Mechanical Design of a Novel Hybrid Power Module

Transcription

1 Prceedings f The Natinal Cnference On Undergraduate Research (NCUR) 2006 The University f Nrth Carlina at Asheville Asheville, Nrth Carlina April 6-8, 2006 Mechanical Design f a Nvel Hybrid Pwer Mdule Chung Bui, Mathew Carlsn, Rbert Fleming, William Lenard Department f Mechanical, Materials and Aerspace Engineering University f Central Flrida 4000 Central Flrida Parkway Orland, FL Faculty Advisr: Dr. Jhn Shen Abstract The purpse f this prject was t redesign an IGBT pwer mdule based n the use f duble metal leadframes, direct leadframe-t-chip bnding, size reductin, and injectin-mlded high temperature plymer encapsulatin. Cnventinal pwer mdules have cmplex material systems cmprised f: multiple semicnductr chips, ne r mre ceramic substrates with metal intercnnect films, a metal base plate, Al r Au wirebnds cnnecting the semicnductr chip t the substrate, sldered jints, epxy casing, silicn-gel ptting material, and external metal psts. This multifaceted material system jins dissimilar materials, which cause reliability issues at high temperatures r under thermal cycling cnditins as well as increases material csts. Figure 1. Mitsubishi IGBT Mdule (Reference 1). Figure 2. Wirebnds f the IGBT Mdule (Reference 2). Figure 2 Wire bnds are used t distributed electrical current and heat. The prpsed design includes the eliminatin f wirebnds by using dual purpse cpper plates that sandwich the chips and prvide a flat surface fr which the heat generated by the chips is dissipated effectively. This wire-free design will nt encunter the parasitic impedance that is present frm the use f metal wirebnds. The reductin in size will be accmplished bth thrugh the height lss frm the plate-t-chip layut and frm the epxy encapsulatin ring arund the sides f the mdule that eliminates the need fr bulky casings. This ring will eventually be injectin mlded t frm an air-tight seal withut any air pckets and will prvide the flexural rigidity needed t withstand external stresses. The design has been accmpanied by Finite Element Mdeling and Thermal analysis fr structural develpment. The thickness f the bdy surrunding the silicn chips including the epxy was apprximated t be 6mm with a cpper plate thickness f 1mm. The manufacturing f the cpper leadframes and the cpper plates will be in the frm f stamping in rder t mass prduce the cmpnents. With the imprvements in size, in impedance and selectin f fewer materials this design shuld be highly sught after. Keywrds: Mechanical, Design, Hybrid

2 1. Intrductin f the Design IGBTs are an imprtant cmpnent t many energy efficient systems including, A/C systems, hybrid Autmbiles, and large scale battery backup systems. Hwever, their current design system is plagued by a cmplex material cmpsitin, inefficient cling, high impedance, high weight, relative large size, and relatively shrt lifespan. IGBTs are a pwer delivery cmpnent which prvides a high vltage and high amperage supply. The design gals fr this prject are t create an epxy supprt fr the micrchips and their cnductive plates which serve as pwer cnnectins, as a heat cnductin surface area, and mechanical supprt. The epxy will serve as a binder fr the cnstruct, as a prtectin frm external stresses, and as an electrical insulatr between the ppsitely charges plates. The challenging aspect f the epxy shape design is that it must prtect the chips while still allwing the installatin f heat cnductin cling devices in the future. The cnceptual design aspects selected require the mdule t have a flat heat cnductin surface area t allw a variety f cling slutins t be munted in the future. An extruded epxy ring will prvide flexural rigidity and heat cnductin area simultaneusly t the mdule. By aligning the chips side by side, the cnductive plates were able t replace the wirebnds frm the riginal design. This reduces the parasitic impedance, eliminates the failures f chips due t wirebnd failure, and reduces the size and weight f the mdule significantly. These design imprvements shuld significantly imprve the efficiency, reliability, and make IGBT mdules suitable fr mre applicatins. Applicatins where size, weight and reliability cnstitute critical characteristics, such as satellite applicatins, may find IGBTs a mre attractive slutin. These design imprvements prpsed will make these next generatin IGBT mdules sme f the best n the market Analysis f the Design In the prcess f designing the Insulated Gate Biplar Transistr Mdule, there are several prblems that need t be satisfied fr the design. The main cncerns f the design are t lwer the impedance, increase reliability f the mdule, increase the dissipatin f heat frm the mdule, and t reduce the size f the mdule. These technical bjectives f the mdule were selected with the guidance f Dr. Shen. Additinally, research was perfrmed t understand whether thse bjectives are cmpatible with the need f IGBT current users. The results frm the research were very similar t the technical bjectives that were determined Technical Objectives The fllwing are the technical bjectives f the imprved mdule design. Limit the uses f many materials. The purpse is t eliminate the uses f many different kinds f materials. Such a mdule cnstructin methd wuld unnecessarily increase the mdule s weight, size, and thermal resistance. Smaller thickness. The expected new mdule will have a apprximate thickness f 3-5 mm. Eliminate Wirebnds. Wirebnds tend t fail by creep defrmatin at high temperature and cause the mdule t be unreliable. The new design mdule will be free frm the uses f wirebnds, and shuld significantly reduce parasitic impedance. Increase Cling Capability. The leadframe design prvides cling n the tp and bttm f the mdule which shuld enhance the reliability f the mdule. 300 degrees Celsius r higher perating temperature. The new mdule s targeted steady state perating junctin temperature is 300 degrees Celsius r higher with an ambient temperature f degrees Celsius. This can be achieved thrugh an imprved material selectin. T achieve these bjectives a stress and thermal analysis with finite element sftware must be perfrmed. Als, a preliminary characterizatin f materials fr high temperature and high temperature cycling must be perfrmed. 2325

