Introduction of PCB aluminum substrate thermal conductivity

As the name implies, the thermal conductivity of PCB aluminum substrate is a kind of heat dissipation parameter of aluminum substrate. It is one of the three major standards for measuring the quality of aluminum substrate (thermal resistance value and withstand voltage are the other two properties). The thermal conductivity of the aluminum substrate can be obtained by testing the test equipment after the plates are laminated. At present, the high thermal conductivity is generally ceramics, copper, etc., but due to the consideration of cost, most of the aluminum substrates on the market are The thermal conductivity of the aluminum substrate is a parameter that everyone is concerned about. The higher the thermal conductivity, the better the performance. Aluminum substrate is a unique metal-based copper-clad aluminum substrate, which has good thermal conductivity, electrical insulation properties and machining performance.

The role of thermal conductivity of PCB aluminum substrate


I. Use of PCB aluminum substrate:


1. Audio equipment: input, output amplifier, balanced amplifier, audio amplifier, preamplifier, power amplifier, etc.


2. Power supply equipment: voltage regulator, regulator, DC-AC adapter, etc.


3. Communication electronic equipment: high-frequency amplifier `filter electrical appliances` reporting circuit.


4. Office automation equipment: motor driver, etc.


5. Computer: power supply unit, floppy disk drive, motherboard, etc.


6. Power module: converter `solid relay` rectifier bridge, etc.


7. Electronic control: cooling of relays, transistor bases, components in various circuits;


8. Switch, microwave: heat sink, semiconductor device insulation heat conduction, motor controller;


9. Industrial automobile: igniter, voltage regulator, automatic safety control system, light conversion system;


10. LED display (two modes: LED display; display using LED light source)


11. Lamps and lanterns: With the promotion and promotion of energy-saving lamps, various energy-saving and gorgeous LED lamps are welcomed by the market, and the aluminum substrates used in LED lamps have begun to be applied on a large scale.


Second, PCB aluminum substrate performance:


(1) Heat dissipation


At present, many double-sided and multi-layer boards have high density, high power, and difficult to radiate heat. Conventional printed circuit board substrates such as FR4 and CEM3 are poor thermal conductors, which are insulated between layers and cannot emit heat. The local heating of electronic equipment is not ruled out, resulting in high temperature failure of electronic components, and the aluminum substrate can solve this heat dissipation problem.


(2) Thermal expansion


Thermal expansion and contraction are the common nature of materials, and the thermal expansion coefficients of different materials are different. The aluminum-based printed board can effectively solve the problem of heat dissipation, thereby alleviating the problems of thermal expansion and contraction of different substances of the components on the printed board, and improving the durability and reliability of the whole machine and electronic equipment. Especially to solve the problem of SMT (surface mount technology) thermal expansion and contraction.


(3) dimensional stability


Aluminium-based printed boards are obviously more stable than printed boards made of insulating materials. The aluminum-based printed board and aluminum sandwich board are heated from 30 ° C to 140 ~ 150 ° C, and the size change is 2.5 ~ 3.0%.


(4) Other reasons


PCB aluminum substrate printed with shielding effect; replaces the brittle ceramic substrate; rest assured to use surface mounting technology; reduce the real effective area of the printed board; replaced components such as radiators, improve product heat resistance and physical properties; reduce production costs And labor.


III. Structure


(1) Metal substrate


A. Aluminum-based substrate, using LF, L4M, Ly12 aluminum, requires expansion strength of 30kgf / mm2, elongation of 5%. American Beggs aluminum base layer is divided into 1.0, 1.6, 2.0, 3.2mm 4 types, aluminum model is 6061T6 or 5052H34. Japanese Matsushita Electric Works, Sumitomo R-0710, R-0771, AL C-1401, AL C-1370 and other models are aluminum-based copper clad laminates, and the aluminum-based thickness is 1.0 ~ 3.2mm.


(2) Insulation layer


Insulation function, usually 50 ~ 200um. If it is too thick, it can play an insulating role and prevent short circuit with metal base, but it will affect the heat dissipation; if it is too thin, it can dissipate heat well, but it is easy to cause short circuit between the metal core and the component lead.


The insulation layer (or prepreg) is placed on the anodized and insulated aluminum plate, and the copper layers on the lamination surface are firmly bonded together.


