Have you considered the effectiveness and yield performance of your solder paste printing process lately? It has been said that 60-80% of all circuit board defects are attributed to the solder paste printing process.
The following is a list of attributes or key variables that must be under control to maximize solder paste transfer during the stencil printing process. With these attributes under control you will take 90 % of the black magic out of the SMT stencil printing process. Remember all of these variables work together in a dynamic system to provide the print quality you receive on your SMT print line.
LIST OF KEY VARIABLES
PCB Variables
- Board coplanarity – will impact stencil gasketing, paste volume transfer, component placement. Legend, solder level and solder mask can impact coplanarity
- Plug in Via’s- impacts stencil gasketing by raising stencil.
- Solder mask thickness in relation to pads – will impact stencil gasketing and solder paste volume transfer.
- Board warp – impacts stencil gasketing, paste volume transfer, and component placement.
- Tooling holes if used for alignment – impacts board and stencil alignment.
- Board stretch / step and repeat- impacts printing accuracy overall print quality.
- Pad width – impacts stencil gasketing , print quality, solder paste transfer accuracy.
Solder Paste Variables
- Powder size and distribution – impacts print quality, solder paste transfer, bridging and insufficients
- Rheology – impacts slumping which affects deposition height causing bridging and insufficients
- Environmental factors – temperature and humidity will affect rheology.
Stencil Variables
- Aperture width relative to pad width – impacts print quality, paste transfer deposition volume.
- Wall smoothness – impacts solder paste release, solder paste transfer efficiency, deposition volume.
- Aperture and stencil thickness tolerance – impacts deposition thickness and volume causing bridging and insufficients.
- Trapezoidal apertures – impact deposition volume, paste transfer efficiency.
- Stretch during stencil assembly – impacts deposition accuracy
- Foil distance outside image area – impacts print quality.
- Stencil thickness – determines deposition height, print quality, bridging and insufficients
- Fiducial location and fill – impacts board and stencil alignment, deposit accuracy.
- Gasketing – impacts print quality & bridging
Stencil Printer
- Squeegee types – metal, urethane, edge design effect print quality, solder paste transfer, deposition morphology, bridging and insufficients.
- Squeegee speed – impacts print quality, deposition volume, bridging and insufficients paste rheology.
- Squeegee pressure – impacts print quality, deposition volume, paste rheology, bridging and insufficients.
- Pad orientation – parallel apertures to squeegee stroke generally have 5-10 % less paste than those perpendicular to stroke.
- Alignment accuracy – impacts print quality
- Board orientation – 0, 45 or 90 degrees can impact print quality. Especially if coplanarity is poor.
- Quick or slow release – effects print quality, deposition morphology
- Environmental issues – effect paste roll, release, paste rheology.
- Cleaning & wiping stencil – impacts paste deposition quality and cycle time.
- Board stretch – impacts board and stencil alignment, print quality.
- Consistency and accuracy of setup – impacts print quality and repeatability.
- Post print QC & SPC – effects overall quality
If you haven’t heard of these variables or haven’t considered them lately and there are employees performing rework on your circuit boards you should reconsider. Look for additional blogs and communications from the Murray Percival Co. discussing in detail some of these very important printing variables. The SMT Troubleshooting Guide is just one example as to how we may help in your surface mount assembly process.
Happening at Murray Percival Company 
Nice job Mark and Brian, well done!