2nd reflow

Hi:

Generlly speaking, in a reactive mode, nitrogen blankets are used to improve the yield of a process that is marginal in terms of removing, and/or preventing the re-appearence of, oxides on copper features or component I/Os. That marginality can be caused by the flux chemistry (e.g., the early versions of low-solids fluxes), the tentative protection provided by OSP surface finishes, or the requirement for a prolonged process cycle that explicitly resulted in re-oxidation of the copper features on the PWB or I/Os on components. Improvements in low-solids flux formultation as well as more robust OSP coatings have actually led some manufacturers to abandon nitrogen blankets and the associated cost burden.

From the pro-active viewpoint, nitrogen can provide that process margin which allows one to successfully work with "difficult to assembly" product. Nitrogen also reduces the tendency for flux residues to char, thereby reducing furnace maintenance costs.

The major disadvantage are the added costs associated with nitrogen: (1) equipment/storage and delivery systems to provide nitrogen; (2) an increased thermal burden on the furnace as exiting nitrogen takes some heat from the hot zone; and (3) safety measures to prevent asphixiation oin the event of a gross leak of nitrogen into the immediate work area.

Thanks

reply »