The causes of the observed head-in-pillow (HnP) soldering defects in BGA

Transcription

1 ORIGINAL ARTICLE Solder joints in surface mounted IC assemblies: Relief in stress and warpage owing to te application of elevated stand-off eigts E. Suir E. Suir. Solder joints in surface mounted IC assemblies: Relief in stress and warpage owing to te application of elevated stand-off eigts. J Mod Appl Pys. 8;():-9. ABSTRACT Te ead-in-pillow (HnP) defects in Ball-Grid-Array (BGA) packages wit leadfree solders and particularly in packages wit fine ball pitces are attributed by te numerous investigators to tree major causes: manufacturing process solder material properties and design-related issues. Te latter are tougt to be caused mostly by warpage of te PCB-package assembly. In tis analysis te warpage issue is addressed using an analytical predictive stress model. It is assumed tat it is te difference in te post-fabrication deflections of te warped package and its PCB tat are te root cause of te observed HnP defects. Te calculated data suggest tat te replacement of te conventional BGA design wit a design wit elevated stand-off eigts of te solder joints suc as eg column-grid-array (CGA) could reduce te product s propensity to warpage-related HnP defects. It is sown tat owing to te greater longitudinal interfacial compliance of te system employing solder joint interconnections wit elevated standoff eigts significant stress relief and considerable decrease in te relative PCB vs. package warpage can be epected. In te carried out numerical eample te effective stress in te solder material is relieved by about % and te difference between te maimum deflections of te PCB and te package is reduced by about 6% wen te conventional BGA design is replaced by te CGA system. Tere is a reason to believe tat te application of BGA joints wit CGA designs could result in a substantial improvement in te IC product s reliability including te it s propensity to possible HnP defects. Te stress model used in tis analysis is a modification and etension of te model developed by te autor back in mid-eigties. Tese models sed ligt on te underlying materials pysics of te solder joint interconnections in surface mounted devices. Keywords: Head-in-pillow; Column-grid-array; Interfacial searing stress; Solder joints; Stress INTRODUCTION Te causes of te observed ead-in-pillow (HnP) soldering defects in BGA packages wit lead-free solders (-9) are attributed to various processrelated solder-material-related and design-related issues. Te design-related problems are caused by te elevated warpage of te PCB-package assembly. Because of tat in some IC package designs package structures of te type sown in Figure are used. Suc structures are owever too epensive and it is more often tan not tat no metal frames are used in te to-days package tecnologies. A typical today s IC package structure (Figure ) and is addressed in our analysis. Te empasis on te warpage issue. It is assumed tat it is te difference in te post-fabrication termally induced deflections of te warped PCB and te also warped package tat is te root cause of te possible HnP defects as well as of course in an insufficient interfacial strengt of te solder joint interconnections. Te advantages of te elevated stand-off eigts of solder joint interconnections ave been first indicated in application to flip-cip solder joints () (Figures and ) i.e. to te s.c. first level of interconnections and is recently addressed in application to te second level of interconnections () including CGA designs (-9) (Figure 5). ANALYSIS Interfacial searing stress Let a BGA or a CGA assembly be fabricated at an elevated temperature and subsequently cooled down to a low (room or testing) temperature. Te interfacial longitudinal displacements and in te PCB (component #) and in te package (component #) BGA- or CGA- bonded on te PCB can be sougt in an approimate analysis based on te concept of te interfacial compliance [9] as follows: u( ) α t λ T ( ξ) dξ τ ( ) w ( ) u( ) α t λ T ( ξ) dξ τ ( ) w ( ). () Figure : SMT package wit a metal frame to prevent post-fabrication warpage Figure : Typical today s SMT package surface-mounted on a PCB Portland State University Portland OR USA; Tecnical University Vienna Austria and ERS Co. USA. Correspondence: E. Suir Portland State University Portland OR USA Telepone: suire@aol.com Received: January 8 Accepted: February 8 Publised: February 9 8 Tis open-access article is distributed under te terms of te Creative Coons Attribution Non-Coercial License (CC BY-NC) (ttp:// creativecoons.org/licenses/by-nc/./) wic permits reuse distribution and reproduction of te article provided tat te original work is properly cited and te reuse is restricted to noncoercial purposes. For coercial reuse contact reprints@pulsus.com J Mod Appl Pys Vol No Marc 8

