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The Knowledge and Know-how Connection
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June 18, 2013
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Views: 3302
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Going into the reality of pin-in paste requires a little more than applying paste into the hole. There are usually unforeseen issues which occur after the pin in paste operation. The stencil can be designed to prevent inadequate hole fill by forming a cross pattern with the paste around the top side of the hole. The flux will stay mostly on the top of the pcb under the part instead of flushing out to the bottom pad. The flowed solder will only take what is required to do the job. Making the use of expensive preforms is not necessary. People are asking for advice or suggestions as to which way to perform their particular task at hand. They may not be aware of the pitfalls of following this advice. I know I would prefer to also know what I shouldn't do as well as what I should.
Mark A. Maheux Sr., Sr. Manufacturing Engineer, Honeywell Life Safety, USA
Readers may be interested in the free ebook on "Pin In Hole Intrusive Reflow Design and Assembly" at http://www.pihrtechnology.com
The most common problem with the process is the hole to lead ratio, at the design stage its easy to change and it gives design engineers more tracking space. Changing the hole to lead ratio on existing design is easy too as its not going to impact reliability either.
Bob Willis, Bobwillisonline.com, UK
There are numerous tradeoffs to consider when pin in paste hole fill is the goal. First, overprinting can be problematic due to the possibility of mid chip solder balls. Step stencils are more expensive than standard stencils, and will likely result in reduced throughput and material expense due to the need to increase underwipe frequency.
One technology which is becoming very popular is the use of solder preforms in tape and reel packaging. The preforms come in standard sizes, such as 0402, 0603 and 0805 to name a few. They are packaged exactly like chip capacitors and resistors.
Placement is done after paste printing. Only 20-25% of the preform needs to contact the solder paste. Since the preform is solid metal, a preform as large as four times the volume of the paste volume can be added and still achieve perfect reflow results.
Paul Koep, Cookson Electronics