Reducing joint embrittlement in lead-free soldering processes

The present invention relates to the technology of soldering and, in particular, to improved lead-free solder joints for electronic soldering applications, involving printed wiring boards or chip scale packaging and ball grid arrays .

- For environmental reasons, there is an increasing demand for lead-free replacements for lead-containing conventional alloys. These alloys typically contain silver and copper additions to tin, and are used in many of the soldering processes required for the manufacture of components and the joining of components to printed wiring boards.

Molten solder alloys tend to dissolve the substrate and to form an intermetallic compound at the interface with the substrate. For example, tin in the solder alloy will react with the substrate at the interface to form an inter metallic. If the substrate is copper, then a layer of CUeSn ₅ will be formed. Such a layer typically has a thickness of from a fraction of a micron to a few microns. At the interface between this layer and the copper substrate an intermetallic compound of Cu ₃ Sn may also be present. Such intermetallic compounds may result in a brittle solder joint. In some cases, voids occur, which may further contribute to premature fracture of a .stressed joint. This may particularly be the case when the intermetallic interfacial layers grow during the ageing after soldering and during service.

It has been found that certain alloying elements inhibit the growth of the interfaces, thereby prolonging the effective life of a solder joint. These elements include, for example, Ni, Co, Cr, Mn, Zr, Fe and Si. The inventors have found, however, that there can be problems associated with the incorporation of these elements into solder alloy compositions. In particular, problems can arise during fabrication of the alloys into the form desired for their use.- For example, high volume manufacture of solder spheres is typically achieved using liquid metal jetting. In this process molten alloy is required to flow through a small diameter hole in a nozzle prior to being broken up in to droplets of a fixed size and then solidified as a sphere in a cooling column. The inventors have found that the presence of certain alloying elements at higher concentrations can result in unmelted or undissolved intermetallic compounds. These intermetallic compounds can block the nozzle, causing loss of jet or spray stability, and sometimes a complete cessation of liquid metal flow.

The present invention aims to address at least some of the problems associated with the prior art and to provide improved solder alloy joints.

Accordingly, in a first aspect the present invention provides -a method of forming a solder joint, comprising:

(a) providing a lead-free, tin-containing solder alloy;

(b) providing a printed wiring board substrate or a ball grid array substrate, said substrate being formed from a material comprising copper, optionally also zinc, and one or more of Ni, Co, Cr, Mn, Zr, Fe and Si and/or alloys thereof;

(c) heating the solder alloy; and

(d) either before, during or after step (c) contacting the solder alloy with the substrate.

In a second aspect the present invention provides a method of forming a solder joint, comprising:

(a) providing a lead-free, tin-containing solder alloy;

(b) providing a printed wiring board substrate or a ball grid array substrate, said substrate being formed from a material comprising copper and optionally also zinc;

(c) depositing one or more of Ni, Co, Cr, Mn, Zr, Fe and Si and/or alloys thereof on a surface of the substrate;

(d) heating the solder alloy; and

(e) either before, during or after step (d) contacting the solder alloy with said surface of the substrate.

The step of depositing the one or more of Ni, Co, Cr, Mn, Zr, Fe and Si (including alloys of any one or more thereof) on said surface of the substrate may be achieved by plating, spraying and/or vapour deposition. For example, the spraying may compris,e plasma, arc or flame spraying. The vapour deposition may comprise chemical vapour deposition or evaporation, sputtering or ion implantation.

In a third aspect the present invention provides a method of forming a solder joint, comprising:

(a) providing a lead-free, tin-containing solder alloy;

(b) forming the solder alloy into spheres or other preform;

(c) depositing on the surface of the spheres or other preform, one or more of Ni, Co, Cr, Mn, Zr, Fe and Si and/or an alloy thereof;

(d) providing a printed wiring board substrate or a ball grid array substrate, said substrate being formed from a material comprising copper and optionally also zinc;

(e) heating the solder alloy spheres or other preforms; and (f) either before, during or after step (e) contacting the solder alloy with said surface of the substrate.

The step of depositing one or more of Ni, Co, Cr, Mn, Zr, Fe and Si (including alloys of any one or more thereof) on the surface of the spheres or preform may be achieved by plating, spraying and/or vapour deposition. The spraying may comprise plasma, arc or flame spraying. The vapour deposition may comprise chemical vapour deposition or evaporation, sputtering or ion implantation.

The presence of the Ni Co, Cr, Mn, Zr, Fe and/or Si introduced to the solder-substrate interface by means of the present invention has a beneficial effect in reducing joint embrittlement .

Heating of the solder alloy results in the alloy becoming fully or substantially molten.