2 Suir Figure : Solder joint modeled as a sort circular cylinder ( column ) clamped at its end planes and eperiencing aisyetric elastic deformation caused by te local termal mismatc of te solder material wit te Si material of te substrates. Te induced maimum stresses and strains are in te vertical direction wile te eternal mismatc strains act in te orizontal planes Figure : Solder joints configured as sort cylinders eperience muc lower stresses and strains tan tose configured as pancakes Here t is te cange in temperature α and α are te effective CTEs of te component materials (tese sould be assessed in an approimate fasion for te composite structures of te PCB and package) and ν ν are teir ticknesses λ and λ are te aial compliances E E of te components E and E are te effective Young s moluli of te component materials (assessed in an approimate fasion for te composite structures of te PCB and package) ν and ν are teir Poisson s ratios 5 Figure 5: BGA vs. CGA designs and G are te longitudinal interfacial compliances of G E ( ν ) E te components G and G are sear moduli of teir ( ν ) materials T( ) τξ ( ) dξ () are te forces acting in te cross-sections of te assembly components(tensile - in te PCB and compressive - in te package) τ () is te interfacial searing stress l is alf te assembly lengt and w ( ) w ( ) are te deflections of te PCB and package. Te origin of te longitudinal coordinate is in mid-cross-section of te assembly. Te first terms in te equations () are unrestricted (stress-free) displacements. Te second terms are displacements caused by te termally induced forces acting in te cross-sections of te assembly components. Hooke s law is used to structure tese terms assuming tat te displacements of all te points of te given cross-section are te same. Te tird terms are in effect corrections to suc an assumption. Tey consider tat te longitudinal displacements of te points at te interface are somewat larger tan te displacements of te inner points of te given cross-section. It is assumed tat tis correction can be sougt as a product of te known interfacial compliance of te component and te tus far unknown interfacial searing stress. Te last terms in () are due to bending. Te condition u( ) u( ) τ ( ) () of te compatibility of te displacements () considers tat tese displacements differ because of te interfacial compliance () G of te bonding (solder) layer by te amount epressed by te second term in E te rigt part of te condition (). In te formula () G is te ( ν ) sear modulus of te bonding material (solder joint system) E is its Young s modulus and ν is its Poisson s ratio. Introducing te epressions () for te longitudinal interfacial displacements into te condition () te following equation for te interfacial searing stress τ () can be obtained: τ ( ) ( λ λ) T ( ξ ) dξ w ( ) w ( ) ( α α) t. (5) Here is te total longitudinal interfacial compliance of te assembly. By differentiation we find: τ ( ) ( λ λ) T( ) w ( ) w ( ) ( α α) t. (6) J Mod Appl Pys Vol No Marc 8

