Qualification and Application of Pressure-less Sinter Silver Epoxy Loh Kian Hwa, Nadzirah Yahya, Chin Siew Kheong, Lee Ken Hok Carsem Technology Centre Carsem (M) Sdn. Bhd S-site. Lot 52986 Taman Meru Industrial Estate, Jelapang 30720. Ipoh. Perak. khloh@carsem.com, Tel: 6012-5833098 Abstract Semiconductor industry is getting momentum to use pressure sinter silver to replace solder on high power application due to need to replace the lead in the solder. But pressure sinter silver epoxy requires heavy investment on the pressure molding and oven cure equipment. The existing pressure sinter silver is also need to dedicated to identified package type and this will causing the package flexibility concern. So, we would like to explore the pressure-less sinter silver epoxy. There are already many paper presented by the semiconductor industry and research centre and still a lots of work need to done in order to qualify this pressureless sinter silver epoxy. In this paper, we will discuss the collaboration work between supplier and customer to qualify this pressure-less sinter silver epoxy. Now we are understand the behavior of this pressure-less sinter silver epoxy and can predict the application to suit this epoxy. 1. Introduction 1 st table comparsion we are comparing lead free solution technology, namely lead free solder and Transient Liquid Phase Sintering (TLPS) mixed metal technology.. different is that pressure sinter need to apply pressure on the dies with direct contact during curing. Fig 2 : sinter silver process flow Fig 3 : pressure-less sinter silver process flow Since there is heavy equipment investment on the pressure sinter silver technology. Also coupled with package dedication which is against semiconductor sub-contractor flexibility concept. On top of that customer also concern on the direct pressure on the dies (~ 0.021 MPa). We will focus on the pressure-less sinter silver technology. 2. Material Characterization Carsem Technology Center established in 2007. It consists of material laboratory that equipped with material analysis equipment. For the material technical datasheet that we obtain from material supplier, we can carry out the actual measurement in our laboratory to verify the value compare to technical datasheet claimed value. Fig 1 : Table comparison on lead free technology 20000 19500 Viscosity (cp) Lead free solder biggest problem is solder remelt during convection reflow. This will causing solder ball instrusion and reliability concern. For Transient Liquid Phase Sintering (TLPS) mixed metal technology biggest problem is poor adhesion on leadframe surface that causing poor Moisture Sensitivity Level (MSL) performances. The die attach delamination will caused RDSon failure of the devices. Base on the table comparison 1, we select the sinter silver technology. But sinter silver technology is categoried to 2 type, namely pressure and pressure-less sinter silver. The process flow can refer to Fig 2 and 3. The main 19000 18500 18000 17500 3 hrs 24 hrs 48 hrs 5.0 rpm Fig 4 : Chemical work life - viscosity curve for pressure-less sinter silver
Fig 5 : Physical worklife ThermoGravimetric Analysis (TGA) check on the filler content Chemical work life is defined by the allowable chemical advancement until a 25% increase in viscosity is reached at 25 C. We can use the viscometer to measure the viscosity and plot the graph. Physical work life is defined by the allowable physical separation of adhesive components before significant changes in cured physical properties can be detected. Filler sinking down tend to happen due to density difference between resin (specific gravity approximate 1) and silver filler (specific gravity of silver = 10.5). We can use the ThermoGravimetric Analysis (TGA). Work life is defined by the shorter of the two. Pressure-less sinter silver Differential Scanning Calorimetry (DSC) curve is different from the normal silver paste. Normal silver paste you will have distinct curve with information of starting temperature of the curing reaction, heat of cure and end temperature of the process. You can obtain the temperature that maximum silver paste reaction and can determine the silver paste fully cured. But for pressure-less sinter silver the interpretation of the curve is not straight forward. The reason is that sinter silver concept is curing of the resin, solvent evaporation and sintering. Since DSC measurement is raise the temperature at constant rate (example : 10ºC/mins). It will not able to predict the sintering profile. Fig 7 : Normal silver DSC curve For the TherMomechanical Analysis (TMA) and Dynamic Mechanical Analysis (DMA) curve, we will able to obtain the Coefficient of Thermal Expansion (CTE) and modulus of the epoxy for our simulation model. Fig 8 : Pressure-less sinter silver TMA curve Fig 9 : Pressure-less sinter silver DMA curve Fig 6 : Pressure-less sinter silver DSC curve Thermal Conductivity calculation = Thermal Diffusivity x Specific Heat x Density Thermal diffusivity measurement using laser flash meter. Specific Heat (Cp) measurement using Differential Scanning Calorimetry (DSC) machine and density measurement using densitometer.
