GB2154489A

GB2154489A - Soldering semi-conductor material chips to a metal strip

Info

Publication number: GB2154489A
Application number: GB08504314A
Authority: GB
Inventors: Marino Cellai; Pieramedeo Bozzini; Via Correggio Marzio Fusaroli
Original assignee: ATES Componenti Elettronici SpA; SGS ATES Componenti Elettronici SpA
Current assignee: STMicroelectronics SRL
Priority date: 1984-02-23
Filing date: 1985-02-20
Publication date: 1985-09-11
Also published as: KR850006778A; JPS60195947A; GB8504314D0; IT8419749A0; FR2560438A1; IT1213144B

Abstract

Metal substrates for a plurality of devices are connected together in strips. A low fusion soldering alloy is disposed on the strips by plating. During automatic assembly, the strip is heated to the melting point of the alloy whilst the chips are supported on predetermined zones of the strip.

Description

SPECIFICATION Improvements in or relating to methods of soldering semiconductor chips to metal substrates The present invention relates to semiconductor devices and, in particular to a method of soldering semiconductormaterial chips to metal supports during the automatic assembly of such devices.

A typical semiconductor device is constituted by an active component, for example a suitably processed silicon chip, the larger surface of which is metallized, a metal substrate component on which the chip is soldered, and a plurality of metal components in electrical contact with predetermined areas of the chip which act as the electrodes of the device. The whole of this, excepting the terminal portions of the electrodes and a surface area of the metal support component, is embedded in a plastics body (see, for example, the Patent Application No 24533 A/82 in the name of the applicants and the corresponding British Patent Application No 8332106).

During the assembly of devices of this type by automatic apparatus, the substrate components for a plurality of identical devices are obtained, usually by shearing, from a metal sheet, for example nickel copper, and remain connected together in strips by means of a tongue of sheet metal until the end of automatic processing.

In the soldering of chips to sheet metal strips, a known method involves the use of a metal alloy having a low melting point, for example a 90% lead and 10% tin alloy which melts at a temperature of approximately 300 C. A alloy of this type is advantageous with respect to other alloys having higher melting points as it enables fastenings which are not too rigid and therefore suitable absorption of the mechanical stresses which are exerted on the chip during the temperature variations involved in the normal operation of the device.

However, in order to use this type of alloy, the metal surfaces of the components to be connected must be capable of being completely bathed in the alloy at its melting point. For this purpose, the silicon wafers from which the chips are obtained are treated such that their base surfaces are covered by a layer of noble metal, for example gold, which is usually the last of a series of layers of different metals. The metal sheet is also plated, in many cases, with a noble metal, at least in the zones to which the chip is to be soldered.

Soldering is carried out by placing a small disc of soldering alloy on the sheet, on which the chip is then placed, the entire structure then being heated to the melting point of the soldering alloy.

Plating of the sheet metal with a noble metal may also be omitted, with obvious savings in materials and labour, although this requires the use of particular measures to eliminate the oxide layer which is normally located on the surface of the strip before the soldering process (see, for example, Italian Patent Application No 24328 A/82 filed in the name of the present applicants on 19.11.1982 and the corresponding British Patent Application No 8320040). In any case, the assembly process described above involves high-precision operations which, in order to be automated, make it necessary to use relatively complex machinery which is therefore readily susceptible to malfunction and hence requires painstaking maintenance work.

In order to simplify the process and make automatic assembly more rapid and reliable, it has been proposed to apply the soldering alloy directly onto the rear of the silicon wafer. This solution has not however proved to be particularly satisfactory as it gave rise to new causes of wastage during cutting of the wafer as a result of the comparatively thick and soft layer of soldering material.

According to the invention, there is provided a method of soldering semiconductor material chips with a metallized surface to a metal strip by the application of a low-fusion soldering alloy between the metallized surface of the chip and predetermined zones of the metal strip and by heating the strip so as to cause the soldering alloy to melt when the chips are disposed on the metal strip, in which the soldering alloy is applied by plating of the metal strip at least on the predetermined zones.

It is thus possible to provide a method of soldering semiconductor material chips to a metal substrate which is suitable for automatic assembly and does not require additional operations on the wafer of semiconductor material from which the chips are obtained.

The plating may be obtained, for example, by electrochemical deposition and may be restricted, by suitable masking, solely to the zones provided for the fastening of the chips. It may be carried out during processing of the meta sheet, either during a stage before or after shearing of the sheet.

The use of strips of sheet treated in this way enables operations for the collection and positioning of the soldering alloy disc and for the elimination of the oxide layer, which may be present on the metal surfaces to be joined, to be avoided and obviously also enables any further treatment of the wafer to be avoided. This provides a simpler and more reliable automatic assembly method providing a more economic method of production.

