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Board-Mount Evaluation Tin-Plated Component Leads
Douglas Romm, Donald Abbott, Bernhard Lange, Muhammad Khan ABSTRACT solderability performance (Sn)-plated integrated circuit (IC) component leads assessed this paper, with nickel-palladium-gold (NiPdAu)-finished leads used control. test methods were wetting balance board mount. wetting-balance test done with eutectic SnPb (tin-lead) solder. Board-mount tests were done using tin-lead-silver (SnPbAg) Pb-free, tin-silver-copper (SnAgCu) pastes under three different reflow conditions. Solder joints were inspected visually, lead-pull data collected, cross sections were made. conclusion that Sn-finished leads perform well with both Pb-bearing Pb-free solder pastes. Their solder wetting performance equivalent other Pb-free finish that used control, NiPdAu. Standard Linear Logic
Contents Introduction Components Evaluated Evaluation Methods Results Wetting Balance Board-Mount Evaluation Test Board Solder Stencil Component Placement Test Matrix Reflow Profile Reflow Profile Reflow Profile Evaluation Methods Visual Appearance Cross Sections Solder Joints Lead-Pull Test Method Results Lead-Pull Test Lead-Pull Data Lead-Pull Failure Mode Conclusions References
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List Figures SOIC Package Isometric View Typical Wetting-Balance Curve Wetting-Balance Curve Sn-Finished SOIC Component Lead Wetting-Balance Curve NiPdAu-Finished SOIC Component Lead Test With Thermocouples Attached Reflow Profile SnPb Soldering Process Reflow Profile SnAgCu Soldering Process, Reflow Profile SnAgCu Soldering Process, High Typical Wetting Sn-Finished Leads With SnPbAg Solder Paste, 215°C 220°C Peak Reflow Typical Wetting Sn-Finished Leads With SnPbAg Solder Paste, 235°C 240°C Peak Reflow Typical Wetting NiPdAu-Finished Leads With SnPbAg Solder Paste, 215°C 220°C Peak Reflow Typical Wetting NiPdAu-Finished Leads With SnPbAg Solder Paste, 235°C 240°C Peak Reflow Typical Wetting Sn-Finished Leads With SnAgCu Solder Paste, 235°C 240°C peak reflow Typical Wetting Sn-Finished Leads With SnAgCu Solder Paste, 255°C 260°C peak reflow Typical Wetting NiPdAu-Finished Leads With SnAgCu Solder Paste, 235°C 240°C peak reflow Typical Wetting NiPdAu-Finished Leads With SnAgCu Solder Paste, 255°C 260°C Peak Reflow Cross Section Typical Sn-Finished Component With SnPbAg Solder Paste, 215°C 220°C Peak Reflow Cross Section Typical Sn-Finished Component With SnPbAg Solder Paste, 235°C 240°C Peak Reflow Cross Section Typical NiPdAu-Finished Component With SnPbAg Solder Paste, 215°C 220°C Peak Reflow Cross Section Typical NiPdAu-Finished Component With SnPbAg Solder Paste, 235°C 240°C Peak Reflow Cross Section Typical Sn-Finished Component, SnAgCu Solder Paste, 235°C 240°C Peak Reflow Cross Section Typical Sn-Finished Component, SnAgCu Solder Paste, 255°C 260°C Peak Reflow Cross Section Typical NiPdAu-Finished Component, SnAgCu Solder Paste, 235°C 240°C Peak Reflow Cross Section Typical NiPdAu-Finished Component, SnAgCu Solder Paste, 255°C 260°C Peak Reflow Leads Bent Pull Test Leads Pulled Vertical
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Average Lead-Pull Values Sn-Finished NiPdAu-Finished Units, SnPbAg Solder Alloy Average Lead-Pull Values Sn-Finished NiPdAu-Finished Units, SnAgCu Solder Alloy Typical Lead-Pull Failure Mode High Magnification Typical Lead-Pull Failure Mode List Tables
Component Details Evaluation Control Groups Solder Paste Reflow Profile Matrix
Introduction
need lead (Pb)-free electronics well documented, with European legislation influence Japanese market forces.[1, industry made strides developing options eliminate major sources from electronic assemblies: solder printed wiring board (PWB). specific class tin-silver-copper (SnAgCu) solder alloy been embraced industry alternative tin-lead (SnPb) solder.[4, Several Pb-free finishes available: organic solderability preservatives (OSPs), immersion immersion plating. SnPb lead finish last least-significant source board-mounted integrated circuits (ICs). Texas Instruments (TI) provided nickel-palladium (NiPd) nickel-palladium-gold (NiPdAu) lead finishes Pb-free options more than years.