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.
Garry McGuire
Sr. Engineer, NASA/Marshall Space Flight Center
Garry McGuire is a manufacturing process engineer and Chair of the IPC J-STD-001 and IPC/WHMA A-620 Space Addendum committees.
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.
John Vivari
Application Engineering Supervisor, Nordson EFD
Mr. Vivari has more than 15 years of electronic engineering design and assembly experience. His expertise in fluid dispensing and solder paste technology assists others in identifying the most cost effective method for assembling products.
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:- Install and route the jumper; bond the jumper to the board as required
- Solder the jumper at the unoccupied land. The end to be soldered at the capacitor location is laid against the existing joint, but not soldered at this time.
- Pre-heat the assembly in a box oven to 110 C, or the highest temperature allowed as a "storage temperature" for the installed components,whichever is lower
- Remove the assembly and immediately solder the jumper using an iron set to a conservative, controlled temperature. The minimum usable iron temperature will be determined by the heat transfer characteristics of the board and jumper.
- While soldering the jumper, the "heat bridge" should be formed between the iron and jumper; the iron should not touch the capacitor body.
The above process will minimize the thermal shock to the component. I'd suggest that thermal profiling will be useful to understand the actual thermal shock seen by the component.
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.
Fritz Byle
Process Engineer, Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
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.
Paul Austen
Senior Project Engineer, Electronic Controls Design Inc
Paul has been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 45 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes as well as moisture sensitive component storage solutions.
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.
Kris Roberson
Manager of Assembly Technology, IPC
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.
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.
Mike Green
Design Engineering, Lockheed Martin Space Systems
Mike Green is co-chairman of the IPC Terms and Definitions Committee. He has been working with board design and manufacturing for 33 years.
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