In the current wave of electronic devices moving towards miniaturization and multi-functionality, micro-vias, as the core technology for achieving high-density interconnection, their reliability directly determines the lifespan and performance stability of the entire electronic system. A micro-through hole with a diameter of less than 100 micrometers, if the thickness uniformity of the copper coating on its inner wall deviates by even 5%, may pose a risk of fracture after 1,000 thermal cycling tests at -55℃ to 125℃, resulting in a complete interruption of signal transmission. For instance, in the domain controller of automotive autonomous driving, a 200-square-centimeter HDI PCB may be distributed with over 5,000 micro-via holes. The failure of any