- Introduction
Bonding wire is used to connect the silicon chip to the brass leads in the vast
majority of integrated circuits made around the world (see Figure 1). The microelectronics market
consumes 14 billion feet of bonding wire every year (at an increasing rate from year to year). That
is a $400,000,000 market.
Most applications use wire in the 0.0013²
to 0.0010² diameter range (with the sizes decreasing from
year to year). Current wire-making technology is limited to sizes down to
0.0008². Below that size wire production cost increases to
such an extent that the market cannot tolerate it.
The Hydraw Method, developed by Metalforming, can produce wire down to
0.0004² in diameter at reasonable prices. This will enable
microchip makers to produce smaller (or denser) chips, which is the ongoing goal of the entire
industry.
More importantly, as the wire size decreases the Hydraw process becomes much
more efficient than conventional manufacturing methods. Consequently, a manufacturer using the
Hydraw process can produce cheaper wire than those using conventional processes. It is this price
advantage that justifies the formation of a Hydraw-based bonding wire manufacturing business.
Metalforming's goal is to go into that business.
Figure 1: Open View of a Simple Integrated Circuits.
- Comparison of the Conventional Wire Making Method and the Hydraw Process
Bonding wire is a fairly simple product. Its price on the market is based upon two
key elements: the manufacturing cost of the wire and the price of gold. Since gold is a commodity,
gold price to all makers is essentially the same. In addition, all the major bonding wire manufacturers
use the same manufacturing processes. Consequently, their manufacturing costs are almost the
same. Because of this wire makers differentiate themselves by the quality and variety of services
they provide to their customers.
Gold bonding wire undergoes several manufacturing steps before it is ready for
the customers: 1. gold ingots are cast, 2. the ingots are extruded into rods, 3. the rods are drawn
down to the size of wire using a draw bench or bull block and 4. the wire is drawn using the wet wire
drawing process.
The weight of a small ingot is 200 grams. That can produce 45,000 ft of wire that
is F0.0010² (or 25
microns) in diameter. When the cost of all the processing steps (needed to transform the ingot into
wire) are taken into consideration, each thousand feet of
F0.0010² wire costs
about $7.00 to produce.
Beyond that point wire processing cost begins to rise dramatically. Currently, 20%
of the market uses sizes below 1mil (25 microns). That is Metalforming's market niche, because at
these sizes the Hydraw process has significant advantages.
In wet wire drawing machines, the wire is threaded, back and forth, through dies
stationed between two cylinders. At standard sizes the wire can be reduced by 8% at each of the
wet wire drawing machine's 8 to 12 dies. That gives a total reduction in area of 50~60% at a speed
of 100m/min.
At diameters smaller than 0.0010² the wire
can only be reduced 4~5% at each die (because larger reduction would cause tearing). Furthermore
the speed must be reduced to 40m/min. Consequently, a 10 die wet wire drawing machine can reduce
the wire by less than 40%.
By comparison, the Hydraw process uses one die whose reduction can reach 25%.
More importantly, the speeds are significantly greater: 250m/min at those sizes. As a result, the cost
to manufacture 0.0008² wire by conventional methods is about
$7.50 per thousand feet of wire. In comparison, the manufacturing cost of 1000 feet of wire
produced by Hydraw (in the last stages of manufacturing) is only $6.80. This is a 9% cost savings.
When the wire is reduced further, to 0.0006², the savings are
much more significant: $21.80 for wet wire drawing compared to $13.40 for Hydraw. The calculations
that follow are based on these costs.
- Annual Consumption of Bonding Wire (25m and below)
Wire
Size
(m)
|
(xxx)
Consumption
km/month
|
(xxx)
Consumption
kft/month
|
Worldwide
Consumption
kft/month
|
| 25 | 10,200 | 3,108,960 |
18,288,000 |
| 23 | 5,000 | 1,524,000 |
9,000,000 |
| 22 | 50 | 15,240 |
90,000 |
| 20 | 4,000 | 1,219,200 |
7,170,000 |
| 19.5 | 500 | 152,400 |
896,000 |
| 18 | 500 | 152,400 |
896,000 |
| 15 | 500 | 152,400 |
896,000 |
- Market for 'Hyper-Fine' Wire
Fine wire has two significant advantages over course wire. Firstly, it is smaller so it
takes up less space when bonded to the surface of a chip. Secondly, the same length of wire should
be cheaper because it contains less gold than courser wire. However, fine wire has one significant
disadvantage when compared to course wire: it is weaker.
Bonding wire is most often used to connect the leads on the silicon chip to metal
leads. In such applications the wire is "looped" from the chip to the lead frame. Later, a plastic resin
is injection molded over the die, the bonding wire and the bonded end of the lead frame. The resin
dries into a hard layer that encapsulates and protects the chip and the leads from the outside
environment. During the molding operation, the viscosity of the resin drags the wire in its path. With
weak wires this may cause the wires to short or tear.
Figure 2: Bonding Wire "Looped " from Silicon to Lead Frame.
A new technology called Chip Scale Packaging (CSP) eliminates such problems. In
one types of CSP a very short loop is used to connect the chip with the lead. Instead of a metal
lead, the wire is bonded to a special tape (see the diagram below). CSPs were designed to enable a
very high pin count chips in very small packages. Since this is also the goal of 'hyper-fine' wire it is a
good match.
Some of the semiconductor industry's major companies are already using this kind
of CSP technology: Sharp, Fujitsu, Toshiba, Sony, Rohm, Hitachi, Oki, T.I. and LSI Logic to name a
few. We know that several of these companies are very interested in fine pitch applications.
Figure 3: Bonding Wire "Looped " from Silicon to Substrate Lead on CSP.
- Comparison of Pad Pitch to Wire Size
The perimeter of a silicon chip has a fixed length. The number of leads that can fit
on the periphery is limited by the size of the pads used as leads. In typical applications the pad size
is 50m wide. This enables the 40m
'bump' of a 25m wire to attach itself with room to spare. Given a
30m gap between pads, the pad pitch would be
80m. Smaller wires need smaller pads. Consequently, more leads can
fit on the perimeter of a chip when the wire is smaller (see the figure below). The trend toward more
powerful chips means that more leads are needed. To be able to limit the size of the chip, the pad
pitch will have to be kept at a minimum. This means smaller wire will be needed.
Figure 4: Comparision of Wire Size with Pad Pitch.
- Cost of Wire
The main attraction, of 'hyper-fine' wire, to potential customers is cost savings.
With Hydraw fine wire costs less. The graph below compares wire size to cost.
Figure 5: Bonding Wire Cost.
