Rel Assess Using Finite Element Techniques ADA216907

8/8/2019 Rel Assess Using Finite Element Techniques ADA216907

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This re po rt has been review ed by th e RA DC Pub lic Affairs Offic e (PA) and isre le asa b le to the N at iona l Tec hn ica l In fo rma t ion Serv ice (NTIS) . At NTIS i t wi l l bere leasab le to the genera l pub l ic , inc lud ing fo re ign na t ions .RA DC TR -89-281 has been rev iewed and is approv ed fo r p ub l ic a t ion .

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ANT HONY J F E DUCCI AChief, Rel ia b i l i ty & Engine er ing Div is ionDi r e c to r a t e o f Re l i a b i l i t y a n d Co m p a t ib i l i t y

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WORK UNITACCESSION NO.2X11. TITLE (Include Security Classification)

RELIABILITY ASSESSMENT USING FINITE ELEMENT TECHNIQUES12. PERSONAL AUTHOR(S)

Gretchen A. Bivens13a. TYjPE OFREPORTTn-Hor--[ouse 13b. TIME COVEREDFROM .Ian 88 TO lu l 88 14. DATE OF REPORT (Year, Month, Day)November 1989 15. PAGE COUNT76

16. SUPPLEMENTARY NOTATIONN /A17.

FIELD1*20

COSATI CODESGROUP0*13

SUB-GROUP18. SUBJECT TERMS (Continue on reverse if necessary and identify by block number)Rel ia b i l i t y Pred ic t ion Techn iquesFini te Element Analys is M ate r ia l Fai lure

19. ABSTRACT (Continue onreverie if necessary and identify byblock number)Fini te e lement analys is (FEA) is a computer s imu la t ion techn ique tha t is used to p red i c t the t h e r m a land mechanical response of a s t ruc tu re . Rome Air Deve lopment Center (RADC) has used FEA top red i c t the response in var ious e lec tron ic assembl ies. This par t icula r s tudy was concerned w i th ther e l i ab i l i t y of surface mounted microwire boards. A FEA of this assembly was s imu la ted and the resul tsused to es t imate its useful l i f e . This repo rt g ives a deta i led acccount of the f a c t o r s to be consideredwhen per fo rming a FEA and the procedure used to t rans fe r the results to a r e l i ab i l i t y f i gu re -o f -me r i t .

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21 . ABSTRACT SECURITY CLASSIFICATIONUNCLASSIFIED22a. NAME OF RESPONSIBLE INDIVIDUALGRETCHEN A. BIVENS 22b. TELEPHONE (Include Area Code)(315) 330-4891 22c. OFFICE SYMBOLTJABCi " " " " "DD Form 1473 . JUN 86 Previous editions are obsolete. SECURITY CLASSIFICATION OF THIS PAGE

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LIST OF ILLUSTRATIONS

Figure1234567891011121314151617181920212223242526272829303132

TitleFour Surface Mounted Package DesignsMicrowire ConstructionMicrowire ConstructionMicrowire Construction3-D Solid ElementsTypical One-Quarter GeometryStress-Strain Curve for a Ductile MaterialBounded Plastic RegionCoffin-Manson ModelCoffin-Manson Curve for CopperCoffin-Manson Curve for SolderS-Lead System FEM GenerationS-Lead System Superelement ModelS-Lead System FEMLeadless System FEM GenerationLeadless System Superelement ModelLeadless System FEMGull-Wing System FEM GenerationGull-Wing System Superelement ModelGull-Wing System FEMMicrowire Board FEMS-Lead Superelement ModelS-Lead FEMSolder Joint: X-Y PlaneSolder Joint: X-Z PlaneSolder Joint: Y-Z PlaneLeadless FEMGull-Wing Superelement ModelGull-Wing FEMWire Within a BlockMeshed BlockMierowire/PTH FEM Generation

Page678811121415161819232425262728293031323334353637383939414143

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33 Microwire/PTH FEM 4434 Modified Microwire/PTH FEM (20 Mil Via) 4535 Modified Microwire/PTH FEM (10 Mil Via) 4536 Stress Contours for S-Lead FEM 5037 Stress Contours for Leadless FEM 5338 Stress Contours for Gull-Wing FEM 5539 Stress Contours for Microwire/PTH (20 Mil Via) FEM 5840 Stress Contours for Microwire/PTH (10 Mil Via) FEM 59

