment might occur) Surface finish Cu OSP gave best results wetting/performance Imm Ag / ENIG / Imm Sn acceptable wetting/performance Bake-out (If any bake process is needed) 12 hours at 125 C too harsh 12-15 hours 90 C (Recommended if baking process is need it) De - Panelization More adequate fixturing / board support (Due to fragility of the Bi containing alloy) Slide #17
Critical metric for paste print performance is the Area Ratio The area of stencil aperture opening divided by the area of aperture side walls Simplifies to D / 4T (circles and squares) > 0.66 widely accepted as industry standard < 0.66 stencil clogging Transfer Efficiency Volume of solder paste deposited divided by the volume of the aperture (actual vs. theoretical) The Experiment Optimized stencil, tooling, and printer settings were utilized from previous experimental designs to evaluate need to change from current Sn63 or lead-free set up Type 3 powder, 57Bi / 42Sn / 1Ag alloy, no-clean flux, 90% metal load Print 8 boards (no under stencil wipe) Evaluate using Koh Young paste measurement tool 18
Area ratio 0.67 Included the most data points; 6400 Slide #19
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Larger apertures (16mil) circles performed better than squares At 14 mil circles and squares nearly equal < 14mil (area ratio approaches 0.66) squares outperform circles For the same area ratio: circles vs. squares Larger apertures rely on stencil gasket to PCB for best transfer efficiency Smaller apertures have less surface area for adhesion of solder paste to PCB pad to help pull the solder paste from the stencil aperture The walls of the solder mask on SMD pads may increase surface area for adhesion of the solder paste improving release from stencil aperture Slide #22
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Test vehicle Imm Ag pads Components Imm Ag Via in pad 5 x 5 11 x 11 Reflow X-ray Printed paste 1:1 on ground plane to exhibit worse case scenario Slide #24
Bi Sn Ag can reduce thermal induced stresses Lower cost option Concerns Pop Corning effect on BGA Delamination issues Ternary alloy formation in presence of Pb Similar mechanical properties to Sn63 (in absence of Pb) Low melting eutectic Paste volume critical Printing process same as Sn63 or Pb-free (SAC305) Reflow characteristics similar or equal to Sn63 or Pb-Free (SAC305) (But lower surface tension and baking issues) Slide #25
Dúvidas? Perguntas? Ivan Castellanos Technical Manager for Latin America Indium Corporation icastellanos@indium.com
How Did We Get Here? Sn/Sb Sn/Cu Sn/Ag Melting Point Sn/In/Ag Sn/Bi/Ag Sn/Ag/Cu Sn/Pb Melting Point Bi/Sn
Sn / Ag / Cu (SAC) was the best current option SAC305 ranked slightly higher than other SAC alloys due to cost and performance. Sn / Cu alloys good enough for wave soldering But SAC solders DO have challenges
Process Temperature Challenge Wetting Challenge Voiding Challenge Fragility Challenge High Operating Temperature Challenge
Lower hardness (such as low Ag SAC) Dopant which reduce IMC thickness, scallop size, or fragility, such as Mn, Ti, Y, Bi, Ce, Ni, Co, Pt Dopants which reduce Kirdendallvoid formation, such as Ni, In, high Cu Alloys with low tendency of forming large IMC plate needed (Reduced Ag content or rapid cooling)
Low Ag primarily being adopted for cost reduction Thermal cycling decline typically outweighs drop test improvement Simple math calculation of cost savings is deceiving A better option is desired
Still Inferior to Sn Pb
SAC Brittle Fracturing Brittle Fracture Ductile Fracture
Although SAC105 is better than SAC305 at mechanical shock resistance, segments of the industry need better Many can t live with the reduced thermal cycling performance of SAC105 SACM provides better drop test than SAC105 without losing the thermal cycling performance SACM is a SAC alloy with 0.5-1.0% Ag, 0.5-1.0% Cu, and dopant levels of Mn
Test Conditions: -40 to +125 C with 10 min dwells. Pre-aging of 250 hours at 150 C
Fresh Aging 150C for 1000 hours SAC105 SAC305 Stabilized Intermetallic SACM Stabilized Intermetallic Suppressed grain coarsening
Melting Point Tensile Strength Yield Strength Young s Modulus Elongation Solidus Liquidus PSI PSI KSI % SACM 217 C 227 C 5625 3590 2110 15.7 SAC105 217 C 227 C 5640 3359 2150 13.4 SAC305 217 C 220 C 7200 5289 2410 19.3
SAC305 SAC105 SACM
Drop Test SACM exhibits finer and thinner IMCstructure at interface. Inclusion of dopants in IMC may also alter the crystallinity, hence may reduce the brittleness of IMC layer. TCT Test A stable and fine IMCstructure may be the primary contributing factor, and the stabilized grainstructure resulted may be the secondary cause for SACM to exhibit a high TCT reliability.
Dúvidas? Perguntas? Ivan Castellanos Technical Manager for Latin America Indium Corporation icastellanos@indium.com