[7, evaluating packages built with matte (Sn) finish component leads. first package with matte Sn-finished leads evaluated small-outline integrated circuit (SOIC). bulk SOICs supplied built using NiPdAu finish component leads. Sn-finished SOICs were evaluated another Pb-free option. work reported here focuses board-level soldering performance matte Sn-finished leads SOIC packages. data also applicable other packages built with matte Sn-finished leads. This study evaluates soldering performance Sn-finished leads using three different reflow profiles tin/lead/silver (SnPbAg) lead-free SnAgCu solders. Cross sections solder joints were made before after temperature cycling. Lead-pull testing measured strength solder joints. Wetting balance (meniscograph) data compared soldering performance matte Sn-finished leads with different solder alloys. NiPdAu-finished leads were used controls.
Components Evaluated
package used this evaluation small-outline integrated circuit (SOIC). Sn-plated units were built package assembly subcontractor external Internally built NiPdAu-plated units were used control. isometric view SOIC package used this evaluation shown Figure
Board-Mount Evaluation Tin-Plated Component Leads
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Figure SOIC Package Isometric View Features units evaluated listed Table Table Component Details Evaluation Control Groups
Evaluation
Count Package Type Base Metal Finish Plating Thickness Grain Size SOIC Copper (matte) 10.9 mm-17.2 SOIC Copper NiPdAu mm-2 (Ni), 0.02 mm-0.15 (Pd), 0.003 mm-0.015 (Au)
Control
Evaluation Methods Results
Wetting Balance
wetting-balance test used test solder wettability leads. However, wetting-balance test classified ANSI/J-STD-002 "test without established accept/reject criterion."[11] This test method recommended engineering evaluations only production pass/fail monitor. wetting-balance test measures forces imposed molten solder upon test specimen when test specimen dipped held into solder bath during test. This wetting force measured electronic gauge function time. typical wetting-balance curve shown Figure Initially, force negative, indicating that solder begun specimen and, fact, shows buoyancy effect. force exerted solder approaches zero solder begins specimen. commonly used performance measure time cross zero axis wetting force, This point indicates transition from nonwetting (F<0) wetting (F>0). second point comparison time t2/3 needed until wetting force reached equilibrated wetting force. wetting balance test method used compare solder wetting performance lead finish (see Figure NiPdAu lead finish (see Figure
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Equilibrated Wetting Force
Force
Zero Force Line
Buoyancy Associated With Initial Nonwetting Time
Figure Typical Wetting-Balance Curve wetting-balance test, automatic system with solder globule heat control, automatic immersion specimen into solder globule, electronic gauge recording wetting force used. specimen leads were immersed liquid flux (25% pure rosin ethanol) prior test. specimen immersion depth into liquid solder globule (SnPb) good thermal heat transfer 235°C solder bath into specimen under test. immersion speed mm/s.
Figure Wetting-Balance Curve Sn-Finished SOIC Component Lead
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Figure Wetting-Balance Curve NiPdAu-Finished SOIC Component Lead wetting balance results Sn-finished NiPdAu-finished SOICs showed equivalent results wetting time ~0.8 seconds. Also, t2/3 results showed good wetting performance tested lead finishes SOIC. t2/3 wetting time seconds NiPdAu finish showed slightly quicker wetting performance than Sn-finished SOIC. measured differences, however, show impact board-wetting performance, which described following paragraphs.
Board-Mount Evaluation Test Board
double-sided used dimensions 15.24 20.32 with thickness 35-mm-thick copper pads have dimensions SOIC components. Finish copper pads nickel-gold (NiAu), with solder mask separating individual pads.
Solder Stencil
solder stencil laser-cut stainless-steel mask, thick. solder paste printed using stainless-steel tool.
