High Temp Solders and their use

You are asking a very good question which has been an issue in the industry for a long long time. For operating at 225C, you will need a solder with melting point no lower than 270 or even 280C. HMP solder is very difficult to come by. At this stage, lead is still the primary ingredient of choice, mainly based on the melting range consideration. However, lead is not very good on solder wetting, due to lack of reaction with base metals. If a better wetting is desired, lead has to be replaced. The challenge for that is already hinted by the difficulty in finding a Pb-free alternative for high melting solders.

Fluxes for high temperature soldering are also an issue, particularly when cleaning or cosmetic appearance is considered important. Most organics will start to decompose at around 300C. The result of that is (1) poor fluxing, (2) poor cleanability, (3) charred residue appearance for no-clean applications. Although new fluxes have been worked at constantly, a breakthrough in performance will not be easy unless the acceptance criteria have been revised. You may want to check with major vendors with the progress from time to time.

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