The electronic components industry is continually searching for new composite materials for use as electrical or electronic components. Such new materials are being developed in accordance with the need for miniaturized parts which are capable of performing various operations.
Many of these composite materials include metallic portions which alone could easily be electroplated utilizing state of the art electroplating baths. When such electroplatable portions are combined in a composite material in combination with a non-conductive, non-electroplatable portion of an inorganic material such as glass, ceramic or the like, it becomes very difficult to selectively plate only the metallic electroplatable portions of the composite without adversely affecting or concurrently electroplating the inorganic portions of the composite.
For example, when the common tin/lead fluoborate baths are utilized to achieve such selective plating, it is found that fluoboric acid attacks the inorganic portions of the composite. This attack may be in the form of solubilizing (i.e., etching or dissolving the inorganic material) or cracking. If the inorganic portion of the composite is etched to a significant extent, then the tin/lead alloy will deposit on the inorganic portion, thus causing a short circuit of the metallic (electroplatable) portions.
Use of tin/lead electrolytes which are not based on fluoborates, such as those disclosed in U.S. Pat. No. 4,459,185, are also not suitable. While baths such as these do not cause cracking of the inorganic portions, they will deposit tin/lead alloys upon such portions, thus causing short circuiting of the metallic portions.
A specific type of electronic component which utilizes a composite structure is a dual inline package (DIP). These components, which are well known in the art, are generally prepared by adhering a die or a chip to a surface and connecting the circuitry to a plurality of pins. The die is then encapsulated with ceramic pieces that are adhered together with a soft lead oxide glass containing about 50% lead oxide that melts at relatively low temperatures. The soft glass binder used to encapsulate the circuit oozes around these pieces and hardens during the fabrication to become part of the package. The structure is thus composed of metallic pins, the encapsulating ceramic, and the hardened soft lead oxide glass encapsulation. In order to continue processing of these packages, the metallic portion (usually an iron-nickel alloy) must be electroplated to facilitate soldering connections to the pins.
The common practice in the industry today is to plate these DIPs with pure tin from a solution containing stannous sulfate and sulfuric acid, since the soft lead oxide glass portions do not accept a tin deposit from such solutions, nor do such plating solutions adversely affect the soft lead glass portions. Pure tin electrodeposits, however, have been known to produce whiskers which grow out from the surface in various directions. These whiskers have the appearance of very fine hairs of tin metal which can bridge adjacent metal parts and cause a short circuit. The industry would like to avoid utilizing pure tin deposits for such packages for this reason.
Attempts have been made to plate DIPs with a tin-lead alloy containing 5% lead or more, since it is known that these tin-lead alloys do not have a tendency to produce whiskers. The tin/lead plating baths that are commonly used in the industry today, as mentioned above, cannot be used for this purpose since the soft lead glass binder has a tendency to become electroplated, thus causing short circuiting of the pins. Also mentioned above, a further problem is that the soft lead glass binder is soluble in or otherwise adversely affected by such prior art plating solutions.