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February 20, 2018

Guidelines for Pin-in-Paste

Is there a set of design guidelines for pin-in-paste technology?

We've tried a combination of 150 micron thick stencil and 1.6 mm thick board, but it seems we never manage to get the right amount of solder into the hole, and at the end of the reflow process, we have a very weak solder joint. The pin is square with 0.6 mm side length and the hole is 1 mm in diameter. The paste is a standard type 3 SAC305.



J. R.

Experts Comments

Many applications, using a solder preform in conjunction with solder paste is an easy fix to achieve 100% hole-fill. These preforms come in standard pick & place shapes- 0603 / 0402 and are packaged in tape & reel.

Process: Print paste, Place preform by offsetting it away from the hole; place through-hole component; reflow

Based on the PCB & connector dimensions, it is possible to calculate the volume of solder needed to form a reliable joint and hence the volume of solder paste needed. We have developed a spreadsheet to calculate the optimal stencil aperture dimensions based on the PCB & connector dimensions
  • Use an optimized flux chemistry to maximize solder paste transfer and minimize paste trapped in the stencil apertures
  • Using tapered connector pins to minimize the paste volume pushed out from the PCB hole
  • Overprinting (typical is 10%) on the annular pad; this is also a function of the connector pitch and spacing available and ability of the paste flux to pull back solder during reflow
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Karthik Vijay
Technical Manager - Europe
Indium Corp.
Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.

I have a Through Hole stencil design guideline that we use to make stencils for intrusive reflow stencil customers. Input data is 1-board thickness, 2-pin dimensions, 3-diameter of through hole, 4-dimensions of annular ring.

From this data we will calculate paste volume required to achieve positive fillets and design the stencil.

Three possible stencil designs are possible depending on paste volume required: Overprint apertures with single thickness stencil; Overprint apertures with step-up stencil; Two-Print Stencil. I have a recent paper on Intrusive reflow of Lead-free paste. Get in touch if I can help with your requirements.

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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.

In order to calculate the volume of solder paste needed, you will first have to calculate the volume of the pin then the volume of the plated-through-hole (PTH). Subtract the pin volume from the PTH volume.

This will give you the volume of solder needed. Additionally, you have to remember that the volume of the paste deposited has to be double the volume required to fill the solder gap. Stencil aperture design is also important for pin-in-paste applications.

A split aperture design stencil should be used if possible. A printed disc over the PTH will lead to inconsistent volumes as some of the solder paste will randomly fall out of the bottom side during the insertion of the lead.

A split print with a gap just large enough to make some minimal contact with the pin will yield a more consistent fill. On large fills AIM has been successful with prints up .250 inches distance from the edge of the PTH to the outside of the print. These will wick consistently down the PTH.

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Karl Seelig
Vice President of Technology
AIM
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

I do not have any fixed guidelines except to say that this is essentially a volume calculation. If you know the size of the pin and the hole; then, it is a simple calculation for the volume of metal to fill the hole.

Furthermore, you need to allow for a fillet (I use 20% as a rule of thumb) to get the volume of metal you require. You will then need to double this because solder past is only approximately 50% by volume. This will give you the volume of paste you need to apply. A 1.6mm thick board will probably require a step stencil to get sufficient material deposited to fill the barrel.

A further point is that if you simply fill the barrel with paste much of this will get pushed out when you insert the component; usually this will drip off the pin prior to reflow. It is often advantageous to put a bar across the center of the hole (typically about 30% of the diameter), this forces the paste down the edges of the barrel and reduces the tendency for push through.

The Department of Systems Science & Industrial Engineering of the State University of New York, Binghamton and the Surface Mount Technology Laboratory of Universal Instruments Corporation did an extensive study on the pin-in-paste process using a DEK screen printer and Vitronics Reflow oven.

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Doug Dixon
Global Marketing Director
Henkel Electronics
Mr. Dixon has been in the electronics field for over twenty years and is the Global Marketing Director with the electronics group of Henkel. Prior to joining Henkel, he worked for Raytheon, Camalot Systems, and Universal Instruments.
Reader Comment
You can also replace the screen printer with a jet printer for solder paste, such as MY600 from Mycronic, or use the jet printer as add-on solution after the screen printer depending on your production requirements. With optimized solder paste volumes, jet printing can reliably handle difficult components such as QFNs and pin-in-paste components.
Joergen Lundberg, Mycronic AB, Sweden
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