|Ask the Experts|
December 13, 2019
Bottom Side Chip Bonding
When should we use adhesive to bond SMT components to the bottom side of a double sided PWA before going thru reflow soldering? Component size, weight, etc.
|Expert Panel Responses|
As a well known IPC member likes to say, "It depends." (D. Pauls - Rockwell Collins)
Typically in a double sided reflow SMT adhesive isn't required. One side of the assembly is pasted, placed, and then reflowed. Then the second side is run through the same process of paste, place, then reflow. The reflowed solder for the components hanging down on the bottom of the board should be sufficient to hold the components in place. I haven't seen much usage of adhesive in double sided reflow.
Having said that, there are exceptions to everything. If you have a relatively heavy component and you are having issues of the component falling off in reflow, then an adhesive may be appropriate.
As usual, it falls to the designer/manufacturer to determine when an adhesive is appropriate.
Manager of Assembly Technology
This is a really good question. In fact, there is a bunch of incorrect information out there on this. There are a number of variables that affect "inverted adhesion" during a second reflow operation, but the prime variable is the ratio of the total peripheral length of all solder joints to the component mass.
The second-most important variable is the solder alloy being used; SN63 has a much lower surface energy, so a lower retention force, than most Pb-free alloys. It's more than 30% lower than SAC305, for instance.
The "retention force" due to the surface energy of the solder normally should be at very least 1.5 times the part mass, preferably 2 times the mass. All this assumes that the solder joints are distributed relatively evenly around the center of mass of the part; if not, a simple ratio calculation will fail to predict accurately. For most components this condition is at least approximately met.
It can be challenging to do this calculation for a lot of parts, but normally you can do it by part class, and determine that "parts larger than X" should be affixed.
I have done quite a bit of work in this area, and because this calculation is needed on a repeated basis, I have put together an Excel worksheet to do the calculation. If you would like a copy of this worksheet,please e-mail me directly, I would be glad to provide it.
There should not be a normal situation where the components are glued to the bottom side of a PCBA if only seeing double sided reflow. The chip bonder epoxy is used for direct contact for wave solder processes.
In unique situations with large components and thin PCBs the use of chipbonder can assist in reducing stress during second side reflow but the larger components should be on the second side not the first. A pallet can be used to reduce the thermal impact on the second side if need be.
President/Senior Technical Consultant
Unfortunately this is another questions that bring the "it depends" answer. Usually the components placed on the first side of the board do not have to be bonded since the "surface tension" of the solder will hold them in place.
Every rule has exceptions and those will have to be evaluated separately based upon component weight, number of leads and contact surface with the board.
On a typical reflow equipment with bottom and top side heaters, you can set up profiles that have the bottom zone(s) setting(s) slightly cooler than the top and that will prevent components from falling during the second reflow. I'm referring here to the above liquidos temperature zones. This will eliminate the need for component bonding. I've tried it for years and it works very good.
Engineering and Operations Management
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