Thermal Diffusivity Samples (mm2/sec) 1 60.61 2 54.37 3 53.78 4 57.15 average 56.475 Fig 10 : Thermal Diffusivity raw data on pressureless sinter silver Thermal Conductivity = 3.69 x 0.497 x 56.475 = 103 W/m.K Fig 12 : Sinter silver versus control shear stress on top lead surface post heating up from 25ºC to 260ºC. The thermal conductivity calculation is jive with the supplier Technical DataSheet (TDS) value of > 100 W/mK. We are also cross section and SEM to check on the silver distribution to confirm that various silver thermal conductivity value that jive with the silver distribution. Fig 13 : Sinter silver versus control shear stress on top lead surface post cooling down from 260ºC to 25ºC Fig 11 : Silver distribution comparison SEM image (mag: 5kX) 3. Material Simulation Base on the material characterization data, we will input the data to our siumlation model. Carsem Technology Centre simulation laboratory is using Ansys Mechanical software to predict the shear stress on the top lead surface. We will select one appropraite test vehicles to represent the whole range package family. Fig 14 : Sinter silver versus control shear stress on top lead surface result. The simulation show that pressure-less sinter silver has caused 17% drop of shear stress @ top lead surface against control. 4. Process Characterization on Dispensability We will make sure pressure-less sinter silver can fulfill both nozzle dispense and writing tool dispense. This is due to the nozzle dispense will have much more throughput with higher Unit Per Hour (UPH).
Fig 15 : Nozzle and writing epoxy pattern for pressure-less sinter silver We have run enough touch down (~ 20K) to confirm that no dispense issues for both nozzle and writing epoxy pattern. There is no serious epoxy tailing observed although the viscocity value is high ( ~ 20kcp). The dispense parameter need to optimize and cannot use the existing normal or high thermal silver paste parameter for potting. Fig 18 : Optical image, xray image and epoxy peeling image 6. Process Characterization on Quick Kill Test Quick kill test is to assess the epoxy adhesion to leadframe surface with accelerate condition. It is carry out with bare leadframe with die attach epoxy without molding. With Pressure Cooker Test/ Autoclave (PCT) 24hours to saturate the moisture into the epoxy to leadframe interface. Then use hot stage 260ºC to die shear the units to compare each epoxy adhesion level. Fig 16 : Serious tailing on high thermal epoxy with nozzle dispense and un-optimize parameter 5. Process Characterization on Open Time Study Select an appropraite test vehicle to carry out the open time study. Target is to meet the minimum 180 minutes open time although longer is better. Optical inspection, xray inspection and epoxy peeling inspection to confirm good coverage post 180mins dispense. Epoxy peeling is post die attach, snap cure at on then peel off the dies to inspect die attach area and die back to confirm good coverage. Fig 19 : Quick kill test illustration 7. Process Characterization on Curing Condition Some pressure-less sinter silver epoxy need to use oxygen gas during curing. For nano scale silver, ligand is use to protect molecules to prevent the sintering of nano particles. During sintering, oxygen need to consumed to remove the ligand from the surface. In other case, we have explanation of oxygen will accelerate the sintering. But due to the concern of leadframe oxidation issues, we will not select this type of pressure-less sinter silver technology. Fig 20 : Illustration to explain the oxygen function in the curing of pressure-less sinter silver epoxy Fig 17 : Xray image on 0mins to 180mins
8. Process Characterization on Stencil Printing For special application with 2mils thin die with 100% coverage and < 50% fillet height. Pressure-less sinter silver epoxy is able to stencil print on the leadframe which can fulfill the die attach quality. 10. Test Data on C-Scan Inspection Fig 23 : Pressure-less sinter silver versus control c- scan image comparison Base on test vehicles QFN8x8 package, post MSL 3, 260ºC and Temperature Cycles 500cycles result show that sinter silver epoxy delamination performances is better than control. Fig 21 : Pressure-less sinter silver screen print on leadframe with profile +/- 10um 11. Test Data on RDson data 9. Process Characterization Summary In summary, 2 nd table tabulate the comparison of various pressure-less sinter silver epoxy and we have shortlisted 2 sinter silver epoxy for further work. Fig 24 : Pressure-less sinter silver versus control RDSon measurement comparison There is comparable RDSon performances between pressure-less sinter silver epoxy and control. Fig 22 : Table comparison on various pressure-less sinter silver epoxy.