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0.0 EXECUTIVE SUMMARY

Finite element analyses (FEA) are increasingly being used to predictthe mechanical response of an electronic assembly exposed to thermal andvibrational environments. The results of these analyses are used to estimate a component's life. This technique provides engineers with a methodof assessing the reliability of emerging technologies associated withelectronic systems. This particular investigation was concerned with thereliability of surface mounted solder interconnections and microwireboard's plated-thru-hole (PTH) connections when exposed to severe thermalenvironments. These two interconnections were analyzed using FEA and anestimate of the number of cycles to failure was made.

There are many factors that must be considered before performing afinite element analysis. Are the materials relatively well known and theproper material properties available for this analysis? Are the boundaryconditions, input conditions and environment of the system known? Howcomplex is the geometry to be modeled? Can any assumptions be made tosimplify the model? It is important that the engineer performing theanalysis be aware of the information available in order to decide whetheran analysis is feasible and if so, what the best approach to performing thestudy would be.

When modeling a new, complex system, preliminary modeling becomesimportant. Investing this initial effort reduces the overall analysistime and cost, and increases the results' accuracy. Examples of

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preliminary modeling which were performed in this investigation are documented in the body of this report.

The results of the FEA were plotted on either the Coffin-Manson curvefor copper or for solder, depending on the material being analyzed. TheCoffin-Manson curve relates the material's strain to the number of cyclesto failure. This cycles to failure value gives an estimation of the lifeof these connections. This method of transferring FEA results to anestimated life allows the design and reliability engineer to locate areasof reliability concern and make the corresponding corrections early in theprogram. This reduces the overall cost of the program and allows thereliability of the system to be built in. This technique is especiallyimportant for military systems incorporating new technologies.

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1.0 INTRODUCTION

RADC has used finite element analysis (FEA) as a way to assess thereliability of emerging technologies. In the past , the resulting stressvalues were taken from the FEA results and compared with the strength ofthe corresponding materials. It was soon realized, however , that a technique was needed to transfer the numerical results from the FEA to ameasure of useful life for the system being analyzed. This report documents a reliability assessment of the areas of reliability concern inelectronic assemblies through the use of FEA and a transfer of FEA results

to an estimation of the useful life for the areas of concern.

Two areas of reliability concern were identified for the electronicassembly analyzed. The first area was the solder connection between asurface mounted device and its connecting board. Current use of thesedevices has shown that these connections tend to fail soon after the systemhas been installed. Three different lead designs were evaluated, the S-lead, leadless, and gull-wing designs. The second area of concern was theinterface connection between the microwire and the PTH. Microwire layering is a new technology that is being used to replace conventional signallayers in a surface mounted board. The connection with the PTH is criticalto the use of microwire technology.

Once the areas of reliability concern are identified, the evaluationengineer determines the best way to develop the finite element model. Ageometric model representing the physical system is generated and a data

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file which describes the materials used, the boundary conditions, and theinput conditions is created. This data file is read by the finite elementsimulation code and a thermal analysis performed. From this analysis, thestrain range for the material of concern is obtained. Once the strainrange in a monotonic thermal cycle is known, theoretical relations are usedto estimate the number of thermal cycles this material would survive beforefailure.

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2.0 RELIABILITY CONCERNS IN ELECTRONIC ASSEMBLIES

As technology has advanced and electronic assemblies become more complex, there is an increasing concern pertaining to the reliability of thesesystems. A highly specialized system that cannot perform its function isuseless to the overall system. This is very critical when the system is amilitary aircraft or spacecraft. However, in order to implement many ofthe emerging technologies, unique packaging designs must be developed. Ifthese unique designs are analyzed in the initial phase of a program,considerable time and money are saved.

The military was concerned with the reliability of surface mountedmultilayer assemblies investigated in this study. These boards are usingnew mounting techniques and large packaging densities which have led toproblems in the reliability of the interconnects. In particular, thefailure of the leads of the surface mounted components is currently adriving failure mechanism in the reliability of these boards. Figure 1shows typical surface mounted packages for four different lead designs.Factors which have been shown to effect the solder connection failureinclude the lead design, package size, board material and heat removal.

