solder paste

  • Surface Texture on Solder Joints after Reflow

    surface defects reflowSeeing surface effects when using lead-free solder is not an unusual occurrence. There are many factors that could be causing texture on the solder joints after reflow.

    The following are several of the more common factors that could be contributing to this defect:

    • Thermal profile used, peak temperature and cool down rate
    • Metal finishes on the parts that were soldered, dissolution occurs which can impact color
    • Reflow atmosphere
    • Physical or chemical properties of the flux, some fluxes react more with the solder surface than others
    • Flow properties of the flux during the reflow process, some fluxes flow more rapidly than others, giving extended exposure to oxygen
    • Excessive oxidation of the parts to be soldered
    • Shelf life of the paste or how the paste was handled prior to use

    Keep in mind that while the surface of the board may look frosty, discolored, or be demonstrating shrinkage effects this does not necessarily point to a reliability issue. Instead the solder joint may remain in excellent working condition with adequate intermetallic bonding.

    If you suspect that there may be a reliability issue associated with the appearance you will need to do some pull/shear testing to examine the bond layers. Use a quality pair of shears for the most accurate results. ( If there is a problem with the bond layers, examining the reflow process for problems and / or changing to a different solder paste can correct the problem.

    GoKimco has a wide variety of solder pastes so that you are sure to find one that will sort out this issue. Visit

  • Why does solder paste need to be stored in a refrigerator?


    Refrigerating solder paste significantly extends the life of the paste. In general, manufacturers recommend storing solder their pastes between 0 and 10 °C (32 – 50 °F), but customers should confirm this with each manufacturer’s specific instructions for storage. The paste should be stored within this range, either in a refrigerator or cold room, as soon as it is received from Kimco Distributing Corp.

    Solder pastes are comprised of flux and powdered solder alloy. (Alternatively, Kimco Distributors also carries flux without powder in it.). The flux in solder paste serves two functions. During the soldering process, it efficiently conducts heat. It also removes impurities, both on the components being soldered and the powdered allow. Thus, even when solder paste is being stored, the flux is reacting with the powder suspended in it.

    These chemical reactions that the flux induces can be slowed by storing the solder paste at cool temperatures. Manufacturers recommend the above range of temperatures, because they have designed solder pastes to not react as quickly at cooler temperatures. Their chemists carefully select compounds that will react quickly at higher temperatures but are fairly dormant at lower temperatures.

    When properly stored, most of the solder pastes Kimco Distributing Corp. carries have a shelf life of approximately six months. (Again, check with each manufacturer’s specifications to confirm this). Once pastes reach room temperature, though, they will quickly expire – some in as little as 24 hours. This is why refrigerating solder paste is so important.

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