Board Level Assembly Rework Recommendations 4/19/00 Abstract

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Board Level Assembly Rework Recommendations 4/19/00 Abstract

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APPNote
Board Level Assembly Rework Recommendations
4/19/00
Abstract This application note provides recommendations board level assembly ballgrid array (BGA) package family. These recommendations were derived from work done Universal BGA/DCA (Ball Grid Array/Direct Chip Attach) Consortium. consortium spent years (June 1994 July 1996) characterize processes investigate issues relating attachment yields attachment reliability type components. Introduction industry continues grow with demand small packages diverse packaging options, packages ideal solution. This because connections interior device, improving ratio between count board area. package contains twice many connections quad flat pack (QFP) packages same area. packages compatible with most (Surface Mount Technology) assembly equipment used assemble QFP's onto printed circuit boards. Several leading equipment manufacturers have reported that packages successfully their assembly lines without significant process modifications. advantages technology higher board assembly yield. their larger pitch tendency "self-center" during reflow, BGAs have less demanding placement accuracy requirements. self-centering phenomena results from surface tension molten solder ball acting minimize total energy assembly. Self-centering been observed compensate mils misalignment. addition, BGAs with eutectic solder balls have wider assembly process window solder paste deposition less sensitive paste slumping. following recommendations, Universal consortium observed nominal defect rates DPM.
Assembly Process Recommendation Solder Paste clean type paste flux residue acceptable Moderate slump characteristics acceptable (less sensitive slump than fine pitch components) Nitrogen atmosphere clean pastes Stencil Printing Note: There wide assembly process window solder paste deposition since solder bump volume makes majority joint volume. Adding paste often significantly improves tolerances. Visual print inspection 0.0127 (0.5 mil) taper edge Stencil aperture: chemically etched laser acceptable Circular oblong shapes stencil acceptable Stencil thickness: 0.127 0.203 printing (See PBGA Parameters section) Average paste volume: 1500~4000 mil^3 Components Moisture free (follow package recommendation out-of-bag life bakeout) JEDEC coplanarity specification mils) needs coupled with solder volume requirement Acceptable ball diameter variation: 0.0127 (0.5 mil) standard deviation ball diameter Board size: with component (See PBGA Parameters section) type: define geometry Vias: covered component side coating: Cu/OSP ENTEK ENTEK+ HASL acceptable palletized reflow large thin boards, special requirements thick boards Wave soldering Taped capped vias Does increase package warpage
Component Placement Recommendations: Note: placement accuracy requirement less demanding that QFPs larger lead pitch, larger bump size, self-centering during reflow. Bumps have make contact with printed solder Overall deviation: 0.254 mils) Placement Pressure: grams Preferably bump alignment Self centering compensates 0.356 misalignment (observed) Reflow Soldering Recommendations: Note: cautious profiling insure reasonable temperature difference Preferably between components. Ramp rate (pre-heat zone): Peak Temperature: Time about liquidous temperature (above seconds (preferably about seconds) Dwell time flux activation zone (between minutes Forced convection reflow with nitrogen preferred PPM)
Inspection Pre-process solder bump inspection recommended Visual solder deposit inspection machine vision solder bump identification during placement Using recommended assembly process process parameters coupled with electrical testing, 100% post-process inspection recommended
Rework Process Recommendations noted previously, BGAs have extremely board assembly defects. However, when defects occur, numerous processes packages have been discussed industry trade journals. general, fairly straightforward remove replace suspect packages printed circuits boards. However, reattachment touch-up packages that have been soldered circuit board practical most cases. When selecting rework system, nozzle design often dominating factor determining system performance. Three important criteria should used when selecting quality rework system. system should: Minimize change temperature across solder joint array. (This promotes good solder joints formation, minimizes inter-metallic growth, improves solderability, minimizes component warpage.) Minimize temperature. (This prevents delamination wire bond failure.) Minimize board temperature adjacent rework site. (This minimizes intermetallic growth, prevents secondary re-flow, prevents possible component delamination.) Rework System Requirements: Component alignment technique Heating process Delta across solder joint array temperature Temperature adjacent rework component (should possible) Reflow peak temperature: Dwell time above seconds Full under board heater stage heating process (preheat reflow capability) Component Removal Process: Flux nitrogen reflow necessary Entire assembly must moisture free prevent delamination stages heating Bottom side preheat: minutes
side reflow (joint peak): seconds above before removal
Site Redressing: Remove residual solder Soldering iron with solder wick same flux formulation used initial assembly Fluxless solder wick Removal columns rows residual solder process stroke Precise width solder wick Soldering iron should slightly smaller than solder wick. Component Replacement Process: Moisture free components board Compatible solder paste flux Three stages heating Bottom side preheat: minutes side preheat: minutes side reflow (joint peak): Dwell time above seconds Solder paste recommendation high paste volume possible (without causing bridging) nitrogen depends solder paste PBGA Parameters: Verify specifics Mechanical Drawings, typical information summarized Table
Package Body Size Ball Pitch 1.27 1.27 Ball Diameter 0.75 0.75 0.50 0.50 Substrate Thickness Reference Layers/4 Layers 0.36 mm/0.61mm 0.36 mm/0.61mm 0.36 mm/0.61mm 0.36 mm/0.61mm Mold Height 1.17 1.17 1.17 1.17 Package Thickness Overall Layers/4 Layers 1.93 mm/2.38 1.93 mm/2.38 1.93 mm/2.38 1.93 mm/2.38
Table Total package height soldered [(Ball Diameter reflow collapse ~66%) Mold Cap] Solder Mask Defined (NSMD) size 0.381~0.431 (15~17 mils) 1.00 Ball Pitch size 0.584~0.635 (23~25 mils) 1.27 Ball Pitch

Board Level Assembly Rework Recommendations 4/19/00 Abstract

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