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New PCB Technology for 5G Mobile Phones

Driven by the demand for 5G mobile phones, the global smartphone market is expected to grow by 3% this year, and the sales of 5G mobile phones are expected to surpass the sales of 4G mobile phones within a year.

Obviously, in the foreseeable future, 4G mobile phones will gradually come to an end, and 5G mobile phones will also gradually advance from the high-end market to the mid-range or even entry-level, becoming mainstream.

5G mobile phones involve the technological upgrade of the entire industry chain. What is the difference between 5G mobile phones and 4G mobile phones, and which technologies will play a key role in the popularization of 5G mobile phones? Let’s look at the three key technologies behind 5G mobile phones: motherboard, antenna and heat dissipation.

Table of Contents

The “base” of 5G mobile phones – SLP technology

When you disassemble a 4G mobile phone, you will find that the components accommodated on the PCB have almost reached the limit. SLP technology plays a key role in how 5G mobile phones can maintain the thickness and weight of the body on the basis of more components.

What are SLPs?

Substrate-like PCB
Substrate-like PCB

SLP is Substrate-like PCB. Through advanced welding technology, multilayer PCB can be connected, so as to change the motherboard from “2D to 3D” and make full use of the space of the fuselage. The multi-layer motherboard technology used in the iPhone X, XS series and 11 Pro series benefits from the SLP process.

Why use SLP?

According to the estimates of RF industry giant Skyworks, the RF front-end parts of 5G mobile phones will increase significantly, the number of filters will increase from 40 to 70, and other RF devices such as antennas, PAs, RF switches, and LNAs will almost double.

The SLP process not only makes better use of the three-dimensional space inside the fuselage, but also makes the components of the PCB board more compact, that is, to further reduce the line width and line spacing, so as to put more 5G components.

The minimum line width/line spacing of any Anylayer HDI used in the past is about 40μm, while the current SLP is already smaller than 30/30μm. It is understood that the SLP process used in the new iPhone this year will be further reduced to 25um/30um.

The space release brought by SLP technology is quite obvious. Taking the iPhone as an example, after the introduction of SLP technology in the iPhone X, the volume of the same number of components is reduced to 70% of the original, resulting in more battery space.

The “road” of 5G mobile phones – LCP substrate

As mentioned just now, an obvious difference between 5G mobile phones and 4G mobile phones is the substantial increase in radio frequency components. As the “highway” for signal transmission, mobile phone antennas are also regarded as the “first” element of 5G mobile phone upgrades.

What is an LCP substrate?

LCP uses liquid crystal polymer (Liquid Crystal Polyester), which is widely used in 5G mobile phone antenna substrates.

LCP substrate
LCP substrate

LCP has excellent electrical characteristics: in the entire radio frequency range up to 110GHz, it can almost maintain a constant dielectric constant with good consistency; the tangent loss is very small, only 0.002, and it only increases to 0.0045 even at 110GHz, which is very suitable for mm wave applications; thermal expansion characteristics are very small, can be used as an ideal high-frequency packaging material.

Why use LCP substrate?

Reduce communication loss
For example, the traditional PI flexible board antenna will generate about 2% electromagnetic loss for the 2.4G radio frequency signal, and the loss will increase as the frequency increases. The loss of the LCP base material is only 2‰-4‰, which is 10 times smaller than the former, which can effectively reduce the loss.

The LCP antenna can meet the low loss requirements of the 5G frequency band.

Antenna miniaturization
As the internal space of 5G mobile phones is further reduced, the number of antennas increases, and the miniaturization of antennas has become a trend. LCP has good physical properties, which can improve the miniaturization ability of the antenna. In terms of bending performance, the LCP antenna can fit the middle frame of the fuselage, and has no obvious rebound effect compared with the traditional FPC antenna.

At the same time, the thickness of the flexible circuit board using LCP material is only 0.2 mm, which will reduce the overall thickness and provide more clearance area for the antenna when used to integrate the antenna and coaxial cable.

With the rise of full-screen mobile phones, LCP-related industries started earlier, and many 4G mobile phones have used LCP antennas, which is why we see that the screen-to-body ratio of mobile phones is getting higher and higher. As 5G mobile phones have higher requirements for antennas, ID design also puts forward higher requirements for related industrial chains.

What is a vapor chamber (VC)?

The function and working principle of the vapor chamber is the same as that of the heat pipe. It operates in the cycle of evaporation and condensation of the working fluid closed in the plate-shaped cavity, so that it has the characteristics of rapid temperature uniformity, so that it has rapid heat conduction and function of heat dissipation. The composition of vapor chamber is similar to that of heat pipe, consisting of metal shell, capillary structure and actuating fluid.

Vapor Chamber (VC)
Vapor Chamber (VC)

For example, if the heat pipe is compared to a “line”, then the vapor chamber is the “surface”, and the vapor chamber can be regarded as a heat pipe with a huge area.

How smart phone PCB works All Components in mobile phone PCB

What are the advantages of vapor chambers?

Higher cooling efficiency
Due to the good thermal conductivity of artificial graphite sheets, the horizontal thermal conductivity can reach 1500 W/m‧k, while the thermal conductivity of heat pipes with better performance is as high as 5000–8000 W/m‧k, which is more than four times the former. As mentioned earlier, the vapor chamber can be equivalent to an “enlarged version” of the heat pipe, so it has a larger cavity space than the heat pipe and can accommodate more actuating fluid. The equivalent thermal conductivity of the vapor chamber is about 2–3 that of the heat pipe times, it is expected to reach more than 20000 W/m‧k.

Larger cooling area
In the past, on mobile phones that use heat pipes to dissipate heat, we can see that heat pipes are used on the SoC where heat dissipation is most needed. However, the 5G support, fast charging and other features of 5G mobile phones make the components that need heat dissipation not limited to SoC. There are also components such as external basebands, flash memory chips, and IC power chips that have high heat performance. The VC vapor chamber can cover more heat dissipation elements to cool down the overall temperature of 5G mobile phones.

The advantages of the vapor chamber make it widely favored by e-sports mobile phones and 5G mobile phones. At present, Huawei Mate 20 X, Samsung Note 10 and other models have been equipped with a vapor chamber.

At present, the average unit price of a vapor chamber is 2-3 US dollars, which is 5-10 times that of a heat pipe, and the unit price of an ultra-thin vapor chamber required by a 5G mobile phone is even higher.

Conclusion

The emergence of 5G mobile phones may drive a new wave of replacements in the smartphone market, thereby driving the industry chain to usher in a new round of growth. Whether these cutting-edge key technologies can help 5G mobile phones and bring about an inflection point in the smartphone market against the market, we might as well look forward to it.

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