Connector Corner

July 20, 2018

Soldering Connectors Made Simple

Many connectors and related components are often joined by soldering in some capacity. Soldering is the process of joining two metals with a melted filler metal. This process is very similar to welding, but do not mistake the two. Soldering does not melt the working metals, but instead melts an added metal between the two. Some less-common methods of joining two metals are brazing and utilizing organic compounds (epoxies).

Wetting and Spreading

At the start of the soldering process, a waxy substance called “flux” is applied to the metals. This wets the materials, removing oxide and cleaning the metals. Wetting the metals is necessary to join the molten solder to the solid substrate and join the two. These two metals have different interface energies that require the flux to join them. The energy increases in the solder as surface area increases due to melting. The wetting also compensates for the difference in energies. Wetting can also be explained as using surface tension instead of surface energy as a balance.

As the solder melts and mates with the substrate, it dissolves some of it. The rate of dissolution varies, especially depending on the combination of solder and substrate. The increase in temperature of the solder can change the rate of dissolution as well. When the molten solder interacts with the substrate, it creates an intermetallic compound or “IMC.” These IMCs can often be found in the form of tin-copper and tin-nickel due to the frequent use of these metals in electrical interconnect. When IMCs are created, they can be identified by the use of electronic instrumentation like x-rays. X-ray diffraction (XRD) and scanning electron energy dispersive x-ray analysis (SEM EDAX) are examples of such advanced methods to identify IMCs after soldering metals.

Quality of Contact

Soldering to printed circuit boards is particularly more challenging than other components on the PCB. The size of the connector and the thermal mass makes soldering increasingly difficult. Additional dwell time and higher temperatures may be required to adequately melt the solder and wet the substrate. Although the higher temperatures and dwell times are necessary, it puts additional strain on the PBC and components being soldered. Such strain may be unavoidable, but strict adherence to control parameters may reduce these undesirable effects.

Vital mechanical and electrical principles are both observed while soldering joints on the PCB. Defects in the joints must be identified and avoided. The joints must be strong, with good integrity, and remain uniform in appearance with the substrates and terminations. If attention is not paid to these factors, joints may become unreliable and produce a number of problems. Inadequate wetting at the mating point may leave excessive oxide or contamination that causes delamination. Cracking in the solder can take place when there are voids or irregular growths in the metal. These issues can cause weakness in the joint and must be avoided under excessive thermal or mechanical conditions. Imperfections in the solder make it become increasingly brittle.

Additional Concerns

One consideration that many may be unaware of is the potential damage to contact springs. Although the amount of time and temperature exposed to the contact springs may be too little, there is always the possibility that finishes on the contact interface may suffer damage. The soldering process creates diffusion on the base metal and may oxidize the finish. Increased oxidation on the contact may increase resistance to unacceptable levels. Copper diffusion through gold is extremely common and causes problems particularly under low voltage and current applications.

Before deciding on soldering a contact, the contact must be observed in terms of its ability to be soldered in general. Its shelf life, issues related to diffusion, IMC growth, and corrosion must all be considered. Particularly components involved must be preserved in terms of solderability from the time it is produced from the time they are soldered. Such items include: terminations, solder tails, pads, and plated through-holes drilled into the PCB.

The shelf life and storage of components is more important than many be aware of. Climate control is critical for items stored at extended periods of time, especially in terms of temperature and humidity. Climate control is especially important while being transported over boats. Corrosive agents within the area of stored items can be increasingly dangerous for the products. If the storage environment looks potentially threatening to the solderability of connectors, parts may need to be thoroughly inspected and tested. Dip-and-look methods or wetting balance by meniscograph are suitable options for testing. If such items will be knowingly exposed to such conditions under extended periods of time, proper precautions must be adhered to and adequate testing must be done after the fact.

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One comment on “Soldering Connectors Made Simple”

  • vihan
    July 25, 2018 at 9:43 pm

    This is a really good and useful article for me. I am a regular reader of your articles. And here this one is the best one. You have covered everything. Thanks

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