Troubleshooting the Stencil Printing Process （故障诊断模板印刷过程）.pdf

Troubleshooting the Stencil Printing Process Chrys Shea, Shea Engineering Services Sponsored by: Topics Printing – Solder paste properties – Key elements of the process Inspecting – Phase Shift Interferometry –Typical Control Methods – SPI SPC Overall system check – Will identify root cause 80% of the time Investigating specific defect modes – For the other 20% Solder Paste Properties • Solder paste is not solid, it is not liquid, it is thixotropic: – Yields (moves) when pressure is applied to it – Holds its shape when pressure is not applied – Thins down and stiffens up (like peanut butter) • Viscosity – how easily it flows under pressure • Rheology – how its viscosity changes as the pressure changes Key Elements of a Solder Paste Printing System PWB Stencil Solder Paste Squeegee Mechanical Setup - Support, Alignment, Contact, Gasketing Separation PWB Pads: Flat, Right Solder Mask: No taller than Size, Free of Debris pad, not encroaching on or Solder Mask pad (unless SMD) PWB: Minimal warp and bow Positional Accuracy N O T T O S C A L E Stencil Apertures: Right Size Shape, Smooth Walls, Uniform Thickness, Free of Debris or Solder Paste Positional Accuracy N O T T O S C A L E Solder Paste Formulation, Rheology, Release Characteristics, Particle Size, Paste Temperature, Ambient Temperature Humidity N O T T O S C A L E Squeegee Angle, Speed, Pressure, Stiffness, Surface Energy N O T T O S C A L E A Good