3 Finally, a cst estimatin and cmparative analysis with the cmpeting technlgies must be perfrmed. Fr the testing and evaluating f the mdule when the design is cmpleted, a single-chip prttype will be built t demnstrate its feasibility and prvide preliminary mechanical and thermal evaluatin Design Market Cmparisn In rder t adequately develp a unique design a market cmparisn must be perfrmed between the technical bjective requirements and the designs that are already n the market. These cmparisns will als further ur abilities as designers because the cmpetitin gives us a benchmark that we must reach. The pwer mdule industry cmprises a 1-2 billin dllar a year industry. Sme f the majr players in this industry are Tshiba, Fuji and Pwerex. A few f the cmparable IGBT mdels frm each f these cmpanies are: Tshiba (2MBI400N) - This pwer mdule has a 1200V/300A capacity. It is a dual type (2 circuits) Intelligent Pwer Mdule. It weighs 510g. The retail price is $ Fuji (6MBI 10S-120L) The Fuji mdule has a handling capacity f 1200V/300A. It is a 6-pack type (6-circuits) pwer mdule. The weight is 487g and it has a retail price f $ Pwerex (POW-R-PAK_PP600D120) With a pwer capacity f 1200V/600 this half-bridge pwer mdule is indeed ne f the clsest cmpetitrs. It weighs a hefty 21 lbs and retails fr $2, The Pwerex mdule has the majrity f the design specificatins desired. The imprvement ver it hwever will be shwn thrugh a lightweight design and fitting the budget cnstraints f this prject, since the Pwerex is extremely heavy and very expensive. The cmpetitin-benchmarks will be t design a mdule that is at least as light as the Fuji but cntains the abilities f the Pwerex and is less expensive as the Tshiba. 2. Discussin f the Design The final design f the mdule will be based n the finalized slutin that has been chsen frm several ptins that have been generated frm the design prcess. This final decisin was measured thrugh cmparing all f the pssible slutins and determining which slutin is the mst effective in terms f meeting the requirements and perfrming the functins required. The final slutin was determined based n the fllwing factrs. The first factr requires that the mdule has t satisfy the technical requirements set frth earlier in this paper. Such requirements include the amunt f heat transfer capability, parasitic impedance reductin, size, thickness, pwer capability, and strength f the mdule. The secnd factr requires that the mdule must be easily manufactured and at a lw cst when it is in prductin Cnstraints and Parameters High Pwer and High Current Capability In the beginning phase f the design, the prpsed mdule was expected t have the capability f perating 1200V/200A. In rder t build an IGBT mdule f such vltage and amperage capability, it requires the use f a material with high heat and electrical cnductivity in the leadframe cnstructin and the use f as high temperature perating epxy as a binder. Due t budget limitatins the prttype will be cnstructed f a lwer perating temperature epxy and a cpper lead frame Sufficient cling surfaces The expsed surface leadframes will serve as the heat cnductin surface area. The design criterin requires bth sides f the mdule t cnduct heat frm the silicn chips. Thrugh the design prcess, many calculatins have been made t determine the amunt f surface are that wuld be required t dissipate heat generated frm the silicn chips. The slutin t the prblem was slved by having the cpper lead frames areas match with the entire layut area f the eight imbedded silicn chips. This wuld eliminate the prblem f having any prtin f the chips blcked frm dissipating heat. In rder t have a fully heat dissipating system that can dissipate enugh heat frm the mdule, ther cling cmpnents 2326

