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Solder paste printing for through-hole reflow soldering is an evolving process. The key is to obtain the correct volume of paste to meet the minimum requirement of IPC 610. Although the strength of the through-hole connection is not in doubt, the ideal situation for the joint is to look exactly like a manual or wave solder connection.

Figure 1: A square print on the pins would allow increased solder volume.

 

A 100% fill is achievable if the volume of paste is correctly applied. The holddown features on the bottom of the image have been printed with paste in the holes with a cross-hair aperture in the stencil to limit flow through and place paste in the hole and not on the surface of the pad. The connector body sits on the surface of the pad. Paste in this area could cause solder balls under the connector body after reflow.

Figure 2 shows a close up view of poor solder coverage on the surface of through hole pads after pin in hole reflow (PIHR) assembly. This can be a function of the solderability of the basic board but is more likely on the base of the board to be insufficient paste pushed through or available in the hole. Tests have shown that the strength of the joints are not affected but it would make visual inspection difficult. Examination of the paste volume will be required and possible modification of the stencil design.

Figure 2: An example of poor wetting.

 

Split print solder paste design for pin in hole reflow as shown in Figure 3 would appear to be a good idea as it allows the operator to see the hole for manual insertion of the leaded connector. Unfortunately it often does not work as the paste fails to wet the pads causing separation and solder balls. It is possible to print a paste deposit leaving a slight nick in the print to help guide the operator for lead insertion. DEK Printing Machines have some process guidelines for this technique that may be beneficial.

Figure 3: There are better options than the split prints shown here.

Damaged Stencils	Stencil Blockage	Paste Misalignment	Smudged Paste	Poor Definition
Incomplete Prints	Residues	Bleeds	Ears	Scooping
Shorts	Slump	PIHR Printing