October 19, 2020
Design of embedded capacity and embedded resistance of MEMS microphone
At present, MEMS PCB is generally concentrated in 2-4 layers, among which, high-end smart phones in the market all use 4-layer embedded capacitance or 4-layer embedded capacity and resistance PCB.Capacitors and resistors are passive components in MEMS. When the product becomes smaller and smaller, the surface space of the circuit board becomes tight.In a typical assembly, components that account for less than 3% of the total price can take up 40% of the space on the board!And things are getting worse.We designed the circuit board to support more functions, higher clock rates, and lower voltages, which required more power and higher currents.The noise budget is also reduced with lower voltages, and major improvements to the power distribution system are needed.All this requires more passive devices.This is why the rate of growth for passive devices is higher than for active devices.
The benefits of putting passive components inside the circuit board are not limited to the savings in surface space.Circuit board surface welding point will produce the amount of inductance.The insertion eliminates the welding point and therefore reduces the amount of inductance introduced, thus reducing the impedance of the power supply system.Thus, embedded resistors and capacitors save valuable board surface space, reduce board size and reduce its weight and thickness.Reliability is also improved by eliminating welds, the part of the circuit board most prone to failure.The insertion of passive devices will reduce the length of wires and allow for a more compact device layout, thus improving electrical performance.1. Embedded capacitance
1.1 buried capacitors have obvious advantages in electrical performance and reliability:
A. Improve power supply and signal integrity of high-speed digital circuits.The ac impedance between the power supply and the ground can be reduced to 10 milliohms using buried technology alone.20 times better than traditional PCB.
B. Reduce the jitter of eye map in high-speed data transmission.You can reduce eye jitter by 50%.
C. Reduce EMI interference.Can avoid or reduce the use of shielding cover, while improving EMC, reduce product volume, reduce product weight.
D. Improve the heat dissipation efficiency of PCB.3 times higher than conventional PCB.1.2 at present, buried capacitor technology is mainly applied in three ways:
Structure of capacitor:;Two pieces of metal are sandwiched with a dielectric layer, which is exactly the same structure as the substrate sandwiched with two pieces of copper foil of PCB. The capacitance generated by the thickness of PCB substrate and the residual copper area becomes the most convenient buried capacitor design.
Capacitance formula: C=S * Dk/T (C: capacitance value, S: effective area of two metals overlapping, Dk: dielectric constant of the dielectric layer, T: thickness of the dielectric layer)
(1) inner sheet technology: using double-sided copper foil of PCB to reduce the thickness of the substrate and increase the dielectric constant (DK) to form the required capacitors, mainly represented as:
A. bc-2000 (unit capacitance: 506pf/inch2), base material thickness: 0.05mm.
B. 3M c-ply (unit capacitance :10nf/inch2), base material thickness: 0.015mm.
(2) on the inner board of special PCB, the thick film of photosensitive burned, using the way of exposure and development to make a lot of capacitors used alone.Also known as: CFP: ceram-filled Photopolymer, (unit permittivity up to :20nf/inch2)
(3) bury the independent capacitor directly into the PCB.
2. Embedded resistance
Improve the accuracy of resistance value in high frequency and high speed applications.Although the resistance value accuracy of discrete resistors is usually 1%, parasitic inductance exists in the resistance package itself, as well as in the PCB through hole and the pad in the actual circuit interconnection.For example, when a 0402 encapsulated 50 ohm resistor is attached to a PCB, the associated parasitic inductance is typically 6nH.If the resistor operates at 1GHz, its parasitic reactance is up to 40 ohms.Well over 1% of the resistance itself.But the parasitic inductance of embedded resistance itself is very small, when interconnect with actual circuit, do not need welding pad and through hole, greatly reduce the parasitic inductance.Therefore, although the finished embedding resistance precision is only 10%, the actual precision is much higher than the discrete resistance in high frequency and high speed applications.At present, buried resistance technology is mainly applied in two ways:
Plane minus the diagnosis: (1) to buy back the resistance of the copper foil, pressure on the PCB directly, by the method of etching processing, the resistor is mainly used for design between 50 ~ 10000 ohm resistance range, the resistance tolerance can be controlled within + / - 15%, and is the most mature application technology at present, the most widely, the main representative materials have Ohmga, Trece resistance thickness is only about 0.2 um.
(2) planar addition method: the purchased resistive ink is printed directly on the surface of PCB, mainly used to replace the circuit board resistor. The resistance range is between 300~100kohm.
Because have buried capacitance resistance design, resulting in product production process is very complex, and design of MEMS is miniaturization, lead to the original production of microphone PCB manufacturers gradually exit, some PCB factory has strong technical background, also gradually into the buried buried resistance of mic competition, some of the most representative PCB factory is described below.
Horexs has a strong technical team. It has been involved in the research and development of buried capacity and buried resistance since 2009. It is a leading enterprise in China's PCB industry that applied the buried capacity and buried resistance technology to mass production, and then promoted the buried capacity and buried resistance technology to system products in the following years.In 2011, the company successively invested in IC board research and development. With strong technology and advanced equipment, the company quickly entered into the MEMS PCB market competition and quickly gained the favor of several companies. As a result, the company has occupied a piece of the MEMS PCB market.