In order to minimize the solder connection failure mode, board materials which reduce the board's horizontal expansion were used. This reduc

tion has caused an increase in the board's vertical expansion which has ledto an increase in PTH connection failures. This assembly used microwire

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H.O FINITE ELEMENT MODELING

When the engineer performing the FEA has an extensive knowledge in the

area being analyzed, assumptions about the geometry, materials, and boundary and input conditions can be made. This is not always the case. Anengineer may be given the basic information and asked to investigate thereliability of the system. Engineering judgment can be used to form someassumptions and other assumptions can be made as a result of preliminarymodeling. Preliminary modeling is very important because incorrectassumptions can have a significant effect on the results of the analysis.These models are simple and are analyzed in a short time period compared tothe overall time of the analysis. If the preliminary models result inassumptions that simplify the model, the time spent performing the analysis is reduced. Since most systems are too complex to be modeled in exactdetail, assumptions have to be made so that the analysis can be performedin a timely manner.

Another reason preliminary modeling is useful is in reducing thechances of round-off error when modeling complex geometries. Round-offerrors can result from several different factors including elements thatare too large, are improperly formed, and have unusual geometries. Simplemodels representing the general shape of the complex geometry can indicate

to the engineer whether the model's basic design will have problems beforethe more complex model is developed. The finite element model of themicrowire and PTH connection interface is an example of this.

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4.1 MODELING OF COMPLIANT CONNECTIONS

Three different compliant connections were modeled using the samegeneral procedure. A model representing the pack age, board, and connectionsystem was modeled. This is referred to as the system model. The deflections resulting from a thermal analysis of this model were then inputtedinto a model representing the compliant connection. Because the system hassymmetrical geometries and boundary conditions, only one quarter of thesystem was modeled. The compliant connection did not have symmetricalboundary conditions, therefore , the entire connection was modeled.

During the generation of the system mode l, it became apparent that themodel had too many elements for RADC's computer capabilities. To reducethe large number of elements, the eight layers of microwire board wererepresented by a separate model and the resulting deflections inputtedinto the system mod el. Only the top layer of the microwire board was

represented in the system model. Once both analyses were simulated, acomparison was made between the deflections along the top surface of theboard for the board-alone model and for the system model. The resultsshowed that the differences in the deflections were negligible and, ther efore, this procedure was accurate.

4.1.1 SYSTEM MODELS

When generating a system finite element model , the following procedure was used. The figures given will represent the generation of the S-

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lead system model. The S-lead package was a .45 x .55 inch package with 32leads. First, a two-dimensional drawing representing a segment of thepackage and board and the S-lead was generated (Figure 12a). This modelwas then extended in the third dimension (Figure 12b). Now a second two-dimensional set of elements was created in a direction perpendicular to thefirst set. This set represented the same configuration as the first setand the nodes along the left side were taken from the original set ofelements (Figure 12c). This second set of elements was then extended inthe third dimension in the direction opposite to the first set of elements(Figure 12d). Now two sets of three-dimensional elements existed. Thesesets of elements were defined with different orientations and, therefore,the second set of elements was redefined to conform with the first set ofelements.

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Figure 12: S-Lead System FEM Generation

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Figure 14: S-Lead System FEM

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S i n c e t h e l e a d l e s s c h i p c a r r i e r (L CC ) w as a l s o a .4 5 x . 5 5 i n c hp a c k a g e w i t h 3 2 c o n n e c t i o n s , t h e sa m e p r o c e d u r e w as u s e d t o g e n e r a t e t h es y s t e m m o d e l f o r t h e LCC a s s ho wn i n F i g u r e s 1 5 , 1 6 , a n d 1 7 . T he g u l l - w in gp a c k a g e w a s a . 9 4 x . 9 4 i n c h p a c k a g e w i t h 1 72 l e a d s . T h e s u p e r e l e m e n tm o d e l wa s g e n e r a t e d u s in g t h e s am e p r o c e d u r e a s u s e d f o r t h e S-lead s y s t e m .

