The most common reason I have seen for flux entrapment in SMA is improperly cured glue. A common error is not considering that cure time and temperature refer to the adhesive bond line and not just the oven. For example, if something is to cure for 3 minutes at 150C, it is not sufficient to put in a 150C oven for 3 min if the substrate is bonded to a large metal plate. The adhesive needs to see that time and temperature in order to cure properly. If the glue is not cured prior to the application of the flux then it is more likely to become entrapped. As far as dot size, shape, placement, etc. and flux entrapment what you want to avoid (this is more applicable to a paste/reflow process than a fluxing/wave solder process) is glue in contact or "bleeding" onto the pads. The dot(s) should be placed such that when "smooshed" (not quite a technical term) by the component placement the glue s!
hould wet the bottom of the component, but not spread out onto the pads.
We would recommend coverage on the order 80% of the area between the pads with a dot width to height ratio of 2:1 up to 4:1 (depending on component geometry). Also the height of the dot should be greater than the sum of the height of the pads (w.r.t. board surface) and thickness of metalization/termination (w.r.t. component body) - this would be Rs &Cs. For QFPs and other leaded devices, the height should be greater than the sum of the height of the pads (w.r.t. board surface) and the height of the leads. Once again, these are guidelines, the objective is to place enough glue such that the part is held in place during the process, yet not too much such that it interferes with the soldering process.
Subject:
Re: SMT Glue and Paste with Wavesolder
Brian,
What has been your experience with flux entrapment using a chip bonder/wave solder process? Is there a particular shape or amount of adhesive that adversely affects flux entrapment or reduces it?
Curious Minds,
KennyB
Brian J. Toleno, Ph.D.
Sr. Applications Chemist
Loctite Electronics
15051 East Don Julian Rd
Industry, CA 91746
Ph: 626-968-6511 x304
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