3 Solder joints in surface mounted IC assemblies: Relief in stress and warpage owing to te application of elevated standoff eigts Assuming tat te assembly components can be treated as tin rectangular plates and tat teir curvatures in te and y directions at te given point of te assembly are equal te equations of bending of te PCB (component #) and te package (component #) can be written as w ( ) T( ) p( ξ) dξdξ λ w ( ) T( ) p( ξ) dξdξ λ (7) were p () is te interfacial peeling stress. Te left parts of tese equations are te elastic bending moments. Te first terms at te rigt parts are bending moments caused by te termal forces T( ). Te second terms are te bending moments caused by te peeling stress p ( ). Introducing te formulas λ w ( ) T( ) p( ) d d ξ ξ ξ λ w ( ) T( ) p( ) d d ξ ξ ξ (8) for te curvatures into te equation (6) te following relationsip can be obtained: τ ( ) ( λ λ ) T( ) 6 ε p( ξ ) dξ dξ ( α α ) t. (9) Here te notation ε λ λ () is used. Te formula () indicates particularly tat te role of te peeling stress can be minimized if te relationsip ν E () ν E is fulfilled. Tis circumstance migt ave a favorable effect on te HnP design-related problem. In an approimate analysis below we assume tat te peeling stress need not be considered wen te interfacial searing stress is evaluated i.e. does not affect te interfacial searing stress. Ten te latter can be determined from te simplified equation τ ( ) ( λ λ) T( ) ( α α) t. () At te end of te assembly te boundary condition T ( l) sould be fulfilled. Ten te equation () results in te following boundary condition for te searing stress function α α τ ( l) t. () T ( Te force ) sould be syetric wit respect to te mid-cross-section of te assembly and could be sougt terefore as T ( ) C Ccos k. () Here C and C are constants of integration and k is tus far unknown parameter of te interfacial searing stress. As follows from () te interfacial searing stress τ () is related to te termal force T () as follows: τ ( ) T ( ) C k sin k. (5) Introducing te epressions () and (5) into te equation () one concludes tat tis equation is fulfilled if te following relationsips take place: α α C t λ λ ( λ λ ) k. (6) Te second formula in (6) determines te parameter of te interfacial searing stress. Note tat tis parameter is independent from te eternal strain ( α α ) t and is a caracteristic of te structure and not te loading. From (5) we obtain: τ (7) ( ) Ck cos k. Te boundary condition () considering te second formula in (6) yields: α α α α C t t k cos kl ( λ λ)cos kl and te solutions () and (5) result in te following epressions for te forces T () acting in te components cross-sections and te interfacial searing stress τ () : α α cos k T( ) t ( λ λ) cos kl α α sin k τ ( ) T ( ) k t. (9) ( λ λ ) cos kl Interfacial peeling stress It is natural to assume tat te peeling stress p () in te given crosssection is proportional to te difference in te deflections of te assembly components in tis cross-section: p( ) K[ w ( ) w ( ) ]. () It is tis stress tat is tougt to be responsible for te possible occurrence of te HnP defects. Te stiffness K in tis formula can be assessed using an analogy wit te longitudinal interfacial compliances by te formula K ν ν ν. () E E E Differentiating te relationsip () twice we obtain: [ ] p ( ) K w ( ) w ( ). () Introducing te formulas (8) into tis equation and differentiating te obtained relationsip twice te following equation for te peeling stress can be obtained: IV p ( ) ε β p ( ) β τ ( ) δ () were te notations β λ λ Kδ δ () are used. Te rigt part of te equation () indicates tat te longitudinal gradient of te interfacial stress plays te role of te ecitation force for te peeling stress. Considering te second formula in (9) te equation () yields: IV cos k p ( ) β p ( ) β p (5) cos kl were te notation ε α α t (6) p δ is used. It will be sown below tat tis peeling stress is twice as ig as te peeling stress at te end of a long-and-stiff assembly. Te solution to te equation (5) can be sougt in te form: p( ) AV ( β) AV ( β) AV ( β) AV ( β) cos k A cos kl (7) were te functions V( β ) i V V i ( ) cos βcos β β V ( β ) sin βsin β β) ( cos βsin β sin βcos β) (8) ( ± (8) J Mod Appl Pys Vol No Marc 8 6