12. Test Data on Thermal Scan 13. Test Data on Thermal Resistance Thermal Resistance (Rth) measured Fig 25 : Thermal scan during functional test on sinter silver epoxy unit. Base on test vehicles QFN7x7, thermal scan during functional test reveal that pressure-less sinter silver is comparable with 16SOICW package with eutectic bond (80Au20Sn). This will enable 77% body size reduction. Abother example is the TO263 can be replaced by QFN8x8 with 87% body size reduction. ( C/W) Pressure Sinter Silver 4.8 Pressure-less Sinter Silver 4.8 AuSn 5.0 Hybrid Sinter Silver paste 6.3 High Thermal Silver paste 9.2 Normal Silver paste 17.0 Fig 27 : Thermal scan during functional test on sinter silver epoxy unit. Base on test vehicles QFN5x5, thermal resistance measured reveal that pressure-less sinter silver is comparable with pressure sinter silver. Both sinter silver are as good as AuSn. 14. Test Data on Carsem internal qualification Reliability Test Summary Pkg Type Level MSL uhast (130 C/ 85% RH, 205kPa) TEMPERATURE CYCLE (-65 C/150 C) Thermal Shock (-55 C/125 C) HTS @ 150 C 96hrs 200cyc 500cyc 1000cyc 200cyc 500cyc 504hrs 1008hrs 0/220 0/77 0/87 0/87 0/77 0/25 0/25 0/25 0/25 QFN 5x5 MSL 1 0/220 0/77 0/87 0/87 0/77 0/25 0/25 0/220 0/77 0/87 0/87 0/77 WW17 WW19 0/25 0/25 Remarks : 10 units for post TC 500 pull out for DPA. Fig 28 : Reliability test summary table Fig 26 : Illutration on the body size reduction Using Carsem internal test dies, QFN5x5 is meeting MSL 1, 260degC with pressure-less sinter silver epoxy. 15. Conclusions Carsem is a technology solution provider. We are able to work with material supplier and customer to qualify the pressure-less sinter silver and convert it to mass production mode. Below are some points to take note: a) Pressure-less sinter silver is another family compare to normal silver paste, high thermal silver paste and hybrid sinter paste. It will have it own set of parameter. b) There is fact that viscocity of pressure-less sinter silver is much higher. But process parameter will be adjusted to handle it.
c) With proper material and process characterization and simulation, you can achieve the good reliability result of pressure-less sinter silver. Acknowledgments The authors would like to thank Carsem s Chief Technology Officer, Mr. LW Yong, and Carsem s Senior R&D Manager, Mr. KH Lee, for initiating this project and providing the technical guidance along the way. Also thank you for the support from Carsem Material Laboratory, Carsem Simulation Laboratory and Pilot line. Last but not least customer and supplier which cannot disclose the actual name that work with us on this sinter silver project.