Preferably the plating is carried out by electrochemical deposition of lead and tin in propositions by weight of 95 and 5%, respectively.

1. A method of soldering semiconductor material chips with a metallized surface to a metal strip by the application of a low-fusion soldering alloy between the metallized surface of the chip and predetermined zones of the metal strip and by heating the strip so as to cause the soldering alloy to melt when the chips are disposed on the metal strip, in which the soldering alloy is applied by plating of the metal strip at least on the predetermined zones.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in or relating to methods of soldering semiconductor chips to metal substrates The present invention relates to semiconductor devices and, in particular to a method of soldering semiconductormaterial chips to metal supports during the automatic assembly of such devices. A typical semiconductor device is constituted by an active component, for example a suitably processed silicon chip, the larger surface of which is metallized, a metal substrate component on which the chip is soldered, and a plurality of metal components in electrical contact with predetermined areas of the chip which act as the electrodes of the device. The whole of this, excepting the terminal portions of the electrodes and a surface area of the metal support component, is embedded in a plastics body (see, for example, the Patent Application No 24533 A/82 in the name of the applicants and the corresponding British Patent Application No 8332106). During the assembly of devices of this type by automatic apparatus, the substrate components for a plurality of identical devices are obtained, usually by shearing, from a metal sheet, for example nickel copper, and remain connected together in strips by means of a tongue of sheet metal until the end of automatic processing. In the soldering of chips to sheet metal strips, a known method involves the use of a metal alloy having a low melting point, for example a 90% lead and 10% tin alloy which melts at a temperature of approximately 300 C. A alloy of this type is advantageous with respect to other alloys having higher melting points as it enables fastenings which are not too rigid and therefore suitable absorption of the mechanical stresses which are exerted on the chip during the temperature variations involved in the normal operation of the device. However, in order to use this type of alloy, the metal surfaces of the components to be connected must be capable of being completely bathed in the alloy at its melting point. For this purpose, the silicon wafers from which the chips are obtained are treated such that their base surfaces are covered by a layer of noble metal, for example gold, which is usually the last of a series of layers of different metals. The metal sheet is also plated, in many cases, with a noble metal, at least in the zones to which the chip is to be soldered. Soldering is carried out by placing a small disc of soldering alloy on the sheet, on which the chip is then placed, the entire structure then being heated to the melting point of the soldering alloy. Plating of the sheet metal with a noble metal may also be omitted, with obvious savings in materials and labour, although this requires the use of particular measures to eliminate the oxide layer which is normally located on the surface of the strip before the soldering process (see, for example, Italian Patent Application No 24328 A/82 filed in the name of the present applicants on 19.11.1982 and the corresponding British Patent Application No 8320040). In any case, the assembly process described above involves high-precision operations which, in order to be automated, make it necessary to use relatively complex machinery which is therefore readily susceptible to malfunction and hence requires painstaking maintenance work. In order to simplify the process and make automatic assembly more rapid and reliable, it has been proposed to apply the soldering alloy directly onto the rear of the silicon wafer. This solution has not however proved to be particularly satisfactory as it gave rise to new causes of wastage during cutting of the wafer as a result of the comparatively thick and soft layer of soldering material. According to the invention, there is provided a method of soldering semiconductor material chips with a metallized surface to a metal strip by the application of a low-fusion soldering alloy between the metallized surface of the chip and predetermined zones of the metal strip and by heating the strip so as to cause the soldering alloy to melt when the chips are disposed on the metal strip, in which the soldering alloy is applied by plating of the metal strip at least on the predetermined zones. It is thus possible to provide a method of soldering semiconductor material chips to a metal substrate which is suitable for automatic assembly and does not require additional operations on the wafer of semiconductor material from which the chips are obtained. The plating may be obtained, for example, by electrochemical deposition and may be restricted, by suitable masking, solely to the zones provided for the fastening of the chips. It may be carried out during processing of the meta sheet, either during a stage before or after shearing of the sheet. The use of strips of sheet treated in this way enables operations for the collection and positioning of the soldering alloy disc and for the elimination of the oxide layer, which may be present on the metal surfaces to be joined, to be avoided and obviously also enables any further treatment of the wafer to be avoided. This provides a simpler and more reliable automatic assembly method providing a more economic method of production. Preferably the plating is carried out by electrochemical deposition of lead and tin in propositions by weight of 95 and 5%, respectively. CLAIMS

2. A method as claimed in claim 1, in which the plating is carried out by elecrochemical deposition of lead and tin in proportions by weight of 95 and 5%, respectively.