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Figure 16: Leadless System Superelement Model

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Figure 17: Leadless System FEM

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Due to t h e l a r g e p a c k a g e s i z e a n d t h e l a r g e n um b e r o f l e a d s , t h e m o d e lc o u ld n o t b e m e sh ed so t h a t t h e n o d e p o in t s wo u ld c o r r e s p o n d to t h e l o c a t i o n o f t h e l e a d s . T h e r e f o r e , t h e f i n a l m od el h ad a p p r o x i m a t e l y t h e s am em e s hi ng c o n f i g u r a t i o n a s t h e o t h e r m o d e ls a nd t h e m a t e r i a l p r o p e r t i e s oft h e s o l d e r an d a i r m a t e r i a l s w er e c om b in ed t o g i v e a s e t o f e q u i v a l e n tp r o p e r t i e s . The g u l l - w i n g s y s te m m od el i s r e p r e s e n t e d i n F i g u r e s 1 8 , 1 9,and 20.

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Figure 18: Gull-Wing System FEM Generation

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Figure 19: Gull-Wing System Supereleraent Model

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Figure 20: Gull-Wing System FEM

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the solder joint model contained only solder elements. Each model has aregion representing the lead's contact with the edge of the package and aregion representing the lead's contact with the top surface of the board.Deflections from the system models were inputted along these regions and astatic analysis simulated. The resulting stress values in the solder werethen used to estimate the lead's useful life.

The generation of the S-lead model was straightforward. The entiremodel was completed in a day. First, a two-dimensional (2-D) model of thelead and the attached solder was created. Eight noded 2-D elements wereused in order to obtain the circular geometry, Figure 22a. This model wasthen extended into the third dimension and 20 noded, 3-D elements weregenerated, Figure 22b. This model was then meshed and the final model isdisplayed in Figure 23.

(a) 2-D FEM 3-D FEM

Figure 22: S-Lead Superelement Model

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Xy RX= egR2= 0

Figure 2 3 : S-Lead FEM

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The ge ner a t io n of the lea dl es s so lde r j o i nt was a more complexpr oc es s . This model took app roxim ate ly two weeks to com plete . Drawingsre pr es en t in g the x -y , x-z and y-z plan es were used as guide s in the ge ne rat ion p ro ces s , F igures 24 , 25 , and 26 . Th is mode l re pr ese n t s a jo in t usedon a package with 50 mil spac ing between j o i n t s . The package has c a s t i l l a -t io ns and , th e r e f or e , the so lde r curves in to the package . The segmentrep res en t in g the s o ld e r base was s imple to gen era te s in ce the x -z p lane i sun iform in the y - d i r ec t i on . The f i l l e t e lement , however, had to bei n p u t t e d i n d i v i d u a l l y s i n c e t h e s e e le m e n t s v a r y i n a l l t h r e e d i m e n s i o n s .The base elem ents were meshed and then th e f i l l e t nodes and elem ents werede fin ed. The re s u l t in g model is shown in F igu re 27.

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Iy RX = ea-~x RY= -30RZ= 0

Figure 27: Leadless FEM

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T h e g u l l - w in g m o d e l was a l s o s im p le t o g e n e r a t e . A 2 -D m o d e l wa sc r e a t e d u s i n g e i g h t n od ed e l e m e n t s r e p r e s e n t i n g t h e s o l d e r , l e a d a nd b r a z i n g m a t e r i a l s , F i g u r e 2 8 a . T h i s m o de l w as t h e n e x t e n d e d i n t o t h e t h i r dd im e n s io n a n d 20 n o d e d e l e m e n t s we r e g e n e r a t e d . T h i s m o d e l wa s t h e nm e s h e d , F ig u r e 2 8 b . A s e g m e n t o f t h e l e a d h a d t o b e e x t e n d e d i n t h e z -d i r e c t i o n b e f o r e t h e f i n a l m o d el w as c o m p l e t e d , F i g u r e 2 9 .

F ig u r e 2 8 : Gu l l - W in g S u p e r e l e m e n t M o d e l

F i gu re 2 9 : G ul l -W ing FEM

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4.2 MODELING OF COMPLEX CIRCULAR GEOMETRIES

When generating a FEM, the analysis will create a superelement model

outlining the general shape of the system. Normally the different elementsin the system will be of the same shape, i.e. circular, rectangular,triangular, etc. In the system that was analyzed, however, there were notonly circular geometries embedded within rectangular geometries, but alsocircular geometries intersecting with other circular geometries. Insteadof attempting to use a combination of rectangular and circular elements,only quadralateral elements were used. Because these elements have cornerand midside nodes, rectangular and circular geometries can be defined.

