Thick-film
pastes for the manufacture of low-cost, insulated aluminum substrates suitable
for use as heat sinks for high-power LED applications.
A new paste system from ESL ElectroScience
The ubiquitous incandescent light bulb has long been known to be an inefficient
source of light. The fluorescent light bulb has superior efficiency and has, for
many years, challenged its dominance, with the modern, compact fluorescent light
bulb (CFL) being used to reduce domestic energy costs. It is known that there
is at least a three-fold increase in efficiency (measured in lumens/watt) with
these devices. Light emitting diodes (LEDs) are about as efficient as CFLs but
the light produced resembles that of a spotlight, and a number of them are
needed in a cluster to make a useful light source. LED clusters are used as an
alternative means of lighting in both domestic and, particularly, automotive
applications. The use of high power LED lighting results not only in light but
heat. This heat needs to be rapidly dissipated for efficient lighting. The
first lighting systems were made by mounting the LEDs onto a printed circuit
board (PCB). The thermal characteristics of the glass epoxy system used in
traditional PCBs are not sufficient to do this efficiently and effectively. The
thermal conductivity, k, of glass epoxy, the major component in a PCB, is 0.23 W.m-1K-1.
Copper conductor tracks superimposed on FR4 (PCB glass epoxy) have a thermal
conductivity of 401 W.m-1K-1 and this gives
an overall k of 16-17 W.m-1K-1. Mounting the assembled PCB
onto an aluminum substrate gives superior thermal characteristics but the use of
ESL pastes to make a solderable, insulated aluminum substrate obviates the need
to deploy a PCB in the structure.
2. Materials
Why
aluminum? Pure aluminum has a k of 237 W.m-1K-1 and copper
has even better thermal conductor. If either copper or aluminum can be
insulated in some way and conductors superimposed onto these insulated metals
using an additive process then the thermal advantages could be used to good
effect. Other substrates with good thermal characteristics such as direct
bonded copper (DBC) and insulated metal substrate (IMS) have been used to
fabricate power circuits but the manufacturing costs are high due to the
sophisticated processes involved. Anodizing aluminum has been tried but there
is some debate as to whether the insulation is sufficient to support circuitry.
The insulation of copper with glasses is difficult due to the oxidation that
takes place during high temperature processing in air. Furnaces that accept a
flow of nitrogen can be used but the processing costs can often outweigh the
advantages gained by using this metal. Aluminum at 3mm thickness can be
processed in air at temperatures up to about 600°C without too much deformation
of the substrate. However, the coefficient of thermal expansion (CTE) for pure aluminum is ~25ppm/°C and until now this has been considered to be a barrier for
effective insulation of aluminum. An advantage of using aluminum is that its
density is low at 2.70g.cm-3 – copper, for example, is at least
three times as dense as aluminum.
2.1 Aluminum substrates
Aluminum
alloys are used as the preferred metal base and, in particular, 3003 alloy. All
data sheets quote 3003 as the aluminum alloy substrate used for calibration
purposes. It has been found that, although this alloy is readily available in
the US, not every country has supply of this alloy. 3103, a close match to
3003, is readily available in Europe but Japan has difficulty in obtaining
either 3003 or 3103.
The
International Alloy Designation System (IADS), introduced in 1970, employs a
classification method developed by the Aluminum Association of the United States.
Each aluminum alloy is given a 4-digit number as shown in Table 1.
Table 1
| Alloy |
Major alloying element |
Characteristics |
| 1XXX |
99% Al |
Excellent formability, weldability and corrosion resistance. Low
strength. The last two digits signify the minimum purity of the aluminum –
1145 has a minimum purity of 99.45% Al |
| 2XXX |
Cu |
Excellent machinability and high strength. Poor formability,
weldability and corrosion resistance. |
| 3XXX |
Mn |
Formable, corrosion resistant and weldable. Moderate strength. |
| 4XXX |
Si |
Formable, weldable, corrosion resistant |
| 5XXX |
Mg |
Formable, weldable, excellent corrosion resistance |
| 6XXX |
Mg and Si |
Medium strength alloy, weldable, excellent corrosion resistance |
| 7XXX |
Zn |
Machinable, poor corrosion resistance and weldability. High
strength. |
| 8XXX |
Other(incl. Li) |
Excellent formability |
Suffixes are added to identify treatments:
F = as fabricated
O = annealed wrought products
H = cold worked
T = heat treated
2.2 Screen-printable pastes
The
dielectric part of the system is a two part system designed for low temperature
firing onto aluminum alloy substrates.
2.2.1 4603
4603
is designed to match the TCE of the aluminum alloy substrate onto which it is
printed. The thermal conductivity of the glass is of the order of 1 W.m-1K-1.
2.2.2 4604 and 4604-A
4604 and 4604-A (lower solids version) are designed to match the underlying 4603 and
provide electrically insulative properties. The thermal conductivity of the
glass is of the order of 1 W.m-1K-1. 4604 is used where a
thicker layer of electrically insulating dielectric is required but the use of
this dielectric requires a cofireable conductor to be used as the top layer.
2.2.3 9912-K or 903-A
9912-K
is a solderable conductor for use on top of the insulating system and may be
fired at the low temperatures required to process circuitry on aluminum alloy
substrates. It must be used on 4604-A. 903-A may be cofired on both dried 4604
and 4604-A. Both conductors fired onto fired 4604 will result in cracking of
both conductor and dielectric film.
3. Processing
The
standard drying temperature for all pastes used on aluminum alloys is 125ºC
for ten - fifteen minutes. The standard firing temperature for all pastes used
on aluminum alloy substrates is 580ºC. This profile is for one hour with a
peak temperature held for ten minutes.
3.1 Aluminum preparation
Where
aluminum alloys are supplied with a plastic protective coating, no preparation
is required. Expose the clean surface using gloved hands and all further
contact with the aluminum alloy should be made with gloved hands.
3.2 Screen printing dielectrics
ESL dielectrics 4603 and 4604/4604-A
are screen printed onto an appropriate aluminum alloy using a 145 mesh
stainless steel screen with 0μm emulsion. Double wet passing is used to
minimise the presence of pinholes in the fired film. Two separately fired
layers of 4603 are used. The thickness of the 4603 should be >45μm
after two layers to allow the total dielectric thickness to be in excess of
60μm, the minimum dielectric thickness required for good insulation, after
the third layer is applied. This layer is either 4604 (for cofiring with conductors)
or 4604-A (separately fired from the overlying 9912-K conductor) which are
printed using a 145 mesh stainless screen with 0μm emulsion. All layers
are fired at 580ºC. Measurement of dielectric thickness is carried out using an
Elcometer 345 NF coating thickness gauge.
3.3 Screen printing conductors
9912-K
is a silver conductor and may be used on 4603/4604-A insulated aluminum
alloy where the dielectric layers have been separately fired. 903-A may be
cofired with either 4604 or 4604-A. Both conductors are screen printed with a
325 mesh stainless steel screen with 20μm emulsion. The thickness of the
conductor should be 10-12μm.
4. Testing
4.1 AC Breakdown Voltage
Breakdown
voltages, measured with a Clare flash tester with the leakage current set at
5mA, in excess of 2000V AC can be expected upon correct processing of the
pastes.
4.2 Insulation resistance
Insulation
resistance, measured using a Unilap Iso X meter at 100V DC, is of the order of 108
Ω at room temperature.
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