4 Suir obey te following simple and convenient rules of differentiation: V ( β) β V ( β) V ( β) β V( β) 7 V ( β) β V( β) V ( β) β V( β). (9) Introducing te sougt solution (7) into te equation (5) we obtain: A p () ( ) were te notation β () k is used. Tere are neiter concentrated bending moments nor lateral forces acting at te assembly ends and terefore te boundary conditions w ( l) w ( l) w ( l) w ( l) () for te deflection functions w ( ) and w ( ) sould take place. Using te formula () tese conditions yield: p ( l) p ( l). () Te peeling stress sould be syetric wit respect to te mid-cross-section of te assembly and terefore te coefficients in front of te odd functions in te solution (7) can be put equal to zero. Ten te peeling stress can be sougt in te form cos k p( ) AV ( β) AV ( β) A. () cos kl Te conditions () result in te following equations for te constants and A of integration: A V A V A p ( ) A V A V A tan kl p tan kl ( ) Tese equations ave te following solutions: A p V V tan kl sin βl sin βl V V tan kl A p (6) sin βl sin βl and te solution () yields: A. (5) V V tan kl V V tan kl p( ) p V ( β) V ( β) sin βl sin βl sin βl sin βl cos k p. ( ) cos kl (7) For long-and-stiff enoug assemblies typical in te designs of interest β β V ( β) e cos β; V ( β) e sin β β V ( β) e ( cos β ± sin β) cos k k( l ) tan kl e. (8) cos kl Ten te solution (7) can be simplified: β ( l ) k( l ) p( ) p e (( )cos[ β( l )] sin[ β( l )] ) e. ( ) (9) At te assembly ends ( l) pl ( ) p ( ). () ( ) If te assembly is very stiff in te troug-tickness direction i.e. p caracterized by large values ten te formula () yields: p ( l). Tis result eplains te pysical meaning of te p value introduced by te notation (6): it is te stress tat is twice as ig as te peeling stress at te end of a very long-and-stiff assembly. Wen te ratio canges from zero p to infinity te stress p(l) canges from zero to. As as been sown above te p value can be made low if te relationsip () is fulfilled. Warpage Te bow of te assembly as a wole can be determined from te equation α α ( D D) w ( ) T( ) ( λ λ) () cos k t cos kl were E E D D () ( ν ) ( ν ) are te fleural rigidities of te assembly components. From () te following formula for te assembly curvature can be obtained: cos k w ( ) w () cos kl were ν.6 λ.57 / kg E 79. α α w t () ( D D) ( λ λ) is te curvature in te middle of a long-and-stiff assembly. As evident from te epression () te assembly curvature canges from its maimum value w ma w (5) cos kl in te middle of te assembly to zero at its ends. From () we find te following epression for te angles of rotation: sin k w ( ) w. (6) k cos kl Assuming tat te assembly ends ave zero deflection tis epression results in te following formula for te assembly deflections: cos k w ( ) w. (7) k cos kl In te middle of te assembly ( ) w() w. k cos kl In te case of a long and/or stiff assembly (large kl values) (8) w() w. (9) k For assemblies aving very ig k values (stiff PCB stiff package and stiff solder system) tis formula yields: l w() w. (5) Te assembly size (alf-lengt) is l. and te cange in temperature is t 5 C. Computed data: Aial compliances: J Mod Appl Pys Vol No Marc 8

5 Solder joints in surface mounted IC assemblies: Relief in stress and warpage owing to te application of elevated standoff eigts NUMERICAL EXAMPLE Structural element PCB Package BGA CGA Element # CTE (/ C) Young s Modulus (E Kg/ ) Poisson s ratio (V) Tickness/eigt ( ) Sear modulus (G Kg/) Aial compliance (λ /Kg) Interfacial compliance (K / Kg) PCB: λ ν.6.57 / E 79. kg Package: ν.65 λ 9.5 / kg E.7 Interfacial compliances: PCB:. 7.6 / kg G 69 Package: / kg G 85 BGA system: / kg G kl.7 8. for te CGA design. In eiter case it is significant so tat te assembly can be treated as a long one. Termal force in te mid-cross-sections of te assembly: α α 5 T t 5.9 kg / λ λ 9.77 Maimum interfacial searing stress (at te assembly ends): wit te BGA system: τ ( l) kt kg / wit te CGA system: τ ( l) kt kg / Aial compliance related parameters: ε λ λ kg λ λ δ kg Te doubled interfacial peeling stress at te end of a stiff assembly ε α α p t δ kg / in te case of te BGA solder and is ε α α 79. p t δ kg / 56.9 in te case of te CGA solder. Te interfacial troug-tickness spring constant is K ν ν ν E E E kg / CGA system: / G.7 kg Total interfacial compliance: wit te BGA system: / kg wit CGA system: / kg Parameter of te interfacial searing stress: wit te BGA system: λ λ 9.77 k 7.85 wit te CGA system:.775 λ λ 9.77 k Product kl is kl for te BGA design and is wit te BGA solders and K ν ν ν E E E kg / wit te CGA solders. Parameter of te peeling stress β in te case of te BGA system and Kδ β Kδ in te case of te CGA design. Te ratio of te parameters of te interfacial peeling and te searing stresses β k J Mod Appl Pys Vol No Marc 8 8 in te case of a BGA and is β k.775 in te case of a CGA.

