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September 9, 2014

Pin in Paste Solder Dropping

Can someone help me with Pin in Paste to eliminate solder dropping during reflow process. I have tried a slower heat gradient profile to keep the solder paste viscosity higher, but it did not make a significant difference.

K.S.

Experts Comments

Pin in paste requires modifications to the stencil design as to avoid paste drop-off. It has to be also mentioned that the correct viscosity or rheological properties of the solder paste are maintained as to avoid slumping. The slumping properties of solder pastes are designed at the development of the solder paste flux system and some solder pastes are better at hot slump behavior than others.  

Increasing the metal percent of a solder paste at times helps reduce drop-off. In doing this if it is available, care must be taken as to not impact negatively the other important solder paste properties such as paste roll, potential for aperture clogs and paste dry out depending on the SMT environment.  

The obvious changes which help are thinner deposits of solder paste, step-downs, thermal profile modifications as to remove more volatile solvents from the paste as to decrease its mobility. Here care must be taken to avoid flux deactivation. This will lead to poor wetting among other issues.
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Peter Biocca
Senior Market Development Engineer
Kester
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.
Your main contributing causes to this problem are likely to be as follows, in approximate order of importance:
  • Pin geometry and length
  • Combination of hole size and board thickness
  • Paste properties
Pin geometry is key to this process as it affects how much paste is pushed out of the hole to "hang" on the end of the pin. A large ball of paste hanging on the end of the lead will be very hard to retain. Hole size is important, since the volume of the hole increases with the square of the diameter, and the required paste volume also tends to increase dramatically.

Larger holes will fill to a deeper depth, perhaps more than what is required. Paste properties, specifically hot slump properties can affect how paste is retained. Tweaking the reflow profile may help, but will not dramatically change a paste's slump behavior. Now with all that bad news behind us, what can we do to improve our lot in life? Following are some ideas for optimizing the process to limit paste dropping:
  • Limit volume to only what is needed. If the hole is filling too deeply, tailor the aperture to partially block the hole, limiting the fill. You can also print more on the top surface, and further reduce the fill, if you have room to over-print
  • Reduce the pin length to the minimum. This will limit how much paste gets pushed out and how far, enabling it to more easily draw back. If there is no requirement for the pin being visible, it may be possible to make the pin actually shorter than the board thickness
  • Consider a paste with better hot slump behavior, if you are permitted to change the paste
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Fritz Byle
Process Engineer
Astronautics
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.
In order to maximize the paste available in pin & paste operation there are some key considerations. The pin to hole ratios need to be as tight as possible to reduce the size of the gap that needs to be filled by the solder. The paste volume is reduced up to 50% after reflow and overprinting the paste hole by 50% is a common technique to accommodate this characteristic.​

Increasing the squeegee pressure and decreasing the print speed will help deposit more paste into the hole and some assemblers will even double print the assembly to maximize the volume of paste in the hole.

Also, the lead length should not protrude very far past the substrate as this will tend to remove more paste from the hole due to more surface area available to drag the paste out of the hole ​and to wick away molten solder.
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Tim O'Neill
Technical Marketing Manager
AIM
Tim O'Neill is the Technical Marketing Manager for AIM Products. AIM is a global supplier of materials for the PCB assembly industry including solders, fluxes and thermal management materials. Tim has a B.A. from Assumption College and post-graduate studies in education. He has 20 years of experience in the electronics soldering industry, beginning his career in 1994 with EFD and was key in business development of their fine pitch solder paste dispensing technology. Tim joined AIM in 1997 and has since assisted many clients with assembly challenges, specializing in Pb-Free process development and material selection.
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