Ask the Experts
July 31, 2018
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, "Itdepends." (D. Pauls - Rockwell Collins)
Typically in a double sided reflow SMT adhesive isn'trequired. One side of the assembly is pasted, placed, and then reflowed. Thenthe 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 boardshould be sufficient to hold the components in place. I haven't seen much usageof adhesive in double sided reflow.
Having said that, there are exceptions to everything. Ifyou have a relatively heavy component and you are having issues of thecomponent falling off in reflow, then an adhesive may be appropriate.
As usual, it falls to the designer/manufacturerto determine when an adhesive is appropriate.
Manager of Assembly Technology
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.
Thisis a really good question. In fact, there is a bunch of incorrect informationout there on this. There are a number of variables that affect "invertedadhesion" during a second reflow operation, but the prime variable is the ratioof the total peripheral length of all solder joints to the componentmass. The second-most important variable is the solder alloy being used;SN63 has a much lower surface energy, so a lower retention force, than mostPb-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 atvery least 1.5 times the part mass, preferably 2 times the mass. All thisassumes that the solder joints are distributed relatively evenly around thecenter of mass of the part; if not, a simple ratio calculation will fail topredict accurately. For most components this condition is at leastapproximately met.
Itcan be challenging to do this calculation for a lot of parts, but normally youcan do it by part class, and determine that "parts larger than X" should beaffixed.
I have done quite a bit of work in this area, and because thiscalculation is needed on a repeated basis, I have put together an Excelworksheet 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.
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
There should not be a normal situation where thecomponents are glued to the bottom side of a PCBA if only seeing double sidedreflow. The chipbonder epoxy is used for direct contact for wave solderprocesses.
In unique situations with large components and thin PCBs theuse of chipbonder can assist in reducing stress during second side reflow butthe larger components should be on the second side not the first. Apallet can be used to reduce the thermal impact on the second side if needbe.
President/Senior Technical Consultant
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.
Unfortunatelythis is another questions that bring the "it depends" answer. Usually thecomponents placed on the first side of the board do not have to be bonded sincethe "surface tension" of the solder will hold them in place. Every rule hasexceptions and those will have to be evaluated separately based upon componentweight, number of leads and contact surface with the board.
On a typical reflow equipment with bottom and top side heaters, you canset up profiles that have the bottom zone(s) setting(s) slightly coolerthan the top and that will prevent components from falling during the secondreflow. I'm referring here to the above liquidos temperature zones. This willeliminate the need for component bonding. I've tried it for years and it worksvery good.
Engineering and Operations Management
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at firstname.lastname@example.org.