Recently, due to the shortage of LED heat dissipation substrates and chip resistors, capacitors, and inductors, a long-standing material “ceramic substrate” has been heatedly discussed in public.
When it comes to ceramic, we usually remind of traditional ceramics such as tiles or ceramic utensils. And what we discuss in this passage is “fine ceramics”. It is a kind of refined high-purity inorganic material as raw material, the composition and uniformity are precisely controlled by chemical or physical methods and then formed by dry pressing, slurry casting, or injection molding. After a sintering step, it is processed into a finished product.
With the strength of wear resistance and pressure resistance and other performance, fine ceramics can be divided into electronic ceramics, structural ceramics and biomedical ceramics. And ceramic PCB is one of the types of electronic ceramics. In this passage, we will discuss the ceramic PCB – what is it, why we should use and how to manufacture.
What is ceramic PCB?
Ceramic PCB, also called ceramic circuit boards, is a special type of PCB that copper foils are combined with Al203(alumina) or AIN (aluminum nitride) ceramic substrates under high temperatures.
Through several procedures, the ceramic substrate is possessed with good electrical insulation and high adhesion and large current-carrying capabilities, which allows to etch different patterns on the board like PCB always does.
What are the types of ceramic PCBs?
1.Ceramic substrates has different types based on different materials:
- Alumina substrate : enables to bear temperature 1,550 – 1,600℃, mainly adopts thick film method and co-firing method, substrate for hybrid integrated circuit, substrate for LSI packaging, multilayer circuit substrate;
- Aluminum nitride substrate : The manufacturing cost is 15 times that of alumina substrate, used for VHF (Ultra High Frequency) band power amplifier modules, high-power devices and laser diode substrates, etc.;
- Silicon carbide substrate : The firing temperature is above 2000℃, and it is usually fired by vacuum hot pressing method. It is mostly used for low-voltage circuits and VLSI high heat dissipation packaging substrates, such as high-speed, highly integrated logic LSI with heat dissipation mechanism packaging.
- Beryllium oxide substrate (BeO) : Its thermal conductivity is more than ten times that of Al2O3, which is suitable for high-power circuits. Its dielectric constant is low, which can be used for high-frequency circuits. The BeO substrate is basically produced by dry pressing.
Generally speaking, the materials currently used in LED ceramic substrates are mainly alumina ceramic substrates, and the global alumina ceramic substrate industry is an oligopoly market.
2. Ceramic substrates can be divided into thick-film ceramic substrates, thin-film ceramic substrates, and low-temperature co-fired multilayer ceramics (LTCC) based on different circuit manufacturing methods:
- Thick-film ceramic substrates adopt screen printing technology. Materials are printed on the substrate by a doctor blade, and then dried, sintered, and lasered. Sometimes, because of the screen stretching, the circuit made by screen printing probably brings rough circuit and inaccurate alignment.
Therefore, for high-power LED products that require smaller and smaller dimensions and finer lines in the future, or LED products that require accurate alignment in eutectic or flip-chip processes, the accuracy of thick-film ceramic substrates has gradually become insufficient.
- Thin-film ceramic substrates, in order to improve the problem of stretching in the thick film process and the shrinkage ratio after multi-layer lamination and sintering, thin-film ceramic substrates have recently been developed as heat dissipation substrates for LED dies.
The thin-film heat dissipation substrate is made by sputtering, electro/electrochemical deposition, and yellow light lithography process, which has:
(1) a low-temperature process (below 300°C), avoiding the possibility of high-temperature material damage or dimensional variation;
(2) The yellow light lithography process is used to make the circuit on the substrate more accurate;
(3) the metal circuit is not easy to fall off, etc., so the thin-film ceramic substrate is suitable for high-power, small-size, high-brightness LEDs, as well as common LEDs that require high alignment accuracy. Chip/flip chip packaging process.
- Low-temperature co-fired multilayer ceramic
Take the ceramics as the substrate material, the circuit is printed on the substrate through screen printing. Then it combines with multi-layer ceramic substrate and sinters under low temperatures.
This type of ceramic PCB should consider alignment errors caused by the screen stretching issue.
What materials are commonly adopted in ceramic PCB?
- Alumina (Al2O3)
With distinct strengths on thermal and electrical properties compared with other oxide ceramics, Alumina becomes the most commonly used material. It has high and stable chemical performance that is suitable to etch different shapes on the board. Different alumina content hardly influences its electrical properties, but greatly affects mechanical properties and thermal conductivity.
- Beryllium Oxide (BeO)
It has higher thermal conductivity that usually applies on high thermal conductivity requirements. Its temperature is getting lower when it exceeds 300℃.
- Aluminum Nitride (AlN)
Different from alumnia ceramic substrates, it has higher insulation resistance and withstands voltage and lower dielectric constant. Impacted by the residual oxygen impurities, the oxygen content can be reduced and improve the thermal conductivity of AIN.
To conclude, alumina is the best choice for ceramic PCB material, which is widely used in power electronics and hybrid microelectronics and so on.
What are the strengths and shortcomings of ceramic PCB?
① Large carrying capacity;
② Better heat dissipation performance;
③ Low thermal expansion coefficient;
④ Keep the shape stable without deforming and warping.
⑤ Good insulation, high withstand voltage
① Fragile, which means it won’t be covered too much space on the circuit board.
