【SMT资料】Printing Defect Guide英文.ppt

Defect Guide for Solder Printing,http:/,Guide Index,HandlingSqueegee/head too bigMissalignedPrint speedPressureBoard supportPrint gapUnder stencil wiping,Page 3,Page 4,Page 7,Page 8,Page 10,Page 20,Page 23,Page 29,Solder paste handling guidelines,Note hazard warnings(MSDS)Store at 5-10C for max.shelf-life.Modern pastes can be stored at RT for upto 6 weeks Allow paste to reach room temp.before openingAvoid use of metal toolsEnvironment of 23-27C&40-60%RH is desirableTry to avoid mixing fresh paste with old,Effect of long squeegee/head,Effect of long squeegee/head,Effect of long squeegee/head,Allows paste to be smeared outside of print area.This can lead to drying of paste and reduced paste life due to possible coining and solderballing.Can be improved by using support across the whole width of the enclosed head,but squeegee should same width as board area.Areas of the head not over the printed section can cause friction/drying of the paste in the head which can lead to lifetime/performance of paste being reduced a bit like compaction but only at the edges.,Misaligned Deposit,Stencil/PCB registrationPCB warpageStencil damage,Print Speed,Print speed is mainly dependent on paste typeAlso dependent on print frequency,temperature,age of paste,pitch and squeegeesExcessive speeds may lead to“over-shearing”of paste causing flux bleed&smudged printsLow speeds may lead to poor print quality and slow squeegee drop-off,Flux Bleed,Pressure,Excessive pressure may lead to flux bleedExcessive pressure may lead to“scooping”Low pressure will lead to smearing of paste on the stencil leading to poor print definition and drying of paste,High Pressure,Flux bleed,“Scooping”,Scooping,What causes scooping?Excessive pressureSpeed to slowRubber squeegees,Low pressure,“Smearing”on stencil,“Dog ears”,Paste Smearing,“Smearing”on stencil,Paste Smearing,What causes paste smearing on the whole stencil?Pressure too lowWhat causes stripes of paste smeared along the stencil?Damaged squeegeeWhat causes patches of paste smeared on the stencil?Poor board support,Low pressure,Dog Ears,“Dog ears”,Dog Ears,What is the difference between scoops and dog ears?The height of the deposit is higher than the stencil thicknessWhat causes dog ears?SmearingIncorrect stencil release speedPaste too sticky,Low Pressure,“Excesssupport”,Insufficientsupport,Insufficientsupport?,1 worn squeegee,Board support,Poor board support allows paste to bleed,Board Support,High pressure should not be used to overcome problems due to poor board supportThis will result in poor print quality and stencil damagePut in more support to ensure good contact between board&stencilAlways make sure support is clean before each shift and when changing.,Print Gap,Thick solder resist,Print gap 0,Print Gap,What is the effect of having a print gap?Smudging of printed depositsPaste contamination on underside of the stencilPaste on the resistInconsistent print heights,Print Gap,Printing is usually“on contact”ie:print gap=0This should reduce print“smudging”and flux bleedThick solder resist,labels or inks in the wrong place may prevent true“on contact“printing If printer is not correctly calibrated the print gap may be less than 0 which will result in poor prints and stencil damage,Print gap allows paste to bleed,Board considerations,Pad surfaceHASLGold on nickelOrganic coating,ResistThinner than pads,good,bad,good,bad,Vias,notation and bar codes,Poor understencil wiping,Poor understencil wiping,Under stencil wiping causes paste to dry on underside of stencil.,Understencil wiping,In a perfect world where your printer has a 100%gasket between the stencil and the board.All of the boards are perfectly flat,with flat pads and the resist is correctly placed and of the correct thickness.Then you should never have to use understencil wiping.How ever this being the real world you will have to use it.,Understencil wiping,This is used to clean of flux medium and paste from the under side of the stencil.This builds up if paste is squeezed out under the stencil during the print process.It also helps to remove paste starting to block the apertures in the stencil.It works by having a cloth roll wipe this material from the stencil.,Under stencil wiping,It works best by using a combination of wiping and a vacuum.First use a wet wipe,this helps to remove paste and loosen dried on material.It will not work if the solvent level is low or the application tube is blocked so this much be checked at the start of each shift.Also the solvent has to correct IPA evapourates too quickly and water based too slow.So something like SC01 or an equivalent would be better.A dry wipe helps to remove solvent.If the wiper roll does not remove the paste particles the friction while being rubbed by the roll could cause the balls to cold solder to the stencil potentially causing solderballing defects or bridging.A vac process sucks powder and paste out of the apertures.,Which leads to this,Can lead to this,