I have a board that is partially reflow soldered with a QFN and a couple of 0603 capacitors and resistors. I wanted to test functionality with this stage before I go ahead and place the other components whose working depends on this stage working right. Since adding components will mean repeating the reflow process, I was wondering if its safe to re-reflow the existing components on the board?
There is no generic answer, it all depends on the components involved, let me add a few things to watch out for, and collect for the convenience a few more from the comments.
First of all, read the datasheets of all involved components, what they say about reflow in general and a second reflow possibly.
A lot of things will not be specified in the datasheets though and need to be tested, things to watch out for:
- If time passes between the reflows, moisture could be trapped and cause popcorning
- Some plastic connectors seem to start decompositing after a second reflow, usually this is not mentioned anywhere and needs to be tried out
- Wet electrolytic capacitors may be designed to lose a bit of their electrolyte and vent it during reflow. They would use more than designed for.
- Other kinds of caps change values when heated and cooled again, they may go out of spec if heated a second time. Same for other kinds of parts, maybe crystals too.
thermal stress during heating (while the part is still held by solid solder) might be too much, especially for MEMS parts and maybe crystals too.
Parts might be held in place by the solder, so watch out for upside down. Also the glue used for holding upside down parts may not be designed for a second heating.
It doesn't seem to make much sense for a manufacturer to specify exactly if a second reflow is possible. In my experience it usually doesn't hurt much, especially if you keep the temperature profile exact.
In the datasheet watch out for any preconditions you might have violated, e.g. if it states the storage temperature prior to reflowing should not exceed 85°C then this might raise an eyebrow about why and if reflowing the first time should maybe count as being store above that temperature.
Doing some research indicates some effects which might happen:
Crystal oscillators show a decaying frequency deviation after reflow soldering. This might not be relevant for your board and doing two reflows shortly after the other shouldn't make much difference. The effect takes several days to decay. Which is interesting if you think about calibrating the frequency of a device shortly after soldering.
A research paper on solder joint reliability during multiple reflows indicates that multiple reflows increase the chance of voids forming in the joint and thus decrease the solder joint reliability. The strength of this effect is dependent on the paste used.
There seems to be an effect on multilayer capacitors (MLC). They recommend to reduce the time spent above 230 °C to prevent barrier layer thinning. But they have something in store to counteract the effect, guess you have to pay extra for it. ("DLI offers enhanced magnetic and non-magnetic termination finishes for applications requiring extended soldering time or repeated reflow cycles.")
Lelon write in one of their reflow guidelines for electrolytic capacitors to avoid two reflow cycles whenever possible. If not possible you should contact them with the profiles and ask if it is okay. "Do not attempt to reflow three times."
Murata says for one part (don't know which one) the time spent at high temperature shall be accumulated and less than given in the figure in that document. So at 250 °C it should be less than 20 seconds, so reflowing 2 times means 10 seconds per reflow.
Another research indicates that there might be problems with delamination of PCBs and the capacitance between copper layers after several reflows. So if your design requires a certain inter-layer capacitance, be careful (mostly RF-designs, I think).
For reflow solderable lithium batteries, the number of reflows is also limited to two times.
Panasonic has a small note for SMD connectors: "Double reflow soldering on the same side is possible"
Haven't found any mention for ICs other than the solder joint reliability problem.
My experience so far was that it worked fine to reflow a board twice. Three times and the PCB was starting to look odd some times (board hand made, not a production PCB). If through hole components are used, especially connectors, remove them first. Small components (0805) were usually held in place on the bottom side of the PCB just by the solder if there is no ventilator blowing right at the board.
A signal flow can be very easily interrupted via a jumper wire, thus it makes no sense to subject components to a reflow process, just to isolate one stage from another! Your idea of partially populating a board and testing it, will most likely cause more problems that it solves.