Component Placement
components were placed with mechanical placement tool using microscope, which allows accurate placement components into printed solder paste test board.
Test Matrix
Pb-free SnAgCu solder alloy chosen this evaluation 95.5Sn/4.0Ag/0.5Cu. This alloy been recommended National Electronics Manufacturing Initiative (NEMI) standardized Pb-free solder alternative. Sn/Ag/Cu class alloys also been recommended suggested IPC, ITRI, NCMS.[4, control paste alloy chosen 62Sn/36Pb/2Ag. This SnPb paste with small percentage silver. matrix solder paste alloys, melting points, reflow profiles used shown Table
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Table Solder Paste Reflow Profile Matrix
Solder Paste
62Sn36Pb2Ag 62Sn36Pb2Ag 95.5Sn4.0Ag0.5Cu 95.5Sn4.0Ag0.5Cu
Melting Point
179°C 179°C 217°C 217°C
Reflow Peak Temperature Range
215°C-220°C 235°C-240°C 235°C-240°C 255°C-260°C
Soldering evaluations were performed using three different reflow profiles. profiles varied primarily targeted peak reflow temperature. Components were soldered using full-convection reflow process under nitrogen atmosphere. Temperature measurements were taken four different locations each profile. test well location components thermocouples used profiles shown Figure
Figure Test With Thermocouples Attached Thermocouples locations measured package surface temperature components located middle corner PWB, respectively. Thermocouples locations measured component lead temperature components located middle corner PWB, respectively.
Reflow Profile
first profile (Figure preheat zone 150°C 180°C seconds. peak temperature range achieved with profile 215°C 220°C. Temperature readings taken thermocouples shown black line blue line, respectively. Temperature readings taken thermocouples shown line green line, respectively.
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Figure Reflow Profile SnPb Soldering Process liquidus point eutectic SnPb solder (63Sn37Pb) 183°C, thus range peak temperature SnPb soldering processes typically 215°C 235°C. Profile intended simulate lower current SnPb reflow environments. goal with profile demonstrate soldering performance Sn-finished components current (SnPb) soldering processes. Because melt point 232°C, critical that soldering performance Sn-finished components current SnPb soldering process investigated. liquidus temperature SnAgCu solder alloy quoted 217°C 221°C. minimum temperature used reflow SnAgCu solder alloy surface-mount applications approximately 235°C. Thus, reflow SnAgCu solder with profile feasible. expected that customers using SnAgCu solder will operate with minimum reflow temperature 235°C.
Reflow Profile
second profile (Figure preheat zone 150°C 180°C seconds. peak temperature range achieved with profile 235°C 240°C.
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Figure Reflow Profile SnAgCu Soldering Process, Profile intended simulate lower Pb-free SnAgCu reflow processes. goal with profile demonstrate soldering performance Sn-finished components Pb-free (SnAgCu) soldering processes operating range 235°C 240°C peak temperature.
Reflow Profile
third profile (Figure preheat zone 150°C 180°C seconds. peak temperature range achieved with profile 255°C 260°C.
Figure Reflow Profile SnAgCu Soldering Process, High Profile intended simulate upper lead-free SnAgCu reflow processes. goal with profile demonstrate soldering performance Sn-finished components lead-free (SnAgCu) soldering processes range 255°C 260°C peak temperature.
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Evaluation Methods Visual Appearance
Solder joints formed were photographed document visual solder wetting performance. Figures show typical wetting performance Sn-finished units with SnPbAg solder paste 215°C 220°C 235°C 240°C peak reflow temperatures, respectively. Figures show typical wetting performance NiPdAu-finished units with SnPbAg solder paste 215°C 220°C 235°C 240°C peak reflow temperatures, respectively.
