Method of forming a coated plastic package

The invention is to a semiconductor package and the method of making the package. A moisture resistant coating such as a ceramic, silica or other plastic material is applied over a plastic packaged semiconductor device to seal the package from moisture.

FIELD OF THE INVENTION 
This invention relates to semiconductor devices and more particularly to a 
plastic packaged semiconductor device having a moisture resistant coating 
over the plastic package. 
BACKGROUND OF THE INVENTION 
Plastic packaged semiconductor devices have an cost advantage over 
hermetic, ceramic packaged semiconductors. The plastic in plastic packages 
have the disadvantage that moisture absorbed into the package can lead to 
reliability failures. Failures result from corrosion on the silicon chip, 
the gold wire connections which are connected to the silicon chip, and on 
the supporting structure such as the lead frame. As miniaturization 
components continues, the trend is for thinner and thinner plastic 
packages. This increases the possibility of moisture absorption into the 
plastic package. 
BRIEF DESCRIPTION OF THE INVENTION 
The invention is to a semiconductor package and to the method of making the 
package. A moisture resistant coating, such as a ceramic coating, is 
applied over a finished plastic packaged semiconductor device. A ceramic 
coating is applied using a plasma torch that has a powder feeder. The 
device is masked to prevent the deposition of the coating on leads used 
for electrical connections to the device. 
After a semiconductor device has been plastic encapsulated, the device is 
masked to cover areas that are not to be coated. The coating is applied by 
plasma spraying the moisture resistant material, which may be, for 
example, alumina, silica, or other material that are non-permeable. 
The coating provides a seal impervious to moisture, inhibiting the ingress 
of moisture along metal leads that protrude from the overmolded plastic by 
allowing the ceramic coating to coat a small section of the lead next to 
the plastic to seal this area from moisture. The heat transfer 
characteristics of the package are improved due to the thermal 
conductivity and radiant properties of the coating and its intimate 
contact with the plastic. The coating provides an improved definition of 
the solder edge that forms when a device is mounted on a substrate by 
virtue of the ceramic coating serving as a stop or dam to prevent solder 
from flowing up to the device body. 
The technical advance represented by the invention as well as the objects 
thereof will become apparent from the following description of a preferred 
embodiment of the invention when considered in conjunction with the 
accompanying drawings, and the novel features set forth in the appended 
claims.

DESCRIPTION OF A PREFERRED EMBODIMENT 
The invention is to a plastic encapsulated semiconductor device having a 
water resistant coating over the plastic encapsulant and to the method of 
applying the coating. FIG. 1 is a cross-sectional view of a device 
according to the present invention and illustrates one aspect of the 
invention. Semiconductor device 10 includes a semiconductor chip 9 having 
a plurality of leads attached thereto, represented by leads 13 and 14. 
Semiconductor chip 9 and a part of leads 13 and 14 are encapsulated in a 
plastic material 11. A coating material of, for example, a ceramic, silica 
or other water resistant plastic material 12 encapsulates the plastic 
coating and a part of leads 13 and 14. The coating material extends along 
each lead as illustrated at numeral 16. The coating material provides a 
moisture barrier, and the coating on a part of the leads adjacent prevents 
moisture from entering the package at the lead-plastic interface. The 
device in FIG. 1 is only illustrative of the invention and is not drawn to 
scale. The coating in practice would be very thin, only about 0.15 mm. 
FIG. 2 illustrates the basic steps in making the semiconductor device 
encapsulant it and providing the coating. A completed, wired semiconductor 
chip (20) is encapsulated (21) to provide a finished device. The leads of 
the plastic encapsulated device are then masked (22) to cover the parts 
that are to be used to make electrical connection to a socket or circuit 
board. A coating material is then plasma sprayed (23) onto the masked 
device. 
The coating is accomplished using a plasma torch and a powder feed. The 
powdered material is sprayed onto the device with the plasma torch. Since 
the device is masked to protect the connection portions of the leads, the 
coating is applied only to the plastic body and a small part of the leads 
adjacent to the device body. 
An intermediate step or steps may be desirable prior to spraying the final 
coating to promote adhesion of the coating. 
This step may to abrade the surface of the plastic package to give it a 
slight textures to provide a surface to which the coating will adhere.