It was obvious that the creation of this model would be complex andtake a considerable amount of time. Therefore, preliminary models werecreated in order to check for round-off error problems in the approachused. The first model created was a wire within a block (Figure 30). Ananalysis was simulated and no problems occurred. This model was thenmeshed in all three directions and elements were removed from the centerarea (Figure 31). Once again, the simulation indicated no problems withthis basic design. From this model, it was obvious that only one quarterof the system would need to be modeled for the ideal case where the wirepassed directly through the center of the hole. Generation of the actualmodel now begins.

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Figure 30: Wire Within A Block

Figure 31: Meshed Block

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Using the prop er dimens ions for the sys tem , anoth er 2-D supere lementmodel usin g ei gh t noded elem ents was cr ea ted (Fig ure 3 2a ). This model wasmeshed and a fi n e r model ge ne rate d (Fig ure 32 b) . This f in e r model was thenextended back in to t he t h i r d dimens ion and 20 noded e lemen ts were gen era te d . This model was the n meshed (Fig ure 32c) and th e m at e ri al s werepro per l y id en t i f i ed fo r a l l the e leme nts . Two se c t i on s had to be removed .The f i r s t rep rese n ted the a i r e lements su r roun ding the top o f the p la t in gand th e second rep res en ted t he a i r e lem ents wi thin th e PTH (Figure 32d ) .F igure 32e di sp la ys an enlarg ed view of the ca vi ty se ct io n . The nodesa long th e cav i ty su r fac e were re shaped in t o a c i rc u la r ho le (F igure 3 3) .S ince t h i s su r fac e con ta ined co rne r and mids ide nodes , i t was neces sa ry tochange th e lo ca t io n of both se ts of node s . When resh apin g a sur fac e suchas t h i s one , i t i s im portan t to be sure th at new sur fac es do not ov erla pany e x i s t i n g s u r f a c e s .

When the an al ys is of th is was s im ula ted , some round-off er ro rsoc cu rr ed . Th is was due to improper meshing by th e pr ep ro ce sso r program andwas e a s i l y c o r r e c t e d by a l t e r i n g t h e l o c a t i o n o f s p e c i f i c n o d e s . Thec o r r e c t e d a n a l y s i s show ed t h a t t h e r e s t r a i n i n g c o re e l em e n t s r e s t r a i n e dthe board so wel l th a t zero de fle c t io n could be con s ider ed a lon g the top oft h i s su r f ac e . The re s t ra in in g core and th e e lements below i t were e l i m i na ted and the board m a te r ia l was expanded in bo th ho r iz on ta l d i r e c t io n s .Once th es e e lem ents were expanded, th e lo ca t i o n of the mids ide nodes had tobe a lt e r e d to corr espo nd to th e new dim en sio ns . Fig ure 34 shows th e FEMfo r th e 20 mil vi a , PTH. Thi s model was mo dified to re pr es en t a 10 mil v ia ,PTH. The 10 m il v ia FEM i s di sp la ye d i n Fig ur e 3 5. The second model was

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(a) (b)

a) 2-D Superelenent Modelb) 2-D Finite Element Modelc) Extended 3-D. Modeld) Finite Element Model with Cavitye) Enlarged View of Cavity

(e) Figure 32: Microwire/PTH FEM Generation43


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\

Y RX= 20X RY= 39


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w*m^^' ; \


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analyzed and the results were used to estimate a useful life of the micro-wire interface connection.

4.3 LESSONS LEARNED

After completing a FEA, it is important to check the results to see ifany other simplifications are possible. This information can then beutilized in the future generation of FEM. Two observations were made inregard to this study.

The first observation concerned the package/board/lead system models.The resulting deflections from the three system models showed that the leaddesign had a negligible effect on the expansion of the package and theboard. This indicates that a more complex system model is unnecessary.Only a FEM of the package and a FEM of the board would be generated and twoseparate analyses on the model performed. Since both the package and theboard are rectangular in shape, generation of these models would beextremely simple. In addition, a single model could be generated torepresent several different package and board sizes. The resultingdeflections along different locations could be inputted into the individual lead mode ls. An assessment of the effect package size has on theestimated life could be made.

