01 -- What is Pin-in-Paste?
Pin-in-Paste (PIP), also called Through-Hole Reflow (THR) or intrusive soldering, is a process where solder paste is printed over plated through-holes before the THR component is inserted. When the board goes through the reflow oven, the paste melts, wicks through the barrel by capillary action, and forms fillets on both the top side (component body side) and the bottom side (solder side). One reflow pass handles both the SMT components and the THR connector simultaneously, eliminating a separate wave or selective soldering step.
For the process to work, the component must be rated for reflow temperatures -- peak 260 °C per IPC/JEDEC J-STD-020D. Connector housings are typically LCP (Liquid Crystal Polymer), which is dimensionally stable through reflow. Pins are designed shorter than standard THT: the tip protrudes 0.5 mm or less below the board surface to allow bottom-side fillet formation without pushing paste out of the aperture during insertion.
02 -- How to Use
- Select pin cross-section shape -- round or rectangular.
- Enter pin dimensions from the component datasheet.
- Enter the finished hole diameter and the solder pad (annular ring) diameter from your PCB layout Gerbers.
- Enter your stencil thickness and solder paste metal content (50% by volume is standard for type 3/4 SAC pastes).
- Read the required paste volume and stencil aperture. Use the aperture shape toggle to get the dimensions in whichever geometry fits your layout constraints.
03 -- How the Calculation Works
The required solder volume has three components: fill the barrel, form a fillet on top, form a fillet on the bottom, then subtract what the pin itself occupies inside the barrel.
Vtotal = 2 × Vfillet + Vhole - Vpin
Hole volume is the cylindrical barrel space:
Vhole = π × (Dh / 2)² × Tb
Pin volume -- for a round pin: π × (Wp / 2)² × Tb. For a rectangular pin with cross-section A × B, the same formula is used with an equivalent circular radius r = √(A × B / π), giving Vpin = A × B × Tb.
Fillet volume uses the Pappus-Guldin centroid theorem applied to the curved meniscus cross-section of the solder fillet. C is the fillet reach -- the radial distance from the edge of the pin to the edge of the pad.
Vfillet = 0.215 × C² × (0.2334 × C + r) × 2π
C = (Dpad - Dpin effective) / 2
Because solder paste is approximately 50% metal by volume (the flux vehicle evaporates during reflow), the required paste volume is larger than the required solder volume:
Vpaste = Vtotal / metal content fraction
The stencil aperture area that delivers this paste volume is:
Aperture area = Vpaste / Ts
Paste-in-hole effect: When the stencil aperture sits over the through-hole during printing, some paste falls into the barrel rather than staying on the pad surface. This is not accounted for in the basic formula. For holes larger than about 1.0 mm diameter with standard 0.15 mm stencils, plan for 10-20% additional aperture area, or use a step stencil with a raised plateau over the THR zone.
04 -- IPC-A-610 Acceptance Criteria
IPC-A-610 requires minimum 75% barrel fill for plated through-hole solder joints, across all three product classes. This calculator targets 100% fill plus full top and bottom fillets -- the ideal result. In production, 80-95% fill is typical with correct aperture sizing. If fill is consistently below 75%, check: aperture area (increase it), squeegee pressure over the THR zone (increase slightly), paste metal content (verify), and stencil thickness (consider stepping up over the THR area). Bottom-side fillet absence is a common IPC-A-610 failure mode and is always a sign of insufficient paste volume.