24. Manufacturing difficulties:


Aluminum substrate production:


(1) Oxidation treatment of aluminum plate: strong degreasing and cleaning (sodium hydroxide) ----- dilute nitric acid neutralization ----- roughening (aluminum plate surface formed honeycomb) ----- oxidation (3UM)- ---- Acid-base neutralization ------ Sealed hole ------ Baking. Each process must ensure the quality or affect the adhesion of the aluminum substrate.


(2) The entire production process is not allowed to scratch the aluminum substrate, do not touch the aluminum substrate by hand, get wet and any other pollution, otherwise it will affect the adhesion of the aluminum substrate.


(3) The insulation layer of the aluminum substrate must be kept clean and dry. Small impurities affect its voltage resistance performance, and moisture can easily cause delamination.


(4) The protective film must be flat and free of gaps and air bubbles, otherwise the aluminum plate will be corroded, changed, and blackened by chemicals during the processing of the circuit board.


制作 Aluminum substrate circuit production:


(1) Machining: Aluminium substrate can be drilled, but no burrs are allowed inside the hole after drilling, which will affect the withstand voltage test. Milling shapes is very difficult. And punching the shape requires the use of advanced molds, mold making is very skillful, as one of the difficulties of aluminum substrates. After the shape is punched, the edges are required to be very neat, without any burrs, and not to damage the solder resist on the edge of the board. Usually, the military dies are used, the holes are punched from the circuit, the shape is punched from the aluminum surface, and the circuit board is forced to shear up and down when punching, and so on. After punching the shape, the warpage of the board should be less than 0.5%.


(2) The aluminum production surface must not be scratched during the entire production process: the aluminum production surface will be discolored and blackened by touching it with a hand or by certain chemicals. This is absolutely unacceptable. It also does not accept, so it is one of the difficulties to produce aluminum substrates without bumping or touching the aluminum substrate. Some companies use the passivation process, and some put protective films before and after the hot-air leveling (spraying tin) ... There are many small tricks. Eight immortals cross the sea and each shows its magic.


(3) Over-voltage test: Communication power supply aluminum substrate requires 100% high-voltage test. Some customers require DC power, and some require AC power. The voltage requires 1500V and 1600V for 5 seconds and 10 seconds. Dirt, holes on the board surface, burrs on the aluminum-based edges, wire sawtooth, and damage to any tiny insulation layer will cause fire, leakage, and breakdown of the high-voltage test. The pressure test board was delaminated and foamed, and all were rejected.


Classification of aluminum substrate


Aluminum-based copper clad laminates fall into three categories:


First is a general aluminum-based copper-clad board, the insulating layer is composed of epoxy glass cloth bonding sheet;


Twenty-two is a high-heat-dissipation aluminum-based copper-clad board, and the insulating layer is composed of a highly thermally conductive epoxy resin or other resin;


Thirty-three is an aluminum-based copper-clad plate for high-frequency circuits. The insulating layer is composed of a polyolefin resin or polyimide resin glass cloth adhesive sheet.


The biggest difference between aluminum-based copper-clad board and conventional FR-4 copper-clad board is heat dissipation. Compared with aluminum-based copper-clad board, FR-4 copper-clad board with a thickness of 1.5mm has a thermal resistance of 20-22 ℃ for the former and 1.0-2.0 for the latter. ℃, the latter is much smaller.


Technical requirements of aluminum substrate


So far, there are no international standards for aluminum-based copper clad laminates. In China, the 704 factory is responsible for drafting the military industry standard "Specifications for Flame Retardant Aluminum-based Copper-clad Laminates" for the electronics industry.


The main technical requirements are:


Dimensional requirements, including board surface size and deviation, thickness and deviation, perpendicularity and warpage; appearance, including requirements for cracks, scratches, burrs and delamination, aluminum oxide film;


Performance, including peel strength, surface resistivity, minimum breakdown voltage, dielectric constant, flammability and thermal resistance.


Special detection method for aluminum-based copper clad laminate


The first is the measurement method of dielectric constant and dielectric loss factor. It is a principle of variable Q value series resonance method, which connects the sample and the tuning capacitor in series to a high-frequency circuit, and measures the Q value of the series circuit;


Second is the thermal resistance measurement method, which is calculated by the ratio of the temperature difference between different temperature measurement points and the thermal conductivity.


Aluminum substrate assembly method


The use of aluminum substrate assembly method can eliminate the heat sink, assembly jigs, cooling fans and other hardware used in traditional assembly. This structure can achieve automatic assembly and significantly reduce assembly costs.