The second observation concerned the microwire/PTH interface connection. The results showed that the board material away from the region ofconcern had negligible stress and that the stresses in the region of

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5.0 TRANSFER OF FINITE ELEMENT RESULTS

When stress caused by a static load is a concern, the engineer performs a finite element analysis and compares the resulting stresses to theyield stresses for the different materials. What can be done when the loadis not static, but varies with time? The specific problem dealt with inthis analysis was stresses caused by thermal cycling conditions. Thesestresses eventually resulted in fatigue of the ductile materials. A technique was needed to transfer the results of the FEA to a value of meritwhich could be used to assess the reliability of the design.

The method used followed a simple procedure. First, a FEM was generated. Static analyses were simulated for the temperature change of stressfree temperature to maximum temperature (high temperature cycle) and forthe temperature change of stress free temperature to minimum temperature(low temperature cycle). The resulting maximum Von-Mises stress value forthe material of concern was obtained. In this study, the two materials ofconcern were the ductile materials, copper and solder. The strain rangefor this cyclic condition was calculated by adding the individual strainsfor the low temperature and the high temperature cycles. These strains hadto include both the elastic strain and the plastic strain. Since the exactplastic strain could not be determined without performing a nonlinearanalysis, a range for this strain was determined and it was assumed thatthe actual plastic strain was in between these two values (see section 3.0for a more detailed explanation).

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Once the strain range value was obtained, this value was plotted ontoa curve representing the strain range versus the number of cycles tofailure for the particular temperature range that was used (minimum temperature to maximum temperature). This curve was generated from theCoffin-Manson equation as described in section 3.0. Two values of cyclesto failure were obtained for each analysis. One presented the low plasticstrain and the other the high plastic strain. The resulting range of thenumber of thermal cycles to failure gave an indication of the useful lifeof the component. This procedure will now be applied to the FEM developedin section 4.0.

5.1 S-LEAD CONNECTION

The high and low temperature cycle Von-Mises stress contours in the S-lead connection are displayed in Figure 36. The overall maximum stressdoes not occur in the solder material, but in the lead material. Thisstress value is well below the yield point for the lead material. Thesolder's maximum linear stress and plastic stress are:

SIMULATION MAXIMUM LINEAR STRESS PLASTIC STRESSHigh Temp Cycle 6,000 psi 2,800 psiLow Temp Cycle 3,900 psi 700 psi

The elastic strain is .0029 in/in (3,200 psi/1,090,000 psi). Theestimated plastic strain and the total strain are:

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(a) LowTemperature Cycle

STRESS COtlTOURSUdM-rirSES STRESSVIEW : 1.67E"92RANGE : 7 . 6 2 E * 0 3

S9 S.S

429.4

14.99Erc-ms(VDrsPLAYT

s L xRX - SBR r - -3eRZ - e

STRESS COtlTOURSUON-MISES STRESSVIEW ; 4 .67 E> 9 1RBMGE : S.85E


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High Temperature Cycle

METHOD PLASTIC STRAIN TOTAL STRAIN

1 2,800/1.09 x 106 = .0026 in/in .0029 + .0026 = .0055 in/in

2 .0026 x 2 = .0052 in/in .0029 + .0052 = .0081 in/in

Low Temperature Cycle


1 700/1.09 x 10 = .0006 in/in .0029 + .0006 = .0035 in/in2 .0006 x 2 = .0012 in/in .0029 + .0012 = .0041 in/in

The overall total strain for high and low temperature cycles for theS-lead connection is:

Method 1 .0055 + .0035 = .0090 in/inMethod 2 .0081 + .0041 = .0122 in/in

These overall strain values were plotted on the elastic plus plasticline of the Coffin-Manson curve for solder (Figure 11) and the followingnumber of cycles to failure range was obtained: 11,500 to 60,000 cycles.

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mess commasISH-HISES STOESSWE U : S . 9 6 E <

Z . 3 M

S . 9 HQWC-MISIVDISPLAY

Y HX- 39* ^ RZ- 8

(a) Low Temperature CycleUJN-MT SES ST HESSWE U : 7 . 7 7 E < K

7.7ae

231.5

77. mErc-rilS(vtiKPLfiY

L Rf -38(b) High Tmperature Cycle

Figure 37: Stress Contours for Leadless FEM


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Low Temperature Cycle


1 4,498/1.09 x 10 6 = .0041 in/in .0029 + .0041 = .0070 in/in2 .0041 x 2 = .0082 in/in .0029 + .0082 = .0111 in/in

The overall total strain for high and low temperature cycles for theS-lead connection is:


These overall strain values were plotted on the elastic plus plasticline of the Coffin-Manson curve for solder (Figure 11) and the followingnumber of cycles to failure range was obtained: 120 to 260 cycles.

