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September 25, 2012

Pin-in-Paste Standards

What is the recommended pin length and PTH hole dia for pin-in-paste. What is the recommended pin protrusion on the "B" side of circuit board assembly?


Experts Comments

There is no "standard" per se for pin-in-paste (PiP) soldering. The closest we have is IPC-610 workmanship standard for solder joint quality. That being said, it is often difficult to meet the same requirements for PiP solder joints as for wave soldered joints.

Pin-in-paste often results in more voids and barrel fill may be low especially in thick boards (<0.063in). Since through-hole part leads come in varying diameters and cross-sections, there is no one size recommendation for the PTH. Rule of thumb dictates that less clearance of lead-to-hole is generally favorable for fill since it is often difficult to deposit enough solder paste for -610 passable solder joints.

Beware reducing hole diameter too drastically as outgassing solder paste fractions may result in forcible ejection of the paste or molten solder. Often PiP solder joints do not meet -610 requirements and a workmanship waiver may be necessary. Best to check solder joints by x-ray both laterally and normal to the board surface to assess vertical and circumferential fill and wetting. Remember too that PTH solder joints are many times stronger than SMT solder joints, so the idea of a waiver is not unreasonable.  

As for lead length, there is no requirement for same since PiP does not need lead protrusion for initial wetting as in the wave solder process. There is even a technique (Buried Intrusive Reflow - BIR) which uses a lead which is necessarily and significantly shorter than the thickness of the PCB which makes it attractive for thick PCBs. BIR results in well-wetted, more reproducible solder joints than conventional long-lead PiP PTH soldering and reliability studies with several connector styles show that it has acceptable reliability. Write to this author for more information on BIR.
Gary Freedman
Colab Engineering
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.
Pin #2 .040" FHS - Pin #1 .036" FHS
In order to prevent the pin from pushing paste out of the through hole, the length should not be much longer than board thickness: about 1/3 of board thickness protrusion. Pin to hole ratio is important as seen below. Pin to hole ratios of .625 or greater gave no voiding in test performed.

Pin diameter in photos is 25 mils. .063" think board, .060" annular ring.
Pin #4 .048" FHS - Pin #3 .044" FHS
Insufficient solder volume. Largest hole size. 6 mil stencil.
Bill Coleman
Vice President Technology
Photo Stencil
For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.
Normally you are struggling to get adequate paste volume and required volume increases as the square of the PTH diameter, so you really want to minimize the PTH diameter. The minimum practical diameter is the smallest diameter hole into which the component can be reliably inserted. Insertion is complicated by the fact that if you have over-printed the hole, you can no longer see it. The lead configuration also plays a role; a lead with a tapered end is much easier to insert than one that is cut square. In general, a PTH diameter of about 0.009" larger than the pin (either pin diameter or the diagonal measurement if it is square or rectangular) is about the smallest reliable dimension.

Pin length is another matter. All depends on what the final solder joint requirements are: Class 3, Class 2, etcetera. If you have no requirement for visible fillets or lead protrusion on the back side, then you can in many cases use a lead length that is equal to or even less than the total PWB thickness. For example, it can be very beneficial to use a lead length that is less than the PWB thickness when working with thick boards, since the solder volume required is reduced (you need not fill the open part of the hole). In this case, the pin length should be long enough that the shear strength of the soldered area of the lead surface exceeds the tensile strength of the lead, as a general rule. In cases where visible lead protrusion is required, use a lead length that is equal to or only slightly greater than the PWB maximum thickness. The fillets observed on pin-in-paste joints are normally somewhat concave, and so the end of the lead will still be visible.

One major risk with pin-in-paste is failing to estimate accurately how much solder paste will fill the PTH prior to component insertion. For small holes (<0.020" diameter) the paste may penetrate only a short way. For large holes (>0.040") the paste may actually fill the entire hole and be extruded out the bottom. Process controls are required to manage the amount of fill and avoid contaminating tooling and equipment.
Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
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