GB2260922A - Fluxless soldering - Google Patents

Abstract

A method of fluxless soldering comprising the steps of placing an article provided with solder formations on metallic areas thereof in an atmosphere comprising a mixture of not more than 25 % by volume of hydrogen in an inert carrier gas, effecting a plasma excitation of the atmosphere surrounding the article and heating the article until reflow of the solder occurs. <IMAGE>

Description

Fluxiess Soldering This invention relates to fluxless soldering particularly for electronic components, especially in low volume, high integrity, high reliability applications such as submarine cable systems or equipment for use in space.

The use of soldering processes is widespread in the electronics industry, particularly for mounting of electronic components such as integrated circuit chips on printed circuit boards.

A method of mounting such chips which is known as "surface mounting", in which the chip packages having metal pads on a flat under-surface are placed on a printed circuit having pre-soldered pads and the assembly is then heated to cause melting and re-flowing of the solder.

For many applications it is common to use flux-requiring solders, e.g.

tin/lead solders, but the fluxes used leave undesirable acidic deposits which need to be cleaned off afterwards to avoid subsequent corrosion and weakening of the soldered joints. However thorough the cleaning processes may be there is always the possibility that flux residues will remain trapped between the body of the chips and the board. Also, many flux cleaning processes employ materials or procedures which are environmentally unfriendly.

Many processes and procedures for fluxless soldering of components have been proposed. US patent 4,921,157 (Dishon et al) proposes a method of reflow soldering in a plasma excited fluorine containing atmosphere. The presence of a fluorine containing atmosphere serves to effect a chemical reduction of the surface oxides found on common solders such as tin/lead solders. The use of plasma excitation serves to reduce the temperature at which the fluorine reduction of surface oxides takes place. Many other workers in the field have proposed variations on fluxless soldering in a halogenated atmosphere.

Unfortunately, halogenated gases such as fluorine are themselves aggressive in respect of some semiconductor materials and undue exposure of integrated circuits to halogenated gases can result in damage to the circuits.

Chemical reduction of oxides using hydrogen is a well known technique. However the reaction temperature required to use hydrogen as a reducing agent is high, typically 6000. At such elevated temperatures the semiconductor integrated circuits can readily be damaged as can other circuit components. Also the use of a pure hydrogen atmosphere is difficult and potentially dangerous in an industrial environment.

According to the present invention there is provided a method of fluxless soldering comprising the steps of placing an article provided with solder formations on metallic areas thereof in an atmosphere comprised of a mixture of hydrogen and an inert carrier gas, effecting a plasma excitation of the atmosphere surrounding the article and heating the article until reflow of the solder occurs.

An embodiment of the invention is described with reference to the accompanying drawing which is a schematic illustration of an apparatus providing a plasma excited atmosphere.

In the apparatus depicted a chamber 10 is formed by a base 11, an enclosing wall structure 12 and a closure or lid 14. Within the chamber is located a support member 16 carrying a heated element 18 upon which is placed a substrate 20, such as a printed circuit board.

The substrate has on its upper surface a printed circuit pattern (not shown) parts of which are pads or lands pre-coated with solder, e.g. a tin/lead solder. Placed on the printed circuit are electronic circuit components with metal pads, e.g. packaged integrated circuit modules 22, 24, 26. As an alternative to a presoldered substrate the substrate may have solder pre-forms placed between the relevant parts of the printed circuit and the components. The chamber is then flushed and filled with a gas atmosphere comprising a mixture of, preferentially, not more than 258 hydrogen and an inert carrier gas, e.g. nitrogen or argon, and a plasma excitation of the gas atmosphere is induced by conventional means (not shown) and the substrate is heated by the heating element until reflow of the solder occurs.Solder fillets are thus formed between the metal pads of the components and the pads of the printed circuit. The soldered joints are allowed to cool and solidify before the chamber is opened to the air and the soldered substrate is removed. Preferably the gas atmosphere is provided by so-called "forming gas" containing 108 H2 and 908 N2.

The use of forming gas provides a substantially inert atmosphere (nitrogen) with sufficient reducing hydrogen gas to react with the solder oxides. The result of the reduction reaction is mere water which is not damaging to the integrated circuits. While normally a hydrogen reduction reaction needs to be performed at temperatures which would be damaging to integrated circuits the use of plasma excitation allows the reduction reaction to be effected at a temperature low enough not to affect integrated circuits adversely.

The result is a soldering process which is clean and environmentally friendly.

Claims (5)

CLAIMS:

1. A method of fluxless soldering comprising the steps of placing an article provided with solder formations on metallic areas thereof in an atmosphere comprised of a mixture of hydrogen and an inert carrier gas, effecting a plasma excitation of the atmosphere surrounding the article and heating the article until reflow of the solder occurs.

2. A method according to claim 1 wherein the gas atmosphere comprises not more than 25 % by volume of hydrogen.

3. A method according to claim 2 wherein the gas atmosphere is forming gas being a mixture of hydrogen and nitrogen.

4. A method substantially as described with reference to the accompanying drawing.

5. An article provided with re-flow soldered formations formed by the method of claim 1, 2 or 3.