--- In
Homebrew_PCBs@yahoogroups.com, "Jim Lewis" <jim.lewis@...> wrote:
>
> I have a problem with SMT chips jumping off the board during reflow. I preheat for 2.5 minutes 1/4" above the hot plate and then reflow 1.5 minutes directly on the hot plate at 375F with a cover. Paste is from Stencils Unlimited. Any solutions?
> -------------------------------------------------------
When I worked in the telecomms industry and we switched to SMT boards we had the same problem at first. It was at a very low level(<1% of boards) but when you are making >100k SMT boards a week it becomes a big problem!
At first we thought it was the reflow process, then the paste, but after we installed a high speed camera in the reflow oven and put our best material scientist brains to work we found out it was the homogeneity of the solder paste.
The volatiles were 'clumping' especially if it had been sitting around or in storage for a while.
The solution was to keep the paste "stirred" - continually mixed until just before use. Also, the paste was carefully analysed in the lab for the correct consistency.
If I remember correctly the volatiles that make the solder a paste boil off. If the get too big in size(aggrigate) they explode and blow the chips all over the place.
My suggestion to you is to take your paste and remix it as best you can and see if this helps your problem.
Stephan is correct, we only ever glued when the board was put through reflow for mixed technology and SMT components on the reflow side.
Surface tension of the paste when it melts properly (controlled spike in the oven)is sufficient to hold and correctly align the components.
Components all out of alignment usually indicates poor temperature control through the oven or contaminated parts.
Hope this helps solve your problem. It is much more difficult to control all the variables for high quality and reliability in a "home bre" environment.
Regards, Geoff.