4 wuld need t be added. It either can be phase change r liquid cling. The mdule cannt t dissipate enugh heat frm the chips by itself. Therefre, the analysis n the thermal capability f the mdule is limited. Fr this particular design, the thermal aspect can nly be simulated by having thermal bundary cnditin n the inside and utside f the mdule assuming that there are ther cling cmpnents is wrking n the mdule. The simulated temperatures given fr this design are 150 degrees Celsius riginate frm the chips and 75 degrees Celsius n the cpper lead frames surfaces. The thermal analysis f the mdule can be seen in figure belw. The verall parameter fr the cling aspect f the mdule is t have duble side heat dissipatin areas. T SET E+08 /hme4/classes/sp05/eml5025/mgc/senir.mf1 VALUE OPTION:ACTUAL 3.35D D D D D D D D D D D Reduce parasitic impedance Figure 3. thermal expansin stress Figure 3 Thermal expansin stress Max stress - 39 MPa ~ 9Lbf Max deflectin mm % The design f this mdule will reduce the parasitic impedance that has been fund in previus mdels significantly. By having the chips slder t the lead frames t eliminate the use f wirebnds wuld reduce the parasitic impedances and the thermal brake dwn f the wirebnds when perating under high temperature Size and weight factr This mdule will have a typical thickness at arund 6 mm which is rughly 5 times thinner than the previus mdule as required. And with the use f the thin cpper plates and epxy as the nly tw majr materials within the mdule, the mdule will be much lighter cmpare t the previus mdule. The new mdule will be ten times lighter. The dimensins f the mdule will be shwed later in the reprt t illustrate the ultra thin factr and the shape and size f the mdule Lw cst and manufacturability This mdule can be manufactured by using the lw cst stamping prcess and epxy injectin mlding prcess that are widely used in the electrnic packaging industry. The prcesses cnsist f stamping the cpper plates int the designed shape, and sldering the silicn chips n the lead frames layer by layer. Then epxy will be injected int the mld t build the casing. These prcesses have already determined the lw cst capability f manufacturing the mdule. In additin the materials that will be use t build the mdule are inexpensive. Ten r mre mdules can be frmed n a single sheet f cpper and a galln f epxy mixer. Fr this prject, building a prttype can be very challenging due t the difficulties f building the mlding that can prduce the unique size and shape f the mdule. Hwever, if the mdule is manufactured in the industry, having a mld fr the prcess is nt a majr cncern. Estimatin fr building a prttype f the mdule and the mlding wuld cst rughly at arund $250 in materials and ther related necessities. Figure 4 belw shws the manufacturing prcess f the mdule packaging. 2327

5 Definitin and Selectin f materials Bending f the Lead-frames with Sheet metal bender CAD mdel Develpm ent fr Cpper lead-frame Silicn chip placement Between tp and bttm Lead-frame Cpper lead-frame Cnstructin using Stamping Prcess Fusin f all three Layers with Slder Paste Placement f lead-frame/chip layer inside Epxy mld Injectin f epxy Material thrugh Varius prts Final Prduct Figure 4. Flw chart descriptin f the prductin sequence. 3. Mechanical Analysis The mechanical prperties f the mdule were studied by applying many pssible lads and stress states the mdule wuld encunter. This prcess was dne by using different methds, hand calculatins and IDEAS 9. In general, the mdule is prtected by the epxy casing. Any damage t the epxy wuld be the damage f the mdule. Hwever, because f the cmplexity f the size and shape f the mdule and the interactin between the cpper plates and the epxy under sme circumstance, mechanical behavirs f the mdule had t be studied as a whle. The fllwing situatins were used t determine the mechanical prperties f the material: 3.1. Distributed Lad Under a distributed lad f 200N apply ver the tp f the mdule, the maximum stress and deflectin were fund t be 75 MPa and 0.4mm which is well belw the yield strength f the epxy and cpper. See figure 4 belw fr mre detail. /hme4/classes/sp05/eml5025/mgc/senir.mf1 ET 6E-04 VALUE OPTION:ACTUAL 4.56D D D D D D D D D D D+00 Figure 5. Distributed lad FEM analysis Impact Lading The mdule was als subjected t an impact frce representing the unit being drpped during the installatin r sme utside impact n the entire device in which the mdule is installed. A mdest estimate f the frce encuntered in a 3 meter drp, which calculated t be apprximately 150N. The stress fr this scenari ranges frm 5 t 25 MPa depending n which surface r edge f the mdule impacts the grund Pint Lading Anther scenari was tested with a pint lad directly in the middle f the mdule. An accurate estimatin f the max frce which wuld be applied t the mdule is apprximately 75N. In rder accunt fr any unfreseen situatins a frce f 150N was tested. With this lading, the maximum deflectin n the unit was 0.129mm, which is nly a 0.02 percent deflectin. The maximum stress was 56.7MPa which is well under the yield strength fr any f the materials used (see figure belw) 2328