Figure Typical Wetting Sn-Finished Leads With SnPbAg Solder Paste, 215°C 220°C Peak Reflow
Figure Typical Wetting Sn-Finished Leads With SnPbAg Solder Paste, 235°C 240°C Peak Reflow
Figure Typical Wetting NiPdAu-Finished Leads With SnPbAg Solder Paste, 215°C 220°C Peak Reflow
Figure Typical Wetting NiPdAu-Finished Leads With SnPbAg Solder Paste, 235°C 240°C Peak Reflow
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solder joints formed with SnPbAg solder alloy documented Figures through exhibited excellent heel, side, filleting. Wetting height heel least equal lead thickness. This performance would considered good three classes products identified IPC-A-610C general electronic products, dedicated-service electronic products, high-performance electronic products.[12] Figures through show typical wetting performance Sn-finished NiPdAu-finished units with SnAgCu solder paste 235°C 240°C 255°C 260°C peak reflow temperatures, respectively.
Figure Typical Wetting Sn-Finished Leads With SnAgCu Solder Paste, 235°C 240°C Peak Reflow
Figure Typical Wetting Sn-Finished Leads With SnAgCu Solder Paste, 255°C 260°C Peak Reflow
Figure Typical Wetting NiPdAu-Finished Leads With SnAgCu Solder Paste, 235°C 240°C Peak Reflow
Figure Typical Wetting NiPdAu-Finished Leads With SnAgCu Solder Paste, 255°C 260°C Peak Reflow
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solder joints formed SnAgCu solder alloy documented Figures through exhibited excellent heel, side, filleting. Wetting height heel least equal lead thickness. This performance would considered good three classes products identified IPC-A-610C general electronic products, dedicated-service electronic products, high-performance electronic products.[12]
Cross Sections Solder Joints
Cross sections solder joints were taken document wetting underside leads. Figures through show cross sections Sn-finished NiPdAu-finished units, with SnPbAg solder paste reflowed with different profiles.
Figure Cross Section Typical Sn-Finished Lead With SnPbAg Solder Paste, 215°C 220°C Peak Reflow
Figure Cross Section Typical Sn-Finished Lead With SnPbAg Solder Paste, 235°C 240°C Peak Reflow
Figure Cross Section Typical NiPdAu-Finished Lead With SnPbAg Solder Paste, 215°C 220°C Peak Reflow
Figure Cross Section Typical NiPdAu-Finished Lead With SnPbAg Paste, 235°C 240°C Peak Reflow
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Figures through show cross sections leads Sn-finished NiPdAu-finished units, with SnAgCu solder paste reflowed with each applicable profiles.
Figure Cross Section Typical Sn-Finished Lead, SnAgCu Solder Paste, 235°C 240°C Peak Reflow
Figure Cross Section Typical Sn-Finished Lead, SnAgCu Solder Paste, 255°C 260°C Peak Reflow
Figure Cross Section Typical NiPdAu-Finished Lead, SnAgCu Solder Paste, 235°C 240°C Peak Reflow
Figure Cross Section Typical NiPdAu-Finished Lead, SnAgCu Solder Paste, 255°C 260°C Peak Reflow
Lead-Pull Test Method Results Lead-Pull Test
Lead-pull testing determined force needed pull lead from land pattern after soldering. First, allow access lead PWB, leads were near package body. Next, with leads separated from package body (see Figure 25), fastened test fixture. Finally, lead pulled perpendicular surface until separated from (see Figure 26). rate movement test device mm/s vertically board surface. force needed pull lead from measured recorded. Lead-pull data taken before after exposure temperature cycling.
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temperature cycle -40°C +25°C, 30-minute cycles. This thermal-shock test, with boards being moved from -40°C 125°C chamber. There ramp between temperature extremes.
Figure Leads Bent Pull Test Lead-Pull Data
Figure Leads Pulled Vertical
Lead pull performed Sn-finished NiPdAu-finished leads. Forty leads from each group were pulled define average pull force. Pull force measured Newtons (N). Figure gives average lead-pull values Sn-finished NiPdAu-finished packages, with SnPbAg solder alloy defined reflow conditions. This data PWBs with coating includes data both non-temperature-cycled units units after temperature cycling.
