Copper foil: a negative electrolytic material, a thin continuous metal foil deposited on the base layer of the circuit board, which ACTS as the conductor of PCB. It is easy to adhere to the insulation layer, receive printing protection, corrosion after the formation of circuit patterns. Copper mirror test: a flux corrosion test using a vacuum deposited film on a glass plate.
Copper foil is made of copper plus a certain proportion of other metals. Generally, there are two types of copper foil: 90 foil and 88 foil, namely 90% copper content and 88% copper content, with a size of 16*16cm. Copper foil is the most versatile decorative material. Such as: hotels and hotels, temples, Buddha, signs, tile Mosaic, handicrafts, and so on.
Copper foil with low surface oxygen characteristics, can adhere to a variety of different substrate, such as metals, insulation materials, has a wide range of temperature use. Mainly used for electromagnetic shielding and antistatic, the conductive copper foil is placed on the substrate surface, combined with the metal substrate, has excellent conductivity, and provides electromagnetic shielding effect. Can be divided into: self-adhesive copper foil, copper foil, copper foil single conductor, etc.
Electronic grade copper foil thickness (more than 99.7% purity, 5 um um – 105) is one of the basic materials of the electronics industry fast development of electronic information industry, electronic grade copper foil usage is more and more big, the products are widely used in industrial use calculators, communications equipment, QA, lithium ion battery, civil TV, VCR, CD players, copy machine, telephone, air conditioning, automotive electronics, game consoles, etc.
The demand of electronic grade copper foil, especially high performance electronic grade copper foil, is increasing day by day. Relevant professional institutions predict that by 2015, China’s domestic demand for electronic copper foil will reach 300,000 tons, China will become the world’s largest PCB and copper foil manufacturing base, electronic copper foil, especially high-performance foil market is optimistic.
Application of copper foil roughness in high speed PCB
I. problems caused by skin effect
With the development of communication, cloud computing and cloud storage technology, as well as higher Ethernet and cloud server, PCB will further develop towards high speed/high frequency, and PCB signal transmission performance will also restrict the development of high speed transmission technology to some extent. In the 4G era, the signal transmission rate of PCB single channel has been increased from 10Gbps to 25Gbps, and it is expected to be further increased to over 50Gbps in the 5G era.
Signal high speed/high frequency signal transmission is more and more concentrated in the wire “surface” (called skin effect), when the frequency reaches 1GHz, its signal transmission thickness on the wire surface is only 2.1 microns, if the conductor surface roughness is 3-5 microns, signal transmission only in the roughness of the thickness range; When the signal transmission frequency is increased to 10GHz, the signal transmission thickness on the conductor surface is 0.7 micron, and the signal transmission is carried out in the roughness range. When the signal is transmitted in the roughness range, the standing wave and reflection of the transmitted signal will become more and more serious, leading to longer signal transmission path and increased loss.
Because of skin effect, if high-speed PCB continues to use conventional (STD) copper foil, the result is: with the increase of signal transmission frequency, skin effect caused by the signal “distortion” is more serious. Therefore, Low roughness copper foil is more and more widely used in current high-speed materials. For example, in Mid Loss materials and Low Loss materials, inversion (RTF) copper foil is adopted as the standard copper foil. Although Very Low Loss material is also standard with RTF copper foil, most of the customer’s designs are using ultra-low profile (HVLP) copper foil. For Ultra low loss materials, HVLP copper foil has become standard. The surface morphologies of STD, RTF and HVLP copper foil (0.5oz thick) were observed by scanning electron microscope and metallographic microscope. The surface roughness (Rz) of STD copper foil was about 5 mu, and the surface roughness was 3 mu. RTF copper foil rough surface and smooth surface about 3 micron; The roughness of HVLP copper foil is within 2 microns. According to the understanding of pate PCB small editor, copper foil suppliers are currently developing NP copper foil with surface roughness less than 1 micron. Due to the unsolved reliability problem, the actual product has not been used yet.
Second, skin effect caused by the problem
Transmission line losses in PCB mainly include dielectric losses and conductor losses. Corresponding to conventional FR4 materials, 1GHz is the dividing line between dielectric loss and conductor loss. When the loss is less than 1GHz, conductor loss is mainly accounted for. When the frequency exceeds 1GHz, the dielectric loss accounts for the majority.
However, corresponding to Very Low Loss materials, medium Loss is no longer the main Loss. Conductor Loss at 10GHz frequency accounts for about 60% of the overall Loss of transmission line. The losses calculated by strip line theory are basically consistent with the losses measured. The dielectric loss is much less than conductor loss, no matter it is microstrip or strip.
From Mid Loss to Ultra Low Loss materials, the proportion of conductor Loss gradually increases. When different types of copper foil are used for high speed materials, the measured losses are obviously different
Three, low roughness copper foil processing
Although the surface of HVLP copper foil is relatively smooth, the existing PCB production process will increase the surface roughness of copper foil, affecting the effect of HVLP copper foil. According to the inner line manufacturing process, the inner layer needs to go through dry film pre-treatment and Browning process. After these two processes, the surface roughness Rz of HVLP copper foil will increase from 1.5 microns to about 3 microns. To help solve the problem, there is also a corresponding low roughness on the market process, compared with the traditional brown liquid, the process not to micro HVLP copper foil surface erosion, but on the surface of copper foil after cleaning, heavy on a layer of tin, and surface was modified by siloxane, siloxane with PP timeliness, can play a bridge role, can to a certain extent, increase the adhesion strength of the copper foil and PP.
The surface roughness of HVLP copper foil was compared with that of traditional Browning process. The existing dry film pretreatment and brown chemical technology can increase the surface roughness of copper foil to some extent. After adopting low roughness process, the surface roughness of copper foil is basically the same as that of the original material copper foil.
The test results show that the signal loss can be reduced by 0.03-0.05db /Inch (12.5ghz) with the low roughness process. Objectively speaking, this improvement range is not significant for Very Low Loss materials. However, considering the improvement effect and cost input, the cost performance ratio of this process is not satisfactory, so this process has not been widely used by PCB manufacturers. In addition, HVLP copper, due to its roughness, can only achieve this effect. For future Ultra Low Loss materials, this improvement range may be more meaningful. In addition, when NP copper foil is officially launched for commercial use, this process is believed to play a better role.