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Poor Penetration
For a solder joint to form on the topside of a board flux, needs to be present in the plated through holes. The temperature of the topside of the board needs to have reached a minimum of 100-110oC. The most common process problem today with no clean is limited flux application. It is generally uncommon to have problems with through hole plating provided a board manufacturer's products are evaluated prior to use. It is also firmly uncommon to have contamination in the through hole causing this sort of problem. If flux penetration is present and correct pre heat obtained then have the printed board tested.