WO2001003878A1 - Solder alloy - Google Patents

Abstract

A lead-free solder alloy suitable for use in wave soldering and, in particular with water base VOC-free flux, low-VOC (water base) flux and low solids (solvent base flux consists of tin to which is/are added one or more of Ag in an amount of up to 10 %, Cu in an amount of up to 5 %, Sb in an amount of up to 10 % and Bi in an amount of up to 10 %, which alloy additionally contains phosphorus in an amount of up to 0.01 %, all percentages being on weight basis related to the amount of tin.

Description

SOLDER ALLOY

This invention relates to solder alloys and in particular solder alloys for use in soldering methods that use a reservoir of molten solder.

The most commonly used solder materials have hitherto been tin-lead solders. Apart from the problem of using lead-containing alloys which are environmentally unfriendly, in particular tin-lead alloys, the molten solder oxidises and thereby prevents the surface from looking bright and clear. Phosphorus has, indeed, become a widely used additive to tin-lead solders in amounts of 0.001 - 0.004 per cent. Such addition slows down the rate at which the molten solder oxidises and prevents the formation of coloured interference films of oxide and keeps the surface looking bright and clear. A further advantageous effect is to break up the "wet" dross containing a high proportion of unoxidised solder formed on wave soldering machine solder pots into a more powdery, drier dross with less unoxidised metal. Apart from reducing the amount of oxide and dross formed, phosphorus addition confers no particular advantage in terms of reduced incidence of soldering defects. Wave soldering with the now preferred lead-free alloys such as SnAg3.8Cu0.7 and SnCu0.7, especially when working with low solids (solvent base) fluxes, has led to the identification of a tendency for a tenacious oxide layer to form on the wave which can cause soldering defects such as webbing and bridging.

Overall, there is an increase in defect rates if such oxide layer is able to form. Such problem may also occur with other soldering methods that use a reservoir of molten solder, including drag soldering and methods using a small solder fountain or wave.

According to one aspect of the present invention,

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effect on oxidation and drossing and in particular in suppressing formation of a tenacious oxide layer at the most critical parts of the molten solder, in particular the wave . It also leads with lead free solders and water base VOC-free or low VOC water base, as well as low solids solvent base fluxes to soldered printed circuit boards showing significantly lower defects than when phosphorus has not been added.

Claims

1. A lead free soldering alloy which consists of tin to which is/are added one or more of Ag in an amount of up to 10%, Cu in an amount of up to 5%, Sb in an amount of up to 10% and Bi in an amount of up to 10%, which alloy additionally contains phosphorus in an amount of up to 0.01%, all percentages being on weight basis related to the amount of tin.

2. An alloy according to claim 1, which contains from 0.001 to 0.004% of phosphorus.

3. An alloy according to claim 1 or 2, wherein the phosphorus is present in SnAg3.5-4.0 CuO.5-1.

4. An alloy according to Claim 3, wherein the phosphorus or SnCuO.5-1 is present in SnAg3.8CuO .7.

5. A method of attaching a circuit component to a substrate, which comprises carrying out soldering using a soldering alloy according to any preceding claim in forming a joint between circuit component and a conductive element on the substrate.

6. A method according to claim 5, which is a wave soldering method.

. A method according to claim 5 , which is a drag soldering method or a method using a solder fountain.

8. A method according to claim 5, 6 or 7 wherein soldering is carried out with water base VOC-free flux, low-VOC (water base) flux and low solids (solvent base) flux.

9. A method according to claim 8, wherein soldering is carried out with rosin base flux having a solids content of less than 10% by wt .

10. A solder joint whenever produced using a soldering alloy according to any one of claims 1 to 4 or a method according to any one of claims 5 to 9.