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Part 1. Solder Paste, Stencils & Printing (with in-line SPI)

Printing is not just "applying glue"; it is the foundation of quality in SMT assembly. Studies consistently show that 60-70% of all soldering defects originate here. This chapter breaks down the science of getting the right amount of paste in the right place, every single time.

We dissect the variables that control transfer efficiency:

  • Paste Science: Choosing the right alloy (SAC305, low-temp) and powder size (Type 4 vs. 5) to match your component pitch.
  • Process Discipline: Why strict storage, thawing, and handling rules are non-negotiable for preserving rheology.
  • Stencil Engineering: Using area ratios and aperture modifications to prevent bridging and tombstoning.
  • SPI (Solder Paste Inspection): Closing the loop with 3D volumetric measurement to catch drift before it becomes a defect.

Master the printer, and the rest of the line becomes easy. fail here, and reflow won't save you.

1.1 Paste Chemistry & Alloy Choice

Solder paste is the single largest variable in the SMT process, responsible for over 65% of end-o...

1.2 Storage, Thawing, Handling, and Traceability

Solder paste is a time-sensitive chemical system, not a shelf-stable commodity. Its performance d...

1.3 Stencil Types, Thickness, and Lifecycle

The stencil is not a static plate; it is a precision tooling die that degrades with every print s...

1.4 Aperture Design Tactics

Do not default to a 1:1 match between the PCB copper pad and the stencil aperture. The copper pad...

1.5 Printer Setup, Cleaning, and Verification Checklist

The screen printer is not a "set and forget" machine; it is a dynamic system where variables drif...

1.6 SPI Metrics, Cp/Cpk, and Closed-Loop Rules

Solder Paste Inspection (SPI) is not just a "reject gate" to catch bad boards; it is a process co...
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