The dipping process is convenient for the application of epoxy flux because most pick-and-place equipment already has dipping units installed. So, there is no need for extra equipment or the costs associated with it. However, unless you have more than one dipping unit in the same or multiple pick-and-place machines, the dipping process can be difficult to implement.
The main reason for this is that, for each board, there is typically more than one component that epoxy flux will be used on. Different components typically have different standoff height, bump pitch, I/O count etc... The dipping process should be optimized specifically for each component so that maximum yield and reliability can be achieved. A dispensing or jetting process may be the answer for the multiple component dilemmas.
It is as equally important to optimize the dispensing and/or jetting process as it is to do so for the dipping process. Too much epoxy flux could cause the component to float, and the solder joint may not form, thus resulting in opens. The dispensing/jetting process will also differ depending on if solder paste is printed on the PCB or not. Too little epoxy flux would result in improper solder joint formation/connectivity or non-optimal reliability.
Indium Corporation's epoxy flux formulations are unique compared to the competition as they are compatible with no-clean solder paste. Because it has a fluxing agent, it can also be used alone, but solder paste is often times used for extra solder volume. Paste can also improve reliability and/or to help minimize warpage-induced defects such as HiP and NWO by bridging the gap between the PCB and the warping component with alloy.
If solder paste is used, the epoxy flux should be dispensed in between the solder paste deposits so that the paste is not disturbed by the force of the dispensing/jetting process. If paste is not used, the flux should be applied to the pads. Both lines and/or dots of flux can be used, but it is important to calculate how much you will need under the component so the optimum amount of it is dispensed or jetted onto the PCB. This may take a number of tries to obtain the perfect amount.
Fine-pitch (<0.4mm pitch) components may create a challenge for dispensing/jetting the epoxy flux in between the solder paste deposits. There is not enough room sometimes, and the paste may be displaced by the force of the flux hitting it. In this case, the dipping process may be a better solution, or dispensing/jetting the flux without paste present on the PCB.
Please feel free to contact me at any time with questions.
Keep your eyes open for next week's blog post: Epoxy Flux Rework Procedure and Clean-up.
* This post is part of the Understanding Epoxy Flux: The Need and the Process series.