Aluminum PCBs feature excellent electrical performance, thermal dissipation capability, electromagnetic shielding, high dielectric strength and bending resistance, widely applied in numerous industries like high power LED lighting, power, television backlight source, automotives, computers, air-conditioner variable-frequency module, aerospace electronics, telecommunications, medical care, audio etc. When it even comes to cell phone camera that is the most commonly used, aluminum PCB is an essential part of cell phone. As one type of metal core PCBs (MCPCB), aluminum PCBs share quite a lot of similarities with FR4 PCBs in terms of manufacturing process or technique such as etching of thick copper foil, aluminum surface etching protection, aluminum board manufacturing and solder mask printing etc.
Developed in 1970s, Aluminum PCBs began to be prevalent since their first application in power amplification hybrid ICs (Integrated Circuits). Recent years have witnessed increasingly wide applications and development trend of Aluminum PCBs especially owing to the motivation of LED industry. Therefore, it's necessary to be aware of some important features of Aluminum PCBs so that they can be better leveraged in your products or industry.
Properties of Aluminum PCBs
• Aluminum PCB Structure
When it comes to aluminum PCB structure, it authentically indicates the structure of aluminum copper clad laminate (CCL) consisting of copper foil, dielectric layer, aluminum base and aluminum base membrane (selective). The structure of aluminum CCL is indicated as below.
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.
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.
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.