6 Suir Te peeling stress at te assembly end is pl p ( ).8795 ( ) ( ).8 ( ) kg / in te case of te BGA system and is pl p ( ) 6.7 ( ) ( ).6 ( ) kg / wen te CGA system is employed. Te effective stress is σ p τ kg / in te case of a BGA and is σ p τ kg / in te case of a CGA design. Fleural rigidities of te PCB and te package: E 79. D 57.77kg ( ν ) (. ) E.7 D 5.58kg ( ν ) (.5 ) Curvature in te middle of a long-and-stiff assembly: α α w t ( D D) ( λ λ) Maimum bow: wit te BGA assembly: w() w 8.88 (.95).9 k wit te CGA assembly: w() w 8.88 k ( ) Te difference in maimum deflections pl ( ).659 K.7 w w() w().797 in te BGA design and pl ( ).8 K w w() w() in te CGA design (Table ). Table : Te calculated data are suarized in te following table Stress and bow Searing stress (Kg/ ) Peeling stress (Kg/ ) Effective stress(kg/ ) Maimum bow Mm Δw w -w BGA E-5 CGA E-6 Tese data indicate tat te application of te CGA tecnology instead of te BGA design can result in significant stress relief and ence in a ig likeliood tat te warpage-related HnP defects could be minimized or even avoided. CONCLUSION Te application of te CGA tecnology instead of te BGA design can result in significant stress relief and supposedly in a ig likeliood tat te warpage-related HnP defects could be minimized or even avoided. Future work sould include FEA and eperimental investigations. REFERENCES. Bat J Garcia R Cristoper A et al. Plastic ball grid array package warpage and impact on traditional MSL classification for Pb free assembly SMTAI Conference... Latrop L. BGA Co-planarity reduction during te ball attaces process SMTAI pan pacific conference. 8. Tsai MY Hsu CH Han CN. A note on suir s solution of termal stresses for a die substrate assembly. ASME J Electronic Packaging. : 6.. Tsai MY Hsu CH Wang CT. Investigation of termo mecanical beaviors of Flip-Cip BGA Package during manufacturing process and termal cycling IEEE CPMT Transactions. ; Dudi A Aspandiar R Buttars S et al. ead on pillow SMT failure modes SMTAI Conference Savic J Xie W Islam N et al. Warpage mitigation processes in te assembly of large body size mied pitc BGA coreless packages for use in ig speed network applications SMTAI Conference.. 7. Liu Y Fiacco P Lee N. Testing and prevention of ead in pillow ECTC.. 8. Hig temperature printed board flattness guideline IPC.. 9. Rekers H Eng P. Case study ead in pillow defect on a plastic BGA SMTAI Conference on soldering and reliability. 5;.. Suir E. Aisyetric elastic deformations of a finite circular cylinder wit application to low temperature strains and stresses in solder Joints. ASME J Appl Mec.989;56.. Suir E. Mecanical reliability of flip-cip interconnections in siliconon-silicon multicip modules IEEE Conference Santa Cruz California Marc 99.. Suir E. Analysis of a sort beam wit application to solder joints could larger stand-off eigts relieve stress (EPJAP) 5: 7.. Suir E. Predicted Stresses in a ball grid array (BGA)/column-grid-array (CGA) assembly wit a low modulus solder at its ends. J Mat Science Materials in Electronics.5;6.. Suir E Gaffarian R Nicolics J. Could application of column-grid array tecnology result in inelastic strain free state of stress in solder Material J Mat Sci Materials in Electronics. 5;6. 5. Suir E Gaffarian R Nicolics J. Could termal stresses in an inomogeneous BGA/CGA System be predicted using a model for a omogeneously bonded assembly. J Mat Sci Materials in Electronics 6: Suir E Gaffarian R Nicolics J. Predicted stresses in ball grid array (BGA) and column-grid-array (CGA) interconnections in a mirror like package design J Mat Sci Materials in Electronics. 6;7. 7. Suir E Gaffarian R. Predicted stresses in a ball grid-array (BGA)/ column-grid-array (CGA) assembly wit epoy adesive at its ends J Mat Sci Materials in Electronics. 6;7. 8. Suir E Gaffarian R. Column grid array (CGA) vs ball grid-array (BGA) board-level drop test and te epected dynamic stress in te solder material J Mat Sci Materials in Electronics. 6;7. 9. Suir E. Die attacment design and its influence on termal stresses in te die and te attacment ECTC J Mod Appl Pys Vol No Marc 8