|Ask the Experts|
September 29, 2020
Challenge Wetting Solder to Brass Pins
In our production we have a product with a connector. The connector pins are made of brass, and tin plated. Unfortunately, the connector pin plating is made by the supplier before cutting and forming the connector pins, so some part of the pins remain unplated, leaving exposed brass.
In our selective soldering process, we have difficulty getting solder to wet to the brass exposed part of the pins. Any recommendations to solve the issue, apart from changing the connector with ones which is more wettable?
|Expert Panel Responses|
In order to give a good answer, it would be necessary to know more about your product (IPC Class 1, 2, or 3), the connector lead-form, and the process you intend to use.
Generally speaking, it would not be a good idea to continue trying to solder a component with incomplete or inconsistent tin plating. If the plating bond is as weak as you described, solder joint reliability will be very poor.
I would recommend that you either work with the component supplier to improve their plating process or select a different component with plating that is compatible with your intended process.
Principal Product Engineer
Benchmark Electronics, Inc.
You could run the leads through a tinning operation prior to through-hole soldering. This would cover the brass with solder and make it "solderable" for subsequent through-hole soldering.
Alternatively you could use a high activity flux that will enable soldering to exposed brass. Most water soluble liquid fluxes are high activity, and some will work with oxidized copper and brass surfaces.
I suggest contacting your flux supplier for specific recommendations.
Have you tried superior 75 flux? If not, this might be the solution. Following the manufacturer guidelines is strongly recommended.
Circuit Technology Center, Inc.
Soldering to Brass is almost impossible in the electronic assembly manufacturing. Brass consist of Copper and Zinc a mixture or alloy which is rare in electronic assemblies.
My suggestion would be to retin the brass with tin prior to assembly to any electronic assembly.
The suggested flux would be a Zinc Chloride plumbers flux as this would prepare the brass to be tin by the solder alloy. Dip the parts into a solder pot and tin the leads which will be soldered by the selective soldering process. Follow this tinning process with a high performance cleaning process as the flux must be completely removed. At this point the lead would be more solderable and would be easier to solder.
I would also check the solderability of the lead to verify the goodness of the tinning process.
Vice President, Technical Director
Brass pins of the connector solder not happen because of brass pins has contamination with moisture that Soldering wetting not Happened or it may be partially solder. Suggestion: to the Rh factor Humidity level between 35 to 60% and Shop floor temp 22 to 27 deg. Celsius.
Senior Engineer SMT
Advance Power Display system Ltd Mumbai
I cannot find a requirement that the tip of the lead be covered in solder in IPC-A-610, as long as the barrel fill and fillet requirements are met. In fact, Section 18.104.22.168 give the acceptability requirements for Lead Cutting After Soldering, which would expose the base metal. Section 8.3.13 on Bottom Termination Components (QFNs) shows where wetting will not occur in an exposed base metal condition.
Pre-tinning the leads with a more aggressive flux such as an OA Water Soluble or using such as flux in the selective solder process are about the only alternatives. Be sure that the assemblies are properly cleaned if you use the more aggressive flux chemistry.
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
Brass in normal conditions is very difficult to solder and wet because is a material very sensitive to oxidation. In selective soldering process this issue is hard to fix because typically the fluxes are zero or low halides whit less activity in order to no have reliability issues.
There are some alternatives to solder component lead with exposed brass, however implicate change flux or additional steps on your process and analysis.
Apply more flux. Apply more flux probably helps to increase the solids deposit on the lead and helps to improve the wetting. ( be careful to not exceed the solids deposit of flux recommended by supplier or splash out flux in areas that don't needed).
Change flux type, You can use a more active flux or more solids. ( This solution implicate validations and contaminations test). In selective solder low or zero halides flux are recommended to avoid reliability issues).
Pre-tin process. before assembly your component, you can pre-tin the component lead with a mini solder pot. This is not the best solution but can works as a containment action.
The best solution is request changes to your supplier that fix this issue considering that the soldering process will be selective solder.
Engineering Director / Master IPC Trainer (MIT)
There are several areas that you should investigate:
Global Segment Manager
Air Products & Chemicals, Inc.
In general brass is a difficult base metal to solder since it is an alloy of copper and zinc, with zinc being very difficult to solder. Without knowing more details about what flux you’re using you could use a more aggressive flux but this could present issues with your selective soldering equipment in terms of corrosion of the fluxer and other parts of your machine. A better alternative would be to have the connector pins stripped of the tin plating and re-tinned by an outside component processing house using organic acid flux followed by application of lead-free or tin-lead plating with the aggressive organic acid flux being cleaned off before your in-house selective soldering process.
I would expect brass to be an unlikely material for connector pin, since it is typically hard to solder to. Short of changing to another connector or asking the supplier to change their process to plating after all cutting and forming of the lead, you can investigate more aggressive flux options. Bear in mind, more aggressive fluxes must be fully cleaned away after soldering.
Manufacturing Process Engineer
There is only one solution that I could recommend. As you are trimming and forming the leads you are exposing the underlying base metal. Pre- tinning pre selective soldering is IMO the only solution you currently have available to solve you problem without going into the use of more aggressive flux. Changing connectors will not necessarily resolve the issue as trimming and forming is still going to expose the base metal.
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