Investigation on the new reliability issues of PCB in 5G millimeter wave application

Driven by the rapid development of a new set of technology applications such as mobile phones, AI (Artificial Intelligence), cloud computing, VR (Virtual Reality), IOT/IOE (Internet of things/Internet of Everything), intelligent driving and so on, faster data transmission will be demanded. 5G communication protocol has been proposed to accommodate the continuously increasing desire for data transmission. 5G networks aim to achieve extremely high speed and large data capacity for streaming while maintaining ultra-low latency (Hayashi et al., 2019; Lin et al., 2019; Guo et al., 2022). A more wide frequency band shall be provided to realize the target of a high data transmission rate for 5G derived from “Shannon Theory” which shows the transmission speed is limited by the frequency band. The last four generations of communication technologies have occupied a mass of frequency bands, and there is almost no continuous broadband frequency below 6 GHz. Thus, it is hard to improve spectrum utilization efficiency even with a better coding technology if the working frequency is Sub-6 GHz. However, a continuous large bandwidth is easy to be obtained in mm-wave frequency bands, which can meet the requirement of a high data transmission rate of 5G communication (Juneja et al., 2021). It can be predicted that in the next few years, there will be more and more technical breakthroughs with acceptable costs in mm-wave communication to meet future application requirements (Li, 2020).

It is generally believed that the control of signal integrity shall be more difficult at a higher working frequency (Seahee et al., 2018; Yokesh et al., 2022). To maintain good signal integrity characteristics when the transmission frequency increases from Sub-6 GHz to mm-wave range, not only the printed circuit board (PCB) material performance and design quality shall reach a better level, but the process capability shall be more complicated. Therefore, many unexpected changes and reliability issues occurred (Choi et al., 2019; Tanigawa et al., 2018).

In this study, key technical demands on PCB for 5G mm-wave application were demonstrated based on the feature analysis of 5G terminal products. Then, new reliability issues were discussed through a comparative analysis between requirements on electrical performance and reliability of PCB in theory combined with failure analysis on actual failure cases.

Great demands on high frequency, high integration and large power for modules used in 5G mm-wave application have promoted PCB technology moving toward big size, high density, low loss at high frequency or high speed, hybrid multilayer and rigid-flex, which means product realization of these PCBs will be more difficult (Lamminen et al., 2020; Chi et al., 2020). However, reliability requirements on PCB in 5G mm-wave applications have not been lowered. Usually, a service life of not less than ten years is desired for PCB used in communication systems (Xie and Lu, 2016). To achieve the expected reliability goal on PCB products using more complex technologies in 5G mm-wave application, it will demand more materials, designs or manufacturing techniques compared with conventional PCB products.

Unlike the uniform distribution of current over the conductor cross-section in direct-current circuits, the current shall be crowding around the conductor perimeter in high-frequency circuits due to the skin effect (Imtiaz Kamrul et al., 2023).

According to the formula for skin depth (Pozar, 2013):

where δ is skin depth, f is current frequency, μ is conductor permeability and σ is conductivity. This formula shows that the skin depth shall get smaller at a higher frequency. For copper, the permeability is 4π × 10⁻⁷H/m, and the conductivity is 0.01724 × 10⁻⁶Ω/m at 20°C. The skin depth can be calculated as follows:

where δ_cu is the skin depth (unit: millimeter [mm]) for copper; it can be induced that the skin depth in a copper conductor line is just 0.39 µm at a typical mm-wave frequency of 28 GHz, which suggests most of the current flow occurs in an extremely thin region near the surface of the copper conductor that is even smaller than the bud height of low profile copper, as shown in Figure 1. Therefore, the copper surface roughness will have a greater influence on PCB signal integrity at mm-wave frequency bands. It was reported that the roughness contributed a percentage as high as 25% to total signal loss on common copper foil by ATOTECH (Liang et al., 2006). Generally, the roughness must be smaller than the skin depth at the corresponding application frequency to avoid obvious impedance mismatching and signal loss, and a roughness less than 0.4 µm has been recommended even only at 10 GHz, which suggests that a more smooth surface will be needed at mm-wave frequency bands (Manessis et al., 2021).