② Expensive on manufacturing due to its materials and technology.
What is the difference between ceramic PCB with FR4 PCB?
|Features/Types||Ceramic PCB||FR4 PCB|
|Materials||It adopts ceramic substrates such as alumina ceramic substrates, aluminum nitride ceramic substrates, and silicon nitride ceramic substrates.||FR4 PCB is usually made of fiberglass.|
|Thermal conductivity and Insulation performance||Made of ceramic material, it has good heat dissipation and insulation.|
Poor heat dissipation performance.
It does not have insulating properties. But it can add a layer of the insulating layer to dissipate heat.
|High-frequency characteristics||Ceramic PCB has better high-frequency characteristics.||FR4 PCB does not have high-frequency performance.|
|Application||With comprehensive performance, ceramic PCBs are widely used in high thermal conductivity, high current, high insulation, high frequency and other fields||FR4 PCB is more widely used in different fields like electrical system relays, electrical switch and washer.|
|Price||Ceramic PCB manufacturing cost is high.||Low price compared with ceramic PCB.|
Where can ceramic PCB be used?
- LED lighting commodities with high power;
- Electronic commodities of automotive and aerospace, military field;
- Switching power supply with high frequency;
- Solar panels, electronic modules with high power;
- Communication antenna
How the ceramic PCB is made?
The production process of ceramic circuit boards requires different processes. Common ceramic PCB production processes include thin film circuit technology and thick film circuit technology (LTCC process, HTCC process, DBC process). Two different circuit board production processes are described below.
① Thin film ceramic PCB
Though some procedures and technologies include magnetron sputtering, dry etching and wet etching and electroplating thickening process, an ultra-fine circuit pattern is made on the ceramic substrate.
Specific production process: alumina/aluminum nitride ceramic substrate grinding — magnetron sputtering — electroplating (gold, silver, copper, nickel, etc.) — etching pattern transfer — surface treatment.
②Thick film ceramic PCB
Its manufacturing process includes the LTCC process, HTCC process and DBC process.
③ Ceramic PCB
Drill connecting pipes between metal layers through mechanical drilling machine.
2.Plated through holes
Even though the copper circuits, which are connecting the layer, are drilled, the circuit between different layers fail to be connected. Thus, a conductive layer should be made for connecting the layers. This process also called “PHP” process mainly involves besmearing, electroless copper and electroplating copper.
3.Dry film lamination
To form an etching resistance layer with photosensitive.
4.Image transfer of the inner layer circuit
It transfers the negative film to the surface through exposure.
5.Outer layer circuit exposure
The ceramic PCB board would be exposed and developed again after the attachment of the photosensitive film.
The photosensitive film used in this step is to cover those areas where it does not require electroplating.
Through the positive ions generated by the gas discharging process, it can move the material from the original materials to the substrate for the film deposition.
A technology to cope with the chemical reaction or physical impact. It mainly uses specific patterns to selectively remove some impacts.
When the ceramic PCB plating is finished, next is the process of stripping, etching and tin stripping. The plating resist should be fully stripped and then the copper that needs to be etched should be exposed on the etching solution. Because the top part of the ceramic PCB is protected by tin, the alkaline etching solution should be used to corrode copper.
8.Solder resist coating
Ceramic PCB is to carry electronic components for connecting circuits. Thus, after we finish the circuit board, the electronic assembly parts on the board should be marked, while those non-assembly parts should be made some protection measure through using polymer materials. Usually, the solder is used for electronic assembly.
Challenge in ceramic PCB manufacturing and solutions
The use of ceramic substrates should pay attention to the following aspects:
① Slice size of ceramic circuit board:
After the alumina ceramic substrate is processed, it needs to be separated into independent small units. Since the depth of the substrate basically does not exceed 50% of the thickness of the substrate when scribing, when scribing, the bottom of the scribing line is used as the cracking point.
Due to the slight difference in the melting of each point during laser scribing, there will be a slight error between the direction of the crack and the point of view of the substrate, so there will be a slight gap between the size of each unit after separation and the theoretical scribing distance, generally 0.1~0.15 mm range.
Heating is generally required for wire bonding, and the alumina ceramic substrate has been cut by laser marking, and there are defects in the alumina ceramic substrate. Therefore, marking and cutting on the alumina ceramic substrate becomes a thin defect when subjected to thermal shock. When the thermal stress is greater than the strength of the substrate’s thin defect, the substrate can be damaged.
Countermeasures: During the welding process of the alumina ceramic substrate, it is necessary to preheat the substrate to increase the temperature from room temperature to the welding process, so as to prevent large thermal stress caused by excessive temperature difference.
The heating conditions of the ceramic substrate temperature are generally confirmed according to the actual temperature of the welding wire, environmental welfare and welding process conditions, and the corresponding process parameters are confirmed by measuring the appearance temperature of the substrate at different stages.
Ceramic PCBs provide a better choice for those PCBs that need high thermal conductivity and good insulation performance, which make it widely applies in communication, defense, medical and electronic industry.
IBE, set up since 2005, is a professional ceramic PCB, flexible PCB, multi-layer PCB and other types of circuit boards suppliers. We provide one-stop PCB manufacturing and PCBA (PCB assembly) fabrication services. To learn more information, please feel free to contact us.