5.3 GULL-WING CONNECTION

The high and low temperature cycle Von-Mises stress contours in thegull-wing connection are displayed in Figure 38. Once again, the overallmaximum linear stress does not occur in the solder, but in the leadmaterial and this stress value is well below the yield point for thismaterial. The solder's maximum stress and plastic stress are:

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These overall strain values were plotted on the Coffin-Manson curvefor solder (Figure 11) and the following number of cycles to failure rangewas obtained: 400,000 to 2,000,000 cycles.

5.4 MICROWIRE/PTH CONNECTION

Four analyses were performed on the two connection models. Figure 39displays the Von-Mises Stress contours for the 20 mil via model and Figure40 displays the contours for the 10 mil via model. The majority of theboard has very little stress with the high stresses occurring in the platedcopper and the copper microwire. The highest stresses occurred at theplated barrel where the pad contacts the board. These values were not usedin this analysis because that area was not the area of reliability concern.The maximum linear stress found in the copper wire/PTH interface is listedbelow for the four different analyses.

SIMULATION LINEAR STRESS PLASTIC STRESS

10 Mil Via, High Temp Cycle 19,000 psi 11,000 psi10 Mil Via, Low Temp Cycle 19,000 psi 11,000 psi20 Mil Via, High Temp Cycle 10,000 psi 2,000 psi20 Mil Via, Low Temp Cycle 8,000 psi 0 psi

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STRESS CCMTEURSIWI-tllSES STRESSUICUBOHSE :

=.,,,,]^__.

i 1.816-66 < t t c < . ; * . ?

. 6 3

18.33

6.198

et-aeEJ-FC -MI Sa' D ISPLBY

|y RX- 2I X RY> 38\ Z RZ- 9

(a) Low Temperature Cycle

STUESS corrrouoWM-ruses STRESSUICU i 3.07C-4C*nnsE ! 3 5BE-aa

* W - 37.83

3E-88ETRC -rt KB-'O I 3T. BY

^RX- 2*RY- 38RZ- 8

(b) High Temperature Cycle

Figure 39: Stress Contours for Microwire/PTH (20 Mil Via)FEM58


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srwcs* cawnwsUIEO I 1 . * - WM6E : 4 . U S E - * *

6 C - W2 E - W

t- KZ - (a) Low Temperature Cycle

STRESS COITtM Bicn-mses STBESSBAItSC s S.27E-81

SE-Wiiiis;::;.. S E - B S

g r K C - n i s n - ' D i a ' L f l rk UK- 2RT- 3RZ - (b) High Temperature Cycle

Figure 40: Stress Contours forMicrowire/PTH (10 Mil Via) FEM

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METHOD OVERALL STR AIN

1 . 0 0 1 2 + . 0 0 1 2 = . 0 0 2 4 i n / i n

2 . 0 0 1 9 + . 0 0 1 9 = . 0 0 3 8 i n / i n

20 M i l V ia , H ig h T e m p e r a tu r e Cy c l e

METHOD PLA ST IC STRAIN TOTAL STRAIN

1 2 ,0 0 0 /1 6 x 10 = . 0 0 0 1 i n / i n . 0 0 0 5 + . 0 0 0 1 = . 0 0 0 6 i n / i n

2 2 x . 0 0 01 = . 0 0 0 2 i n / i n . 0 0 0 5 + . 0 0 0 2 = . 0 0 0 7 i n / i n

20 M i l V i a , Low T e m p e r a tu r e Cy c l e

METHOD PLA ST IC STRA IN TOTAL STRAIN

1 0 i n / i n * . 0 00 5 + 0 = . 0 0 0 5 i n / i n2 0 i n / i n * . 0 00 5 + 0 = . 0 0 0 5 i n / i n

* N o t e : T he V o n -M i se s s t r e s s v a l u e d i d n o t e x c e ed t h e e l a s t i c l i m i t ,t h e r e f o r e , t h e r e w as no p l a s t i c s t r a i n .

