|Ask the Experts|
January 18, 2021
Soldering Multilayer Ceramic Chip Capacitors
We are conducting rework that includes soldering a wire to one end of a heat sensitive multilayer ceramic chip capacitor (MLCC).
We need to solder the wire to one end of a ceramic chip cap while the other end is soldered to a PCB land.
Since ceramic capacitors are sensitive to thermal shock, how should we proceed? Which end should be soldered first?
|Expert Panel Responses|
You need a controlled thermal environment to avoid cracking the ceramic for both soldering operations. Use reflow with a controlled ramp temperature to secure the part.
Then, you can use a hot air soldering station to install the jumper wire.
Adding the jumper to the land and drilling out the trace connected to it might be a safer process with less chance to create a thermal gradient within the MLCC which would crack it.
Lockheed Martin Space Systems
The answer isn't so much which end to solder first, the answer is going to be removing the thermal shock.
Thermal shock is the rapid change of temperature on an assembly or component. Chip capacitors can be relatively robust as far as what temperature (maximum) they can withstand. The problem is when the chip is rapidly heated from room temperature it tends to crack.
Contact the component manufacturer to determine the ramp rate that the component can handle. Then I suggest using an auxiliary heater of some kind. Either an under-board heater or other controlled heat source to slowly bring the component and surrounding structure close to the reflow temperature no faster than the component ramp limits. Once the component is heated you should be able to solder either end of the component without damage.
Manager of Assembly Technology
It would be best if you could solder both ends of the cap at the same time, using a hot air reflow rework station. If this is not possible, I'd solder the PCB land end first, since the land is not likely to move as you solder the wire end, second.
To make sure you are not stressing the cap, make sure you run a couple of test runs to measure your process with a good Temperature Profiler (logger) to prove that you did not subject cap to a temperature ramp rates or peak value beyond the specifications for the part.
Senior Project Engineer
Electronic Controls Design Inc
The degree of sensitivity of the ceramic cap depends on several things, including the body size and the type of dielectric. If you take a conservative approach, you can look to what has been done in the wave soldering process for some guidance.
The rule of thumb there is that we preheat at less than 2 C/second to a temperature within about 110 C of the wave temperature.The 110 C "shock" is considered acceptable for all but the largest body sizes (larger than 1812). If we design a process around this, we would envision preheating the entire assembly, then soldering the jumper using controlled heat. The process might look something like this:
An alternative to the above process is to solder using hot air (after preheat). This process will result in a little more heat being transferred to the capacitor, but at a somewhat slower rate.
Ideally, solder both sides at the same time by heating the entire component uniformly to avoid unnecessary thermal stress. One option is to dispense solder paste at both terminations and heat with hot air or a larger, lower temperature soldering iron tip.
If you must solder one side at a time, the PCB can be used as the heat transmission vehicle for the pad joint to slow heat transfer and minimize shock. The wire-to-component joint will produce more shock, unless the component is uniformly preheated in some way.
Application Engineering Supervisor
Preheating the assembly is a must (local to the area being worked is okay ... the entire assembly need not be preheated if the preheating system cannot accommodate the size of the assembly). It would also be beneficial to use a non-contact soldering system such as a hot air pencil rather than a conventional iron.
As far as "which end first?" - depending on the size of the capacitor, both ends may melt when applying heat to either end, so I would recommend soldering the end with the wire last so it doesn't reflow and possibly come out of the connection. I would solder the wire to the land last - using a heat sink on the wire if it is short enough to transfer the heat to the solder connection at the capacitor.
NASA/Marshall Space Flight Center
This may be the purist's approach so bare with me and rest assured I won't be mad if you disagree. My experience is to avoid touching or even worst soldering to a SMCC of any size after it has gone through reflow or wave soldering. If there is a problem, invest in correcting the PWB layout interconnect.Juergen Flamm, Northrop Grumman, USA
Doing this you will prevent undetectable induced latent defects due to "mistreating" the SMCC during rework to show up after the product has been delivered. For me this falls into the category of pay me little now to correct the design issue or pay me big time later.
The method I use is to preform the jumper wire to the shape required for connection between the cap and land. I then would tin the end of the jumper that will be soldered to the cap. This jumper would then be laid into position on the PCB. I would apply flux to the cap/jumper junction and solder paste to the land/jumper junction.Alan Christmas, Ultra Electronics
I then gently pre-heat the PCB to just below the reflow temperature of the solder. I then use 2 hot air pencils to reflow the joints at either ends of the jumper simultaneously. The surface tension of the solder will hold the wire in position during reflow.
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