|Ask the Experts|
August 12, 2020
Soldering Components with Silver Pads
We need to attach a component to a PCB. The component has silver pads. Our normal process is to use a reflow oven. We are concerned that silver will leach into the solder joints.
We have tested conductive epoxy, but the contact resistance is too high. We have also tried silver filled solder SN62 and SN43 with Bismuth. If we use a low temp profile we did not obtain proper wetting and also get solder balls.
If the reflow temperature is too high, the silver dissolves into the solder. What do you suggest?
|Expert Panel Responses|
Ag dissolves at the rate of approx. 50 microinches /sec at soldering temperatures. What thickness of Ag is present on the device pads? The addition of Ag to SnPb solders is actually beneficial to joint strength up to a point.
You need to keep the Ag wt% down below 5 wt.% - this is an easy calculation based on the volume of paste printed and the metal content of the paste etc. Alternatively you could tin the part pre assembly similar to what people do for gold plated devices.
S T and S Testing and Analysis
Solder paste is designed to dissolve some of the metal finish on both the component leads and the circuit board pads in order to form a well bonded solder joint. It will be very difficult to prevent dissolution of the silver into the solder paste.
Normally we would recommend using a solder paste that contains silver, e.g. 62Sn/36Pb/2Ag or SAC305 for lead free application. The presence of silver in the solder paste tends to slow the dissolution rate of silver from the component leads.
I suggest using a solder paste with silver in it, and also modification of the reflow profile. A quick profile with a short time above liquidus will help to minimize the amount of silver that will dissolve. If you use 62/36/2 solder paste then try adjusting the profile length (45C to peak) around 3.0 to 3.5 minutes.
Adjust the time above liquidus to 45 - 60 seconds, and minimize the peak temp at 210-215C. These changes to the profile should help minimize the dissolution of silver, but will likely not prevent it.
Solder paste manufactures make a low peak temperature solder paste. I have done this before using such solder paste, see below what Alpha has to provide. Note please make sure that the operating temperature of the product and your post process do not exclude this temperature.
ALPHA CVP-520 is designed to enable low temperature surface mount assembly technology. The lead-free alloy in ALPHA CVP-520 has a melting point below 140C, and has been successfully used with peak reflow profiles between 155C and 190C. The flux residue from ALPHA CVP-520 is clear, colorless, and provides excellent electrical resistivity, exceeding industry standards.
Process Engineering Manager - Electronics
Altech UEC, South Africa
Since you state that the silver dissolves into the solder, I will assume that there is more than an immersion silver finish on the part. I've dealt with a lot of parts with high-silver thick film terminations (chip capacitors, mostly) in the past. The keys to successful reflow soldering of these components are:
You should strive for a peak temperature of about 205C, and minimize the liquidus time. It is not necessary to have a lot of time above liquidus. I've run reflow process for these components with as little as 25-30 seconds above liquidus.
Once you've formed the solder joint, if the joint appears smooth and not grainy, you have succeeded in limiting the leaching. Graininess will appear first at the boundary of the solder where it meets the component termination, and a fine line of graininess at the very edge is always present. You want to avoid a grainy appearance in the bulk of the joint.
One option would be to avoid the use of a Sn based paste and switch to an indium based paste. The rate of Ag dissolution into indium is much much slower than into tin.
Technical Support Engineer
If you have worked on the oven profile and also changed the solder paste type and the results are not as expected, did you think about components tinning prior to place them on the PCBs?
Engineering and Operations Management
Using Ag containing alloy, such as Sn62Pb36Ag2, will reduce the Ag leeching issue. You may also try a different solder paste which wets better and has less tendency of solder balling.
Director New Product Development
Metallic Resources, Inc
Use of 2% silver solder and the shorter profile is correct. Dissolution is a self-limiting process, so if you have solder with 2% silver in it to begin with, then less silver will leach off of the sintered component terminations. Pure silver component terminations are typically sintered, and are very thin. But two great big CAUTIONs here. Indium-based or other "low-temperature" alloys will NOT have the same physical properties or long-term reliability as Sn63 or Sn62; you don't get something (like a lower melting point) for nothing.
The other CAUTION is that whatever solder alloy you use should be on the Assembly drawing and be in full concurrence with your customer before you build 10,000 assemblies with the wrong solder that may or may not work in their application! They do NOT have to accept that product if you use an alloy other than what is listed without some type of waiver, deviation or ECN signed by them. Get their buy-in by providing some type of qualification with their end.
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