A Comprehensive Introduction on Aluminum PCBs

Properties of Aluminum PCBs

1. Copper Foil Layer

Aluminum CCL features the same copper foil layer with ordinary CCLs and large current carrying capacity is required by circuit layer, which is why relatively thick copper circuit is selected with thickness from 1oz to 10oz. The back side of copper foil has to go through oxidation treatment in a chemical way while the surface side should go through zincing and brass plating in order to improve peel-off strength.

2. Dielectric Layer

Dielectric layer is composed by a layer of thermally conductive dielectric material with low thermal resistance whose thickness is from 50μm to 200μm, which is the core technology of aluminum CCL. It performs excellently in anti-thermal aging and can withstand mechanical and thermal stress.

3. Aluminum Base Layer

Aluminum base layer is actually aluminum substrate material that is the supporting component of aluminum base layer. It is required to be highly thermal conductive and suitable for ordinary mechanical manufacturing such as drilling, punching and cutting.

4. Aluminum Base Membrane

Aluminum base membrane plays a role in protecting aluminum surface from scraping and agent etching. Membrane can be classified into ordinary (lower than 120°C) and anti high temperature (250°C). The latter type meets the requirement of HASL (hot air solder leveling) as surface finish.

· Aluminum PCB Performance

1. Thermal Dissipation

Compared with ordinary FR4 PCBs, aluminum PCBs perform better in terms of thermal dissipation and they are capable of dissipating heat quickly. Take comparison between FR4 PCB and aluminum PCB with equivalent thickness of 1.5mm as an example. Thermal resistance of FR4 PCB is from 20°C/W to 22°C/W while that of aluminum PCB is from 1°C/W to 2°C/W. Thus, aluminum can work well on thermal dissipation.

2. Thermal Expansion

Thermal expansion and contraction is the common nature of substances and different substances carry different coefficients of thermal expansion. Now that aluminum PCBs perform excellently in terms of thermal dissipation, thermal expansion and contraction issues of components on board surface can be therefore dramatically reduced with durability and reliability of complete appliance and electronic equipment. This merit of aluminum PCBs especially works smoothly for thermal expansion and contraction issues of SMT (surface mount technology).

3. Dimensional Stability

Aluminum-based PCBs feature obviously stable size. When they are heated from 30°C to 140°C or 150°C, their size changes by only 2.5% to 3.0%.

4. Other Performances

a. Applicable for power device SMT.
b. Effective in thermal expansion of circuit design.
c. Helpful in reducing operating temperature, improving product power density and reliability and prolonging product's shelf life.
d. Shrinking product's volume and decreasing hardware and assembly cost.
e. Replacing fragile ceramic base, with better insulating property and mechanical endurance.


Classification of Aluminum PCBs

Based on application type and material of dielectric layer, aluminum PCBs can be classified into three categories:
· Universal Aluminum PCB - Dielectric layer is composed by epoxy glass fiber prepreg.
· High Thermal-Conductive Aluminum PCB - Dielectric layer is composed by epoxy resin or other types of resin with high thermal conductive capability.
· High-Frequency and Microwave Aluminum PCB - Dielectric layer is composed by polyolefin resin or polyimide resin glass fiber prepreg.
Manufacturing Difficulties of Aluminum PCBs and Solutions
No matter single-layer, double-layer or multi-layer aluminum PCBs, or MCPCB, they share lots of similarities in terms of manufacturing process of FR4 PCBs. Nevertheless, aluminum PCBs, as a type of advanced PCBs, still feature special aspects of manufacturing process, calling for rigorous and effective management and control.

· Thick Copper Foil Etching

Aluminum PCBs are usually applied in power devices with high power density so copper foil is relatively thick. When copper foil is 3oz thick or more, copper foil etching requires trace width compensation. Otherwise, trace width will be out of tolerance after etching. Therefore, to guarantee optimal trace width/spacing and impedance control that are able to meet design demands, the following work has to be finished in advance.
a. Trace width compensation should be appropriately designed.
b. The effect of trace manufacturing on trace width/spacing should be eliminated.
c. Etching factors and agent parameters should be rigorously controlled.

· Solder Mask Printing

Solder mask printing is regarded as a manufacturing difficulty of aluminum PCB fabrication due to effect of thick copper foil. If trace copper thickness is extraordinary after image etching, a large difference will be generated between trace surface and base board and solder mask will become difficult. To ensure smooth solder mask printing, the following principles should be conformed to:
a. Solder mask oil should be picked up with high-quality performance.
b. Two-time solder mask printing is used.
c. When necessary, the manufacturing method of resin filling first and solder mask second is relied on.

· Mechanical Manufacturing

Mechanical manufacturing of aluminum PCBs contain mechanical drilling, milling and molding and v-scoring and burr tends to be left on internal via, which will reduce electric strength. Therefore, to ensure high-quality mechanical manufacturing, the following principles should be stuck to:
a. Electric milling and professional milling cutter should be picked up for low-volume production.
b. Control technique and pattern should be noticed in the process of die molding.
c. Drilling parameters should be appropriately adjusted on thick-copper aluminum PCBs to prevent burr from generating.