Pull Force NiPdAu (215-220C) NiPdAu (235-240C) NiPdAu (255-260C) (215- 220C) (235- 240C) (255- 260C)
Figure Average Lead-Pull Values Sn-Finished NiPdAu-Finished Units, SnPbAg Solder Alloy
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minimum pull force specified non-temperature-cycled units, industry-standard specification, industry-standard requirement temperature-cycled units that average lead-pull force shall greater than half average pull force non-cycled units.[13, Both Sn-finished NiPdAu-finished units meet industry standard requirement non-cycled temperature-cycled units when using SnPbAg solder. Figure gives average lead-pull values NiPdAu finished packages, with SnAgCu solder alloy defined reflow conditions. This data PWBs with coating includes data both non-temperature-cycled units units after temperature cycling.
NiPdAu (235-240C) NiPdAu (255-260C) (235- 240C) (255- 260C)
Figure Average Lead-Pull Values Sn-Finished NiPdAu-Finished Units, SnAgCu Solder Alloy Both Sn-finished NiPdAu-finished units meet industry-standard requirement noncycled temperature-cycled units when using SnAgCu solder.
Lead-Pull Failure Mode
important understand failure mode seen during lead-pull testing. There four possible modes failure this test method. mode component lead, solder, remain intact, pulled away from base laminate. mode interface between solder gives way. mode joint breaks within solder, leaving solder component lead. mode lead peels away from solder, leaving bulk solder attached pad. lead-pull failure modes were recorded Sn-finished NiPdAu-finished units tested. More than lead-pull failure modes NiPdAu-finished units tested were mode (see Figures 30). lifted from base laminate before failure other interface. Figures show typical lead-pull failure modes where pads have been lifted from PWB. small number units showed mode failure, with joint failing within solder. mode-4 failures were noted, indicating that bond between lead solder stronger than bond between substrate, most cases.
Pull Force
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Figure Typical Lead-Pull Failure Mode
Figure High Magnification Typical Lead-Pull Failure Mode
Conclusions
This evaluation demonstrated excellent solder wetting performance Sn-finished components with both SnPbAg (current) Pb-free (future) soldering processes. Specifically, Pb-free solder paste (SnAgCu) Pb-bearing solder paste (SnPbAg) three appropriate reflow profiles: Lead-pull data as-built temperature-cycled units showed passing results with both solder pastes. Lead-pull data showed that weak point joint lead/solder interface. Cross sections as-built joints showed excellent wetting bottom lead excellent heel filleting both pastes under appropriate reflow conditions. Visual inspection solder joints showed acceptable equal performance both solder pastes. Wetting-balance data, using SnPb solder, showed that t2/3 Sn-finished leads were equal that NiPdAu-finished leads. Sn-finished leads will perform well with both Pb-bearing Pb-free solder pastes. Their solder wetting performance equivalent other Pb-free finish that uses, NiPdAu.
References
Proposal Directive Waste from Electric Electronic Equipment 76/769/EEC, Third Draft, European Commission Directorate General Brussels, July 1999. Suganuma, Works Proceedings, October 1999. Nitta, Proceedings EcoDesign 970. NEMI Group Recommends Tin/Silver/Copper Alloy Industry Standard Lead-Free Solder Reflow Board Assemblies, NEMI press release, January 2000. Roadmap Draft (www.leadfree.org)
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Lead-Free Alloys-The Forward, Soldertec (ITRI) Limited, October, 1999. (www.lead-free.org) Abbott, Brook, McLellan, Wiley, "Palladium Lead Finish Surface Mount Integrated Circuit Packages," IEEE Trans. CHMT, 14:567 (1991). Murata Abbott, "Technology Trends Leadframe Surface Finishing," Technical Proceedings, Semicon Japan, (1990). Romm, Lange, Abbott, Evaluation Nickel/Palladium/Gold-Finished Surface-Mount Integrated Circuits. Romm, Lange, Abbott, Evaluation Nickel/Palladium-Finished With Lead-Free Solder Alloys. ANSI/J-STD-002, Solderability Tests Component Leads, Terminations, Lugs, Teminals, Wires, October 1998. IPC-A-610C, Acceptability Electronic Assemblies, January 2000. SEMI Draft Document #2910A, Test Method Solderability Nickel/Palladium Lead Finish Surface Mount Semiconductor Devices, January 2001. 60068-2-21, Environmental Testing-PART 2-21: Robustness Terminations Integral Mounting Devices, 1999.
Board-Mount Evaluation Tin-Plated Component Leads
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