|Ask the Experts|
April 22, 2021
BGA Placement Paste or Only Flux
What are the risks with using flux-only vs. solder paste for placing BGAs in a hybrid lead-free environment?
|Expert Panel Responses|
The biggest risk for using flux only would be the increased propensity for open joints. If the BGA warps during reflow, bumps can lift and lose contact with the pad. At least if there is 4 to 6 mils of paste on the pad, that will help to bridge any gap that occurs due to component warpage.
Generally speaking the additional solder supplied by the paste will improve the mechanical integrity of the solder joint. It will also help to improve the standoff height which would facilitate cleaning underneath the BGA, if required.
Technical Support Engineer
With a flux only process for placing BGA's you have the potential for unsoldered connections and insufficient solder which will cause an added heat cycle to rework. We use an automated paste dispenser which allows for better process control.
Manufacturing Applications Specialist
When it comes to lead-free I prefer to deposit paste on the PCB BGA pads to maintain a steady process along the SMT line. When you use flux only, you rely on the BGA bump ability to form the solder joint with your PCB. The intermetallic layer formed might not be as reliable as one formed with solder paste.
I have worked with placement systems mostly used for micro-BGA which is "dipped" onto a tacky flux surface and placed on the board. These systems require a lot of maintenance because you need to measure the thickness of the tacky flux film in order to ensure proper ball coverage.
Senior Manufacturing Engineer
BGA's can be soldered using only gel/tacky flux. This process works well assuming there is enough solder volume in the spheres to form a good solder joint on the circuit board pads. If you intend to solder lead-free BGAs then it is recommended to use a lead-free gel flux and a lead-free reflow profile.
Gel/tacky fluxes leave more residue than using a solder paste simply because there is typically more flux applied than what would be present in the solder paste. If you use a water soluble gel/tacky flux then the washing parameters might have to be adjusted to remove all of the flux residue.
When using solder paste, it is recommended to use a lead-free paste along with lead-free BGAs. If that is not possible and leaded solder paste is used along with lead-free BGA's then issues can occur. A leaded reflow profile will not melt the BGA solder spheres completely which can lead to weak solder joints.
In this case we would recommend using a leaded solder paste that is also designed for lead-free applications, along with a lead-free reflow profile. The lead-free profile will ensure complete melting of the BGA spheres and good mixing with the leaded solder paste.
The flux-only scenario has been used for many years for flip-chip and for fine-pitch BGA mounting to some degree. It dies result in a somewhat lower solder volume, thus a slightly reduced stand-off height. This can result in lower thermal-cycle reliability, but whether this is a concern is highly dependent on the specific component, the PWB it is mounted to and the reliability requirements of the application, among other factors.
I do not see a risk with this process. My personal opinion is actually that the BGA pads should have no solder paste deposition and just flux. The flux only will allow a better reflow and "collapse" then the paste deposition.
Engineering and Operations Management
I have used pasteless BGA and flipchip assembly processes for many, many years, with no issues whatsoever. In fact, with much better results. CSPs and flipchips work especially well without paste. You don't really have smaller spheres, because for a .020" diameter ball, the loss of the additional 3 mil thickness of solder paste will only decrease the finished ball diameter by about 1/2 mil, which certainly has no impact on reliability whatsoever.
However, you need a lot of process knowledge to perform pasteless reflow, the most important is learning how to coat the BGA/CSP/FC pads with a eutectic tacky flux that is almost like a very, very thin wax at room temperature but changes to a very tacky thin liquid at temps above 90 degrees Fahrenheit. Yes, Fahrenheit! Standard tacky fluxes simply don't work very well for pasteless assembly. But if the correct process is used, it is extremely simple, much better than dipping.
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