6 eml5025/mgc/senir.mf1 Active Design: DEFAULT FE STUDY I-DEAS 9 : Simulatin 21-Nv-05 11:55:58 Units : SI Display : N stred Optin Mdel/Part Bin: mgc-library1 Parent Part: Part1 /hme4/classes/sp05/eml5025/mgc/senir.mf1 CEMENT_1,LOAD SET : 0.00E+00 MAX: 1.30E-0 VALUE OPTION:ACTUAL 1.30D D D D D D D D D D Design Selectin Figure 6. Pint Lad Stress. Figure 6 Max deflectin mm, % deflectin, Max stress 56.7 MPa. The design alternatives were evaluated heavily n the ability t prvide structural stability against all pssible lad cnditins as well as size cnsideratins. After initial analysis, the final ptins were based n a similar cncept with slightly different details. The main cncept was essentially was a sandwich f cpper lead frames with an electrical layut in between the tw plates. Surrunding these plates and prviding structural stability and supprt was an epxy frame. The ptins varied mainly in their apprach t the intersectin between the plates and the epxy frame. The final ptin was chsen based n the fact that letting the cpper plates g directly int the frame prvides fr additinal stability and structural strength. Als, the design is highly simplified by letting the cpper plates g directly thrugh the epxy as ppsed t bending them arund the epxy as required by sme f the ther designs Optimizatin 0.00D+00 The results btained frm the finite element analysis shwed that the mechanical prperties assciated with the initial dimensins fr the mdule mre than satisfied the requirements fr the mdule. Further analysis and design culd lead t mre ptimized dimensins which wuld reduce the size f the mdule while still prviding adequate strength. Hwever, the initial dimensins were partly determined based n requirements which had n bearing n the mechanical prperties f the mdule. The main dimensins were cnstrained by the need t cnfrm t the dimensins f the electrical layut cntained in the unit Assembly The final assembly f the mdule is fairly simplistic. First the electrnic cmpnents are sldered in between the tw cpper plates. Then thrugh the use f a custm made mld, the epxy is pured. When puring any mld the viscsity f the fluid is f great cncern. After testing the epxy, it was determined that within 20 minutes f mixing the viscsity was lw enugh fr the mld t be prperly cast. Figure 7 and 8 belw shw the Final mdule assembly and the epxy mld design fr the prttype f the IGBT mdule. 2329

7 I-DEAS 9 : Simulatin 13-Feb-06 10:41:51 Database: /hme4/classes/sp05/eml5025/mgc/senir.mf1 Units : MM View : N stred Wrkb_View Display : N stred Optin Task : Master Mdeler Mdel: <nne n wrkbench> Figure 7. Final assembly f the Mdule in layers. Figure 8. Prttype Epxy Mld. 5. Cnclusin The mdule packaging has achieved all the technical bjectives prpsed which included the eliminatin f wirebnds by using dual purpse cpper plates that sandwich the chips. This sandwiching reduced the size, and weight f the mdule, reduced the material cmpsitin f the package, and eliminated wirebnds. The epxy binder prvides the flexural rigidity and electrical insulatin between the lead frame plates while prviding the heat cnductin area n the tp and bttm f the mdule. This imprved the thermal perfrmance f the design. 6. References 1. Dr. Ing. Martin Schneider Ramelw, Assembly Technlgies f Pwer Mdule, Fraunhfer IZM (Nv 2005), 2. Enhanced Reliability IGBT & Bims, Mitsubishi Electric (H.K.) LTD. 17/9/1996,