|Ask the Experts
April 28, 2023 - Updated
February 23, 2022 - Originally Posted
Flux Residue During Pin-in-Paste
Our application is for pin-in-paste reflow using tin/lead no-clean, solder paste. Is it standard practice to increase the bottom side temperature to ensure the flux residue has been sufficiently activated and the flux residue is harder vs. softer and slightly gooey?
|Expert Panel Responses
I would not increase reflow temperature to attempt to harden residues. Extent of hardening of residues is highly dependent on the flux chemistry, and in addition is dependent on time after reflow; the residues react with oxygen and harden over time. Pin-in-paste applications can be challenging because of the sheer amounts of residues present, which increases the time required for any reaction with atmospheric oxygen.
The reflow process should be controlled to produce the needed peak temperature and time over liquidus. Excess time/temperature can be detrimental, for example can enhance formation of a thicker layer of intermetallic compounds, which can impact reliability.
The choice of whether to have hard or soft residues will be influenced if there is a need to electrically test these features that you are using PIP for. Bull nose test probes will typically not penetrate a hard residue and soft sticky residues will gum up the test pins, both ultimately giving the same open type defect.
In the past we had to change pastes to allow for probing through the residue with gumming up the probe tips.
S T and S Testing and Analysis
To my knowledge there is no standard practice for pin in paste reflow. It depends upon the board design, components, and the volume of solder paste applied, which makes every application a bit unique. The amount of heat required for no clean flux residues to be "safe" to leave on the PCBA depends upon the solder paste used.
I recommend measuring the reflow profile with thermocouples on the top and bottom surfaces of the PCBA near the through hole solder joints in question. Then set the oven to give a profile which meets the solder paste manufacturer’s specifications for both top and bottom sides. That would be the best way to handle this situation.
I do pin and paste regularly. I do not necessarily increase the bottom side reflow temperature. That is not to say I wouldn't if the parameters warranted i.e. thickness, ground plane, thermal mass, etc.
Using a thermal profiler and my paste parameters, I place thermocouples on both sides of the board in those locations to ensure reflow temps on the surface and in the barrel. I use a traditional soak profile (as opposed to a ramp) allowing the flux to remove any oxides in the barrel.
In all cases so far, both the top and bottom side oven temperatures are the same.
Interesting question and difficult to answer without additional detail, but I will try to answer in a generic manner.
The biggest issues with pin-in-paste soldering are:
1) Having enough solder paste (especially in thick boards)
2) Keeping solder where it belongs
3) Ensuring proper thermal profile
Manufacturer's recommendation for solder paste profile must be adhered to (minor tweaks to suit the board technology/application). If the solder paste volume is incorrect, then the solder joint may be substandard (insufficiently wetted as in graphic "A", below). One problem with having the bottom side hotter than the top side is that the solder may drain down the pin and in extreme cases my form a ball at the end of the pin (see graphic "C").
Besides not meeting standards, "C" may interfere with test fixtures or clearances for mechanical assembly.
Follow solder paste manufacturer’s thermal profile by careful control of board temperature, top and bottom. A properly instrumented thermal profile board is absolutely required to verify proper profile. This is important, not only for good solder wetting, but also for the quality of the solder paste residue. Ensure that solder paste volume is sufficient for proper solder joint result.
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