As used herein, the term ball grid array also encompasses chip scale packaging.

The lead-free, tin-containing solder alloy will typically also be essentially free of one or more of Ni, Co, Cr, Mn, Zr, Fe and Si. However, the solder alloy may contain small amounts of one or more of these elements. The lead-free, tin-containing solder alloy is preferably based on a Sn-Cu alloy and will typically also include Ag\

Each of Ni, Co, Cr, Mn, Zr, Fe and Si will typically be present in the substrate (first aspect) or on a surface of the substrate (second aspect) or on a surface of the spheres or preform (third aspect) in an amount of up to 3 wt.%, more typically up to- 2 wt.%.

The elements may be present singularly or in any combination of two or more thereof. The elements may be provided in the form of an alloy, i.e. in combination with another alloying element, which may be one of the other elements or a different element, such as copper.

Examples of alloy substrate materials include (wt.%) one of the following: Cu-IZr, Cu-2.35Fe-O .03P-0.12Zn, Cu- 1.5Fe-O.18P-0.8Co-O.6Sn, Cu-O .6Fe-O .2P-0.05Mg, Cu-3.0Ni- 0.65Si-O.15Mg and Cu-3. OSi-I .5Sn-O . ICr. These are known copper alloys which can be readily made and obtained, and can be worked into shapes as required.

If the substrate is a printed wiring board, then it is preferably based on FR4 laminate..

The substrate will typically comprise an electrodeposited foil or a rolled foil bonded to another material. The electrodeposited foil or a rolled foil may have the alloy compositions as described herein. The electrodeposited foil or rolled foil will typically be bonded to a glass fibre epoxy laminate.

The substrate may further comprise one or more of silica, alumina, or other low CTE composite materials and/or highly thermally conductive materials.

The substrate may further comprise a solderability enhancing coating such as tin, gold and/or silver, including alloys thereof, or an organic coating.

The present invention also provides a printed wiring board substrate or a ball grid array substrate, the substrate being formed from an alloy comprising copper, optionally zinc, and one or more of Ni, Co, Cr, Mn, Zr, Fe and Si and/or an alloy of any one or more thereof.

The substrate may be formed from a material comprising copper and optionally also zinc, wherein a surface of the substrate has deposited thereon one or more of Ni, Co, Cr, Mn, Zr, Fe and Si and/or an alloy of any one or more thereof.

The substrate may have the alloy substrate compositions as described herein and -may be provided in the form of an electrodeposited foil or a rolled foil, which may be bonded to a glass fibre epoxy laminate.

The present invention also provides a lead-free, tin- containing solder alloy in the form of spheres or a preform, wherein the surface of the spheres or perform has deposited thereon one or more of Ni, Co, Cr, Mn, Zr, Fe and Si and/or an alloy of any one or more thereof. A subsurface centre portion of the spheres or preform may be essentially free of one or more of Ni, Co, Cr, Mn, Zr, Fe and Si.

Alternatively, the bulk of the spheres or preform may- contain small quantities of one or more of these elements. Whatever the case, the concentration of the one or more of Ni, Co, Cr, Mn, Zr, Fe and Si is preferably highest at the surface of the spheres or preform. The solder alloy may be based, for example, on Sn-Cu, or Sn-Cu-Ag, or Sn-Ag or Sn- Zn.

The alloy according to the present invention or for use in the method according to the present invention may be supplied together with a flux.

In the foregoing passages, different aspects of the invention have been ^' defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Example

The following is a non-limiting example to further describe the present invention.

SAC 305 solder spheres, Sn-3. OAg-O .5Cu (wt%) were fluxed and soldered to a range of substrates in a reflow oven. The coupons so made were aged in air in an oven for 1000 hours, then sectioned and polished metallographically. Micrographs were taken and the thickness of the interfacial IMC measured.

The micrographs in Figure 1 compare the copper rich Cu ₃ Sn growth on C151 and C197 compared with a pure copper substrate. The alloying additions in the substrate are effective in suppressing Cu ₃ Sn IMC growth that is implicated in loss of solder joint reliability. IMC growth on a range of solderable lead-frame alloys after 1000 hour aging is summarized in Table 1.

Table 1

C151: Cu - I wt% Zr

C197: Cu - 0.6 Fe - 0.2 P - 0.05 Mg

C194: Cu - 2.35Fe - 0.03P - 0.12Zn

C7025 Cu - 3.0 Ni - 0.65 Si - 0. 15 Mg

C638: Cu - 2.8 Al - 1.8 Si - 0.4 Co

The provision of alloying additions on the surface of solder pads/UBM can have the same beneficial effect.