|Ask the Experts
November 13, 2023 - Updated
February 19, 2019 - Originally Posted
Components Falling Off During Reflow
Can excessive solder paste volume contribute to components falling off the bottom of the circuit board during topside reflow?
|Expert Panel Responses
Theoretically yes, but there are a lot of factors that would determine the point at which that would happen. The most important factor would be the ratio of solder contact surface area to component mass since the surface tension of the molten solder is what holds the parts in place during the 2nd reflow cycle.
I would not expect relatively small deviations from optimal solder volume to cause parts to fall off unless the mass of the part is too large to begin with.
You should allow for a significant factor of safety - do not expect parts to stay in place if you are close to the limit of the retention force for a given surface area. High mass, low contact area parts can be retained with thermal cure epoxy.
The epoxy would usually be applied between the solder paste screening and component placement of the 1st side process. The epoxy cures below reflow temperature, so it could be applied any time before 2nd side reflow.
Principal Product Engineer
Benchmark Electronics, Inc.
Excessive solder paste volume on the 2nd side (top) should not lead to components falling off of the 1st side (bottom). Low solder paste volume on the 1st side (bottom) may lead to components falling off.
It is best to run the side with the smaller lighter components first, so the surface tension of the solder has a better chance to hold those components in place during 2nd side reflow.
Another option is to use glue dots, a pallet or fixture to hold the bottom side components in place during 2nd side reflow.
No, I do not see excessive solder paste volume contributing to components falling off the bottom of the circuit board during topside reflow.
If anything, more solder means a more robust bond formed.
Technical Support Engineer
Excessive solder will not contribute to component falling, but it may result in bridging, solder balling, and tomb stoning of components. I presume you are facing component falling issue, and on high density components it’s usual to see component falling.
A high temperature in reflow during secondary process can cause this to occur, advise you to try solder paste with higher melting temperature on bottom side, and paste with low melting temperature on Top side. Many suppliers have different alloys and they can help you selecting high and low melting paste considering your board. Hope this should solve your issue!
In a word, yes. That's particularly true if the situation was marginal to begin with (part mass too close to retention force). Inverted reflow adhesion is a widely misunderstood phenomenon. It depends mainly on the ratio of the mass of the component to the retention force.
The retention force, in turn, depends on the peripheral length of all the solder joints at their smallest cross section. It does not depend on the solder joint area. The retention force also depends on the solder alloy, the aggressiveness of the flux employed, and the atmosphere (e.g. air, nitrogen) and needs to be greater than the part mass, including some safety factor.
The reason that excess paste can be a detriment is that it can create a situation where the peripheral length of the solder joints is decreased, for instance by wetting above the foot of a gull-wing lead.
In this case the peripheral length at the smallest cross section is only the distance around the lead (2*W*L) instead of the distance around the foot of the lead. This allows the part to drop away from the PWB surface, the solder to be drawn into an extended drop, and finally to snap off.
For component fall down issue, first we need to check for how many components falling down or particular component is falling down. If particular component is falling down then need to check datasheet of this component for solderability. Also if reducing the temp of bottom side can solve the issue.Ganesh Kulkarni, NMTRONICS INDIA PVT LTD
A visual example of how too much paste (solder) on the bottom side components is a dripping faucet. The drop of water will hold (surface tension) onto the faucet's surface until it gets large enough, and then it drips. If the volume of solder on the bottom side components is high enough, it may drop the component off the board like that drop of water off the faucet. Another possible cause is mechanical: vibration, or a "jerk" or bump in the conveyor system while in the reflow zone(s).
Or maybe the conveyor is warped in width along its length, squeezing the board and warping it while in the reflow zone(s). Checking for vibration or "bumps" and the conveyor's width (changes) along its entire length while heated may be necessary. This can be done visually with a good camera outside the oven look in to see if the board is bumped, squeezed or warped as it progresses.
Senior Project Engineer
Electronic Controls Design Inc
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