In high-frequency circuits, D_k (dielectric constant) is a significant factor in impedance controlling, signal propagating speed and signal loss, and D_f(loss tangent) has a positive effect on the dielectric loss. To satisfy the greater demands on PCB signal speed and loss at the mm-wave frequency band, new resin systems with lower D_k and D_f (D_k ≤ 3.5 and D_f ≤ 0.005 @10 GHz) shall be developed. A tighter tolerance for D_k is also very important since the circuit performance is more sensitive to D_k fluctuations at higher frequency. Generally, a material D_k tolerance of ± 0.05 or better is desired, and having a stable D_k vs. frequency, humidity, temperature or any other environmental factor will be ideal.

Except for key requirements on dielectric properties of resin systems, other properties, including dimensional stability, moisture absorption, chemical resistance, thermal conductivity, decomposition temperature, glass transition temperature and thermal expansion in X/Y/Z, shall also be improved to meet the processability and reliability demands from PCBs in the mm-wave application (Imai et al., 2009; Kim et al., 2004; Hasch et al., 2011).

In addition, the transmission of transmission electron microscope (TEM) wave will be disturbed by resonance interference which is easily generated in microstrip circuits on a conventional thick laminate at mm-wave frequency band. The thickness of the lamination should be less than or equal to one-eighth of the transmission wavelength and maintain a uniform structure and good stability to reduce the generation of stray signals. This means that for mm-wave applications (≥ 30 GHz), the thickness of the PCB dielectric layer should be less than 125 μm (Kim et al., 2004).

Various woven fiberglass fabrics are reinforcement materials in laminates to offer good mechanical and fire-resistant properties at low cost. However, the dielectric property of woven fiberglass fabrics is significantly different from saturated resin. The weave patterns of fiberglass bundles will cause inhomogeneous distribution of dielectric properties. It can be expected a balanced status on differential lines will be hard to achieve since the actual dielectric under each conductor in different positions is not the same, which may cause a time skew and greatly affect the signal integrity (Hasch et al., 2011). To decrease the data transmission failure risk caused by fiber weave effect, a flattened weaving structure is recommended to replace the regular structure, as shown in Figure 2. And NE-Glass fiberglass fabric may need to be used in low-loss-laminates (Owzar et al., 1997).

Filler is an important material to improve the comprehensive performance of laminates. The proportion of filler in laminates for mm-wave application needs to be increased to enhance heat resistance, thermal conductivity or dimensional stability.

The shape, size and component of the filler also need to be modified to meet the requirements of high-performance laminates. One typical example is that a new type of “air-ball” filler with air filling in the filler cavity has been used extensively recently to reduce the dielectric constant of laminates, as shown in Figure 3.

As a key platform part for signal transmission between various components in modern high-frequency or high-speed systems, a good engineering design of PCB will be helpful for improving signal integrity. The main design improving measures are summed up as follows:

• reducing electromagnetic interference (EMI) induced by voltage fluctuations by using new components with lower supply voltage;

• cutting down the interference loops and shortening the length of signal lines by applying component embedding technologies. A typical cross-section view of an embedding PCB is shown in Figure 4;

• replacing conventional single microstrips with differential microstrips with higher tolerance to noise and better resistance to EMI; and

• increasing the wiring density by using stacked vias with lines as few as possible to enhance transmission speed and reduce EMI by the number of “lines.” A typical cross-section view of a stacked vias structure is shown in Figure 5.

To satisfy the main technical specifications of 16∼24 layers with 2∼3 stages high-density interconnect, array holes with 0.4-mm pitch and thickness of 2.4∼3.0 mm on harder laminates with better chemical resistance for PCBs in the mm-wave application, more precision controlling procedures are required on manufacturing processes, which are summed up as follows:

• degrading the failure risk of inner connection defect by setting a lower service time for drilling needles, turning down drilling speed and applying plasma desmear process;

• optimizing laminating and electroplating parameters to obtain better thickness uniformity of copper and dielectric, the accuracy of line width/pitch and layer-to-layer registration;

• mitigating the stub effect on signal integrity by applying an appropriate back-drilling technology; and

• selecting organic solderability preservatives or immersion silver as surface treatment when demanding a lower loss, while soldering reliability shall also be considered (Hsu et al., 2015; Wu et al., 2008).

A balance between various performance items seems to be a great challenge in the whole PCB industry. To meet the signal communication requirements of 5G mm-wave applications, the corresponding PCB needs to have better electrical performance. However, this situation will make normal reliability goals more difficult to achieve.

