Overprinting is common, but there are other options.

Paste-in-hole (PIH) printing (aka through-hole printing, pin-in-paste, intrusive reflow) is a way to accommodate traditional through-hole components on a mixed-technology SMT assembly using reflow soldering, rather than wave, to make the through-hole connections. Paste is printed in such a manner as to fill the through-holes; the components are inserted, and the assembly is reflowed. Assuming that the PTH components can take the heat, successful PIH boils down to three basic issues: 1) determining the amount of metal needed in the hole to create a good solder joint, 2) getting the metal in there, and 3) keeping it in there without loss when the through-hole component’s pins are inserted.

There are as many well-established guidelines for calculating how much actual metal, and thus actual solder paste, must be deposited or printed to properly fill a through-hole. Since paste is generally about 50 to 55% metal by volume, overprinting the pad area to ultimately deliver the calculated volume of solder to the connection may be required; this overprinting is typically 10% on the annular pad.

One other approach suggests the use of preforms to add pure metal volume to the printed paste, where enough paste might not be able to be applied due to PCB design or considerations. This has become somewhat popular in some quarters, with the preforms available in tape-and-reel packaging, just as chip capacitors and resistors. However, preforms are expensive and add an extra (placement) step to the process.

Once the paste is printed, keeping the ideal volume in the hole without loss during pin insertion is not necessarily easy. Although there is no requirement for special paste types, paste formulations do vary, including such attributes as tackiness and viscosity, and this can affect the propensity for paste to be dragged out of the hole by the pin, dripping, or other means. Pin shape may also be a factor.

The biggest issue with PIH is, of course, getting enough metal into the hole. Metal gets into the hole two ways: first, by being printed effectively, and second, by coalescing at the surface in overprinted areas and subsequently wicking down into the hole. Volumes have been written about different approaches to PIH over the past decade, and there are many different formulas and approaches to the implementation of the process. But PIH has become tougher with shrinking board topographies because, often, high paste volume PIH holes are too close to low-volume requirement fine pitch SMT pads. Aperture fill volume percentages are a function of paste pressure, generated by the rolling bead of paste driven across the stencil foil surface by the metal printing blade. More pressure results in better aperture fill for fine pitch, and may be required for more thorough print-filling of PIH holes, but when one has a high paste volume requirement, such as an array of through-holes next to fine pitch, the paste pressure in the bead will be reduced by the through-hole requirement and thus rob needed pressure from the fine-pitch apertures, with the result being poor aperture fill. Also, if the board is thicker than 1mm, multiple print strokes may be required, which increases the possibility of paste deposit defects on the fine-pitch pads.

Three different stencil types have been traditionally used in PIH applications: non-stepped stencils, step stencils, and two-print stencils. Step stencils are expensive and can damage print blades (as well as sustaining damage themselves), and the minimum step-up requirements between step areas (to prevent blade damage) place burdens on the PCB designer. The two-print stencil technique requires a second printer inline to perform the second print! Additionally, step stencils reduce throughput and add material expense due to the need to increase underwipe frequency. In terms of materials alone – wasted paste, wiping paper, and toxic disposal of the waste paste residue – it’s a significant contribution to operating cost.

Typically, overprinting has been a standard and necessary technique; i.e., printing solder paste on a masked radius around each PIH hole. The intent is that the solder will coalesce upon reflow and wick toward the through-hole and fill it with sufficient solder. This was a much more reliable technique before Pb-free solders, which generally don’t coalesce or wick as nicely as pre-RoHS pastes. Thus, stray solder balls are always a concern, as well as wicking to adjacent component leads, causing bridging and other problems.

Another consideration: enclosed media printing, which can provide uniformly controlled solder paste pressure, and eliminate the need for step stencils, overprinting and two-print methods. Paste pressure is not dependent on squeegee angle of attack or speed. Since the pressure can be adjusted, hole fill can be optimized, and other process variables maintained as constant values without the need to tweak.

Michael Martel is product marketing engineer at Speedline Technologies; mmartel@speedlinetech.com.

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