|Ask the Experts|
February 28, 2020
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.
|Expert Panel Responses|
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.
Senior Market Development Engineer
Your main contributing causes to this problem are likely to be as follows, in approximate order of importance:
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:
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.
Technical Marketing Manager
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