The future is orange

Copper's share of the bonding wire market is set to grow rapidly over coming years, writes Hans Martin Buschbeck, product marketing manager of Kulicke & Soffa Industries.

In the packaging and assembly industry, the majority of electrical connections are still done by wire bonding, whereby the bonding wire connects the chip-pad with the chip carrier or chip frame. More than 90% of ICs use wire bonding for interconnects.

Newer technologies such as flip chip or packaging on wafer level still only account for a relatively small percentage of electrical connections compared with wire bonding technology.

The bonding wire market - which had a turnover of more than $1 billion in 2003 (2) - is dominated by gold wire. Unsurprisingly, the material cost of the gold accounts for a significant part of the sector's turnover.

Other materials are used, although in much smaller amounts. Aluminium wire (both thin and heavy) makes up about five per cent of the total volume of wire used.

Heavy aluminium wire is used in some power devices and occasionally for automotive applications. Thin aluminium wire is incorporated into so called low end applications such as chip on boards (COBs).

With less than one per cent of the market last year, copper wire might appear to be an insignificant player but that looks set to change over coming years. Copper's market share is growing rapidly.

The reason for this is simple. Copper bonding wire has several advantages over gold wire. Driven by the never-ending pressure to reduce costs, copper has replaced gold wire in discrete and power devices in many applications over the last few years.

Consumption of 2mm (50µm) gold wire has fallen significantly as a result of the growing use of copper. At this diameter, companies can achieve cost reductions of up to 90% (depending on gold prices) by using copper instead of gold.

Beside the cost factor, copper has other key advantages. It has superior electrical resistivity and thermal conductivity to gold. In addition, copper boasts reduced intermetallic growth, higher reliability of the bonding connection at higher temperatures and better mechanical stability.

Here, we will take a look in more detail at the advantages and disadvantages of copper as a bonding wire.


Lower electrical resistivity
The low electrical resistivity of copper (Fig 3) lends itself in particular to high current applications. But even in lower current applications, fine copper wire might be chosen in place of gold because of its better conductivity (around 23% above gold's).

Copper allows a higher operation current, enhances the performance of power devices and extends the possibilities in the area of fine pitch technology. Most of these very complex packages, with multi pole connections with very small pad openings, require the use of fine wire.

Due to its superior electrical characteristics, copper can still carry enough current even when supplied in the finest wire.


Higher thermal conductivity
Because copper has a 21% better thermal conductivity than gold, the heat distribution within the package is faster and more efficient and therefore the wire cools down faster and the stress relieves faster.

This is important because heat encourages grain growth in the wire, reducing its mechanical stability.

The high thermal conductivity also has a positive affect during the ball formation process, as the heat affected zone (HAZ) - and therefore the mechanically weakened zone - tends to be shorter, ensuring a lower (and therefore better) loop capability.

Demand for low loops is growing, especially because of the increasing need for stacked die applications.


Slower intermetallic growth
Most of a chip's metal usually comprises aluminium alloys. Gold wire and copper wire are used to build an intermetallic (IMC) phase with aluminium that behaves significantly differently.

The thickness of an IMC Al-Au welding system increases continuously - as a result of IMC growth - especially with increasing temperature. At 100¡C, IMC growth will already have started (Fig 4).

With a thicker IMC phase, the electrical resistivity increases which, in combination with a higher current, leads to a higher heat occurrence.

This additional heat speeds up the diffusion process which - in turn - is responsible for further IMC growth. Thus begins a vicious circle that results in deteriorating electrical properties.

IMC growth starts at significantly higher temperatures on copper wire bonds. Copper wire bonds therefore have better electrical reliability. Even after several weeks of temperature cycles, the resistivity of copper wire bonds only increases slightly (Fig 5).

The mechanical stability achieved using copper wire is also greater than gold wire. Standard stability tests (shear test, pull tests) show 25%-30% higher values than comparable gold wire bonds (Fig 6)

Copper bond connection can be so strong indeed that during pull tests the wire does not break and the pad metallisation is ripped off instead (Fig 7). This is why in such cases a non destructive pull test - where only a specific pull load has to be achieved - is recommended.


Disadvantages of copper wire bonding
After so many advantages, there inevitably come some disadvantages. Copper oxidises readily at relatively low temperatures. If there too much copper oxidation on the wire surface, the wire gets too hard to be bonded and a spherical free air ball (FAB) can no longer be formed.

It is therefore essential that the copper wire is stored in an oxygen-free environment (to prevent oxidation) during the formation of the FAB. For this reason, copper FABs are usually formed in an inert atmosphere (N2 or N2H2).

As copper is generally harder than gold, bonding parameters have to be adjusted to avoid cratering.

The best solution would be to bond the copper wire onto pads with similar hardness (eg bonding copper to copper). However, this is not commercially feasible yet as the pad cannot currently be protected from oxidation reliably enough.

Moreover, the technology to examine the effect of the moulding compound on the wire is not advanced enough yet.


The future is orange
Despite such problems, the advantages of copper as a bonding wire are increasingly outweighing the disadvantages.

Not long ago, the use of copper instead of gold wire was driven by cost only. But nowadays copper wire's excellent mechanical and electrical properties are the main reasons for its expanding use within the market.

The growing popularity of copper wire can be observed within the semiconductor assembly or packaging market.

Previously, this market only used copper for thicker wire diameters (38-50µm) for power devices and discrete, but now several evaluations are being carried out using fine copper wire (20-30µm) for ball grid arrays (BGAs) and quad flat packs (QFPs) . Some production sites have even already qualified copper wire for use in such applications.

These applications require copper wire with smaller diameters (eg 25µm), where the cost savings are somewhat smaller compared to the power devices, where mainly 50µm wire is used.

The mechanical and electrical advantages are becoming the focus of the attention. That is the reason why plastic dual in-line packages (PDIPs) and small outline integrated circuits (SOICs) are also starting to be transferred from gold wire bonding to copper wire bonding (Fig 8).

As high pin count ICs such as BGA and QFP far outnumber power devices, the potential growth in the copper wire market is massive. Within a few years, copper could have a bigger share of the bonding market than aluminium.


[1] Tracy DP, Packaging Materials Market Trends, Proc. SEMI Microelectronics Materials Strategy Symposium 2002

[2] SEMI, Packaging Materials Report 2003


Glossary

Chip on board (COB)
A hybrid technology that exclusively employs face-up-bonded chip devices interconnected to the substrate in a conventional manner, say, by flying wires. A generic term for mounting an unpackaged silicon die directly onto the PCB

Free air ball
Ball formation prior to bonding contact

Ball grid array (BGA)
Type of IC package (ceramic, metal or plastic) that uses solder balls instead of leads

Quad flat pack (QFP)
A ceramic or a plastic chip carrier in which the leads project down and away from all four sides of a square package

Plastic dual in-line package (PDIP)
Plastic version of a common type of package used to mount and enclose semiconductor chips

Small outline integrated circuit (SOIC)
An integrated circuit surface mount package with two parallel rows of gull-wing leads

Fig 1

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Fig 3

Fig 4

Fig 5

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Fig 7