The o v e r a l l t o t a l s t r a i n f o r t h e h i g h a nd l ow t e m p e r a t u r e c y c l e s f o rth e 20 m i l v i a i s :

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METHOD OVERALL STRAIN

1 .0006 + .0005 = .0011 in/in2 .0007 + .0005 = .0012 in/in

These overall strain values were plotted on the plastic plus elasticCoffin-Manson curve for copper (Figure 11) and the following number ofcycles to failures ranges were obtained '.

10 mil via: 110,0000 to 10,000,00020 mil via: greater than or equal to 10,000,000

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6.0 CONCLUSIONS

Several Finite Element Analyses (FEA) were performed and the resultsused to estimate the useful life of the various connections. For thesurface mounted lead connections, the results indicated that the leadlessconnections would only survive a minimal number of thermal cycles beforeproblems occurred (less than 260 cycles). The S-lead and gull-wing leads,however, showed very good survival rates. It was estimated that the S-leadwould survive between 11,500 and 60,000 thermal cycles and that the gull-wing lead would survive between 600,000 and 2,000,000 thermal cycles. Thisindicates that the two leaded designs will have superior reliability performance.

The results from the analysis on the microwire and Plated-Through-Hole (PTH) connection indicate that, under ideal conditions, this connection will survive a minimum of 110,000 thermal cycles. These results mustbe carefully considered. Due to insufficient information and time, ananalysis of nonideal configurations was not performed. There are two typesof microwire connections that would be of serious reliability concern. Thefirst type is where the wire does not pass straight through the PTH (180degrees), but is rotated at an angle (135 or 90 degrees). The second typeof connection is where the microwire is allowed the maximum variation fromthe ideal condition. An analysis under these conditions would predict thereliability of the worst acceptable microwire connection.

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In addition to the reliability assessments, conclusions can be madeabout the finite element modeling and how this process can be improved.Listed below are four observations that have been identified:

1. The results of the FEA on the microwire connection indicate thata much smaller, less-refined model might have been sufficient toanalyze this connection. This alternate model would have reduced themodel generation time by at least 50 percent and would have providedthe time needed to generate a model of the nonideal case where thewire is off center.

2. The resulting stress values in the center of the microwire in themicrowire connection FEM were not as harmonious with one another asthese values should have been. This indicates that instead of usingquadralateral elements only and distorting some of the elements to aconsiderable degree, a mixing of element types would be preferential.In this case, the quadralateral elements were so distorted in thecenter of the microwire that the elements took on the approximateshape of a triangle. This would explain the inconsistency of stressvalues. Instead, these center elements should have been triangles.

3. In the FEMs of the individual leads, the deflections across the

width of the leads (25 mils) were essentially constant. These resultsindicated that the leads could have been modeled in two dimensionsversus three dimensions.

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4. A comparison of the FEA results for the three differentpackage/board/lead models showed only a slight variation in packageand board deflections for the three mod els. This occurrence impliesthat the lead type has little effect on the package and board defle ctions. Consequently, a single package FEM and a single board FEMcould be generated and used for the analysis of many different leadtypes.

The work documented in this report is just a stepping stone in thedevelopment of a method to transfer FEA results to a useful estimate ofreliability. There are several areas in this analysis , in both the finiteelement modeling and the transfer of results, that deserve further investigation. Two of these areas are outlined below."

1. The validity of performing a purely linea r, elastic analysis andusing the results to estimate the amount of plastic strain should beverified. A question arises as to the ability to predict the responseof the material once it goes beyond the linear range. There is someindication t hat, in fact , a nonlinear, elastic and plastic analysisshould be performed instead.

2. If these connection models could be significantly simplified, afull-view model, rather than a one-quarter model, could be generatedwithout exceeding the computer's limit. The full-view model wouldthen be used to perform a vibration analysis.

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7.0 REFERENCES

1. Sandor, Bela I., "Fundamentals of Cyclic Stress and Strain", The University ofWisconsin Press, 1972.

2. Bivens, Gretchen A. and Bocchi, William J., "Reliability Analysis of a SurfaceMounted Package Using Finite Element Simulation", October, 1987, RADC-TR-87-177.

3. Bivens, Gretchen A., "Reliability Assessment of Surface Mount Technology(SMT)", March, 1988, RADC-TR-88-72.

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