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Suggested Process changes



Suggested Process changes | 11 November, 2004

Dear Colleagues,

I am working with a new prototype product that is presenting some interesting problems. My Engineering Dept. has made some suggestions that I find a little odd. I am looking for some confirmation from fellow SMD personnel as to the soundness of these suggestions.

The suggestions include: 1st.

"Soldering and unsoldering is strongly recommended to be accomplished with a hot-air pencil, since it is practically impossible to cause thermal shock cracking using hot air pencils."


"Post reflow rework processes will be different than in the past. Due to the size and nature of the component packaging, the PCB and the component both need to be heated to bring the solder joint to melting point, and allowing the removal, replacement or repair of the part.

Processes and tools that will only heat the component will result in some of the problems discussed about moisture sensitivity. Heat sources that directly heat the component (i.e. - soldering irons) can cause thermal shock.

The much preferred method is the so called �SMD rework station�, which heats the PCB from underneath, uses temperature controlled directed hot air to melt the solder at the joints, and a built in vacuum pick up to remove the component." And finally, and most worrisome,

"Add liquid flux to the solder paste post placement and prior to reflow."

The first two suggestions have merit but the final suggestion seems very strange to me. I realize that this has been a lengthy posting and I appreciate your time.

Thank You,


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Suggested Process changes | 11 November, 2004

D.B., You are justified in your concern over the last requirement. I have been imposed by a customer with the same hand solder/rework requirements. It seems the biggest concern is thermally shocking ceramic chips. You are much less likely to do this with a hot air tool. It has always ben by belief to NEVER add flux to solder paste. This will change the rheaology which would be a concern pre-print. Adding flux post print and pre-reflow is just asking for a solder ball problem. You may try it but I think you will see a breakdown of the print definition as soon as you add flux. This could result is solder balling or bridging.

Maybee you could get this Engineer to show you exactly what he/she wants. That way, if there is a problem, the person making this a requirement could see it first hand.

Good Luck


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Suggested Process changes | 11 November, 2004

Adding flux prior to reflow was not an uncommon process on certain packages - a couple of our Universal GSM'S have flux heads for this reason (Not that we've ever used it on anything).

What is the component they are suggesting using additional flux on, and how are they suggesting adding it (flux delivery system)?

Regarding the first 2 comments, no problem with the hot air pencil, and yes a rework station with underside heating is great for BGA's and other such components.



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