PLATING PROCESS FOR AN ELECTRONIC PART

Abstract: Before Au-plating (8) onto the metallic surface (3, 5) of an electronic part, Ni-plating (6) and then Co-plating (7) are applied to form a primer coating for Au-plating (8). But in the steps of Ni and Co-platings (6, 7), hydrogen gas produced during the plating is occluded into a plating layer. On the other hand, when a pellet (9) connected onto an Au-plated (8) metal, an Au-Si alloy is formed, but hydrogen gas in the plating layer remains as voids in the Au-Si alloy during a subsequent heating step of the electronic part, resulting in a serious defect. This invention of a plating method is characterized by annealing after Au-plating (8) of an electronic part to eliminate the residual gas.

Description: BACKGROUND OF THE INVENTION The invention relates to an electronic part having gold-plating on its metallic surface. Gold is widely used in various electronic parts because of its excellent heat resistance, corrosion resistance, solderability, wire bonding property and pellet-bonding property owing to formation of Au-Si eutectic alloy. Regarding gold-plating, the manufacturing course of an electronic part provided with pads of metallized layers on a ceramic substrate board is described below. After metallized pads have been formed on a ceramic substrate board, Ni-plating is applied to the pad, and leads are connected to the pad which require lead connecting by soldering. Then coatings by Ni-plating, Co-plating and further Au-plating are applied onto a conductor by electric plating in that order. Next, semiconductor pellet is subjected to die-bonding the on pad. After the pellet is connected to other pad by a wire bonding process, the semiconductor is sealed by glass having a low melting point, or the like. Ni-Co plating as primer coating for Au-plating herein, is applied to prevent problems which are caused by peeling of Au-plating during seal heating after wire bonding. But in the step of further Co-plating onto Ni-plated film, hydrogen gas produced during the plating is occluded into plated layers, and thus remains as voids in Au-Si alloy formed on pellet upon pellet-bonding by heating at the step of pellet-bonding and the sealing by a low melting point glass, and this results in serious a defect regard- ing the reliability of the pellet-bonding. Moreover, Ni-Co-Au plated layer consists of three layers, and consequently strains in the plates between the layers is so large that the problem of peeling of the plated layers may arise when a lead is bent. In addition, the techniques concerning this field are described in Nos. 34692/80, 4955/83 and l55950/84 of Japanese laid open patents. SUMMARY OF THE INVENTION The object of this invention is to provide a plating process for an electronic part which has no voids in Au-Si eutectic crystals formed upon pellet-bonding when Au-plating is applied as the primary coating on Ni-Co platings for an electronic part. The characteristic of the invention is the plating process of an electronic part that is annealed after Ni-plating, Co-plating and a further Au-plating have been applied onto its metallic surface in that order. BRIEF DESCRIPTION OF THE DRAWING Fig. l is the sectional view of an electronic part. DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of this invention is explained in Fig. l. Fig. l is the sectional view of a finished electronic part in which l, 9, l0 and 5 are a ceramic board, a semiconductor pellet, a wire treated with wire bonding and a lead extended from this electronic part, respectively. 2a, 2b and 2c are conductor pads formed on ceramic board l, i.e., a pad connected with pellet 9, a pad connected by a wire bonding and a pad connected with lead 5, respectively. ll is a sealing cap. The method of the working example is explained below in due order. Example First, pads constituted of a metal having a high melting point such as W or Mo, etc. are formed on the ceramic substrate board l by a screen printing process, etc. When the pad is sintered with the ceramic substrate board l, the pads 2a, 2b and 2c are formed by consolidat­ edly mutual sintering of ceramic and the metal. Then Ni is plated on pads 2a, 2b and 2c to form Ni-plated film 3. Next, pad 2c is soldered with lead 5 by a soldering material 4 such as eutectic silver. Then Ni-plated film 6, Co-plated film 7, and further an Au-­ plated film 8 thereonto are formed in 2 - 5 µm, 0.5 - 2 µm, and l - 3 µm of thickness, respectively, by electric plating. Then the electronic part is annealed by heating up to 450°C, for l0 minutes under N₂ atmosphere. As usual afterwards, pellet 9 is die-bonded to pad 2a, and pellet 9 is connected to pad 2b through wire by wire bonding, and a sealing cap ll is fused to the ceramic board l with a low melting point glass to seal pellet 9, and finally lead 5 is coated with solder. When the lead was bent to make an angle of 90 degrees after annealing as described above, no peeling appeared on the plating. And the observation by X-ray did not indicate the generation of voids in the pellet-bonded part. The reliability of an electronic part could be thus improved significantly. In addition, the results of soldering property, etc. are normal and excellent. This invention can give a reliable plating process for an electronic part, which yields no voids in Au-Si eutectic crystal because it can eliminate residual gases in a plating by an annealing process after plating.

Claims: 1. A plating process for an electronic part, comprising applying a Ni-plating (6), a Co-plating (7) and a further Au-plating (8) onto the metallic surface (3, 5) of the electronic part successively in said order, characterized in that the electronic part is annealed after said plating steps. 2. A process according to claim l, wherein the electronic part is annealed by heating up to 450° C for l0 minutes under N₂ atmosphere. 3. A process according to claim l or 2, wherein the Ni-plating (6), the Co-plating (7) and the Au-plating (8) are formed with a thickness of 2 - 5 µm, 0.5 - 2 µm and l - 3 µm, respectively, by electric plating. 4. A process according to anyone of claims l to 3, wherein metallized pads (2a, 2b, 2c) composed of W or Mo metal are formed on a ceramic substrate board (l), which is followed by sintering, and said metallized pads serve as the metallic surface (3) of the electronic part onto which the Ni-, Co- and Au-platings (6, 7, 8) are applied.

Inventors: MIYAZAWA, OSAMU, OHTA, TOSHIHIKO, SEKIBATA, MASAO
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: C25D5
IPC codes: H05K3, C25D5, H01L23, H01L21, H05K1, C25D7