The invention relates to the bonding of an electronic component to its molded package to render the package fluid-tight.
While various types of protective coatings for electronic components have been known and used for various reasons for a long time, it is only recently because of changes in technology and materials that it has become critical to seal completely these units against fluid penetration.
In the past, protective resilient coatings have found utility in providing stress and shock relief for components mounted in hollow shells. Resilient coatings have been used also with potted units to provide protection from vibration, thermal shock, and contamination (including moisture) and from the potting or encapsulating compositions.
Protective precoats have been used with electronic components because of differences in thermal expansion between the units and molded cases and to prevent contamination of the units by encapsulating materials.
As fully molded units became more popular, the seal between the leads or pins became more of a problem as the molding materials contain mold release agents to facilitate removal of the units from the molds. Unfortunately, these agents also prevent good adhesion to the metal leads or pins.
As water-soluble fluxes are replacing rosin fluxes in the reflow soldering of these units to circuit boards, because of environmental concerns, the sealing of the leads or pins to the covers or cases has become even more critical as these water-soluble materials are much higher in ionic material than prior rosin-based fluxes and eventually cause internal contamination and corrosion leading to failure of the unit. Thus, a need has arisen to find sealant materials which are compatible with the materials already in use in molded components and yet resist penetration by fluxes and ionic contaminants. One such solution to the above problem, described in copending application Ser. No. 863,545 filed Dec. 22, 1977, is to fill in the space between the unit (including leads) and the case with a sealant layer.