Usually, the design and manufacture of PCB are based on a given value of complex permittivity provided by laminate suppliers. Hence, a function failure may occur on a finished PCB product if a complex permittivity fluctuation exceeds the maximum acceptable tolerance. However, it was found that the complex permittivity exactly varies with frequency and temperature in a typical high-frequency laminate. A curve of fluctuation of dielectric constant and loss tangent versus temperature is shown in Figure 6; it is seen that the dielectric constant and loss tangent increase by approximate 1.9% and 170%, respectively, when the temperature changes from −50°C to 150°C. A curve of fluctuation dielectric constant and loss tangent versus frequency is shown in Figure 7; it is seen that the dielectric constant decreases by approximate 0.9% while the loss tangent increases by approximate 80% when the frequency changes from 2 GHz to 40 GHz. In addition, it can be deduced that the complex permittivity also will change with the increase of serving time since aging behavior is excepted to happen on polymer resin materials in various environments, including temperature, humidity, light, oxygen, chemical medium, mechanical stress, radiation, etc.

The adhesion ability between copper and dielectric materials is positively correlated to the copper surface roughness. To reach a better electrical performance, copper conductor lines with a lower profile are needed due to the skin effect; thus, a good adhesion strength shall be very hard to obtain simultaneously. If taking electrical performance as a priority, the peel strength tends to be decreased. It has been proved that the peel strength of high-speed laminates (0.43∼0.45 N/mm as received; 0.40∼0.41N/mm after solder float) was usually weaker than common epoxy laminates (1.28∼1.30 N/mm as received; 1.24∼1.25 N/mm after solder float) without specific dielectric performance requirements from statistical results of tests reports issued by the Reliability Research and Analysis Center in recent years, as shown in Table 1. A poor peel strength will bring potential reliability risks such as pad falling off, delamination or flaking of traces.

Conductive anodic filament (CAF) growth occurs wherever fiber/resin separations have been created, and voltage gradient is present between adjacent conductors (hole to hole, hole to track, track to hole or track to track; He et al., 2018; Ramachandran et al., 2013). The resin molecular structures shall be designed toward a lower polarity, or even nonpolarity to meet the severe electrical performance demands for mm-wave application. However, this developing trend will make the resin more difficult to wet glass fibers and therefore increase the likelihood of separation between resin and glass by induced thermal, mechanical or chemical stress from PCB/PCB assembly fabrication process. In addition, the increased additives and fillers to the resin system may create extra opportunities for CAF path formation along their interfaces with the resin.

The above deduction on increased CAF failure risk for mm-wave PCBs had been verified widely in actual evaluation projects for famous 5G communication companies: the CAF seemed to be the most difficult to pass in all of the test items. Analysis views of a typical CAF failure case on PCBs for 5G core routes are shown in Figures 8, 9 and 10. It is seen from Figure 8 that the initial decade dropping time of insulation resistance on all channels is even less than 100 hours at a specified 85°C with 85% relative humidity and 50V direct current voltage (50VDC) bias applied referring to IPC-TM-650 2.6.25A 2005 (the hole wall to hole wall distances of channel 2#, 4# and 5# were 0.3 mm, 0.4 mm and 0.5 mm in X alignment, respectively, and the hole wall to hole wall distances of channel 1#, 3# and 6# were 0.3 mm, 0.4 mm and 0.5 mm in Y alignment, respectively). Decade drops in resistance are always counted as CAF failures; thus, it can be concluded that CAF forms in a short period on each daisy-chain net of the CAF test board, which is far from meeting a minimum standard of 500 h. Through locating the failure plated through holes and choosing a typical failure location for microsectioning, it is indeed found an obvious copper-color filament between 0.4-mm spaced plated through holes, as shown in Figure 9. And scanning electron microscope and energy dispersive spectroscopy (SEM&EDS) views from Figure 10 identify that the main composition of precipitates surrounding resin–glass interface contains copper, which further confirms a solid copper filament has been formed.

Solder joint void is a kind of popular process defect in surface mounting technology (SMT) assembly. In low-frequency circuits, weakening of mechanical strength is the main reliability risk needing to be concerned if voids are formed in solder joints, and in mm-wave circuits, the interference on signal integrity will be caused by solder joint void as an additional reliability risk. From the high frequency structural simulator (HFSS) results at 30 GHz transmission frequency shown in Figure 11, it is seen that the void diameter and distribution position in ball grid array (BGA) solder joint both have a great influence on signal loss (S21) of the solder joint, and the loss increases with volume enlarging of the void is revealed as an overall trend, especially an increment of 0.1 dB shall be occurred with a radius of 140 μm for void in location ① (central position). Thus, the pass criteria of void proportion in solder joint for mm-wave application shall not only meet the demands of mechanical reliability but also meet signal integrity limits.

The temperature rising could be increased significantly to a level of 100∼130°C or even higher on PCB with a design of high density and high-frequency, since an increasing power density shall be generated by more components per unit area and more signal working cycles (Kondo et al., 2013). This huge temperature rising brings a greater thermal expansion to the substrate. Meanwhile, the hole diameter is becoming smaller and smaller, driven by the improvement of integrated density, and there will be a reduction in the tensile strength of microvias. When a decreasing tensile strength is subjected to an increasing expansion stress environment, the crack risk is rising up, and a failure may occur even without obvious material or manufacturing process defects. SEM&EDS and focus ion beam (FIB) images of a crack failure case on stacked microvias are shown in Figures 12 and 13. It is seen from Figure 12 that a crack gap with no resin residue is formed between via barrel and target land, which suggests the interface separation shall not be attributed to insufficient desmear or laser drilling energy. Meanwhile, a morphology looking more like a transcrystalline fracture without abnormal columnar crystal features is found on the cross-section of via plating from FIB images shown in Figure 13. Electroplating faults can also be excluded from the possible reasons causing failure. Actually, all crack microvias are located under a big size CPU chip that can generate more heat than other components. Therefore, the most possible reason for a crack is an exceeding thermal expansion stress.

These years, the key BGA chips on assemblies of premium servers or routers have been moving toward high-density integration and big size (≥ 50mm × 50mm). In this kind of BGA, the warpage is getting larger under the same reflow condition, while the pitch between the solder balls is getting smaller (Oliver et al., 2022). Thus a good soldering quality is more difficult to realize. Microscope and SEM&EDS images of an abnormal soldering case at the biggest BGA that is also CPU location on a high-speed PCB for a server are shown in Figure 14 to Figure 17. It is seen from Figure 15 that the typical shape of the soldering joint at a corner is pillar, while at a central location is elliptical round that looks more like a normal collapse, and the soldering joint height value at corner location (355.3 μm) is about 0.08 mm higher than that (276.1 μm) at a central location, which indicates that a worse warpage at the corners has been induced during the reflow process. This obvious warpage even can pull the BGA ball from touching solder paste on PCB pad surface, and it is indeed found in Figure 16 that no obvious intermetallic compound has been formed, and a continuous Au layer with a thickness of 78 nm is still left on the pad surface under an abnormal soldering joint. After separating the failure CPU from PCB mechanically, SEM&EDS was performed on a nonwetting pad surface and corresponding BGA ball (Figure 17). No obvious abnormal element was found on the pad surface, and the black substances probably are flux residues deduced from element characteristic of high-content of carbon (70.2 wt.%) as well as oxygen (10.3 wt.%) coexisting with tin (16.6 wt.%). In addition, a free cooling appearance without a flattened morphology is observed on the BGA ball (Figure 18), which further confirms that the earlier melted BGA ball has pulled the solder paste from the wetting PCB pad surface.

The use of high mm-wave frequency bands opens new possibilities to realize a faster data link while also bringing new challenges to the material performance as well as PCB design and process capability.

To meet PCB signal integrity demands in mm-wave frequency bands, copper roughness must be controlled to a value less than the skin depth at the corresponding transmission frequency (Ref. 0.39 μm @ 28 GHz); high-performance laminates composed of resin systems with excellent dielectric properties and flattened weaving structure fabrics as well as modified fillers shall also be developed. Meanwhile, a design scheme adopting new concepts such as low-voltage circuits for decreasing EMI, component embedding, differential microstrip and stacked vias structure will be more suitable. Additionally, it is necessary to put more precise controlling measures on manufacturing processes, including stub, drilling, desmear, laminating, electroplating and surface treatment.

However, it is difficult to achieve nice signal integrity and reliability simultaneously. Based on theory analysis and investigation of actual failure cases, a series of new reliability issues, including complex permittivity fluctuation as frequency or temperature changes, reduction of peel strength by low profile copper foil, insulation degradation by CAF growth, signal integrity interference by solder joint void, crack failure on microvias and abnormal soldering connection at BGA location were presented in this paper, which will be helpful to identify potential reliability risks and put forward improvement measures for PCB in the mm-wave application.

This work was supported by GuangDong Basic and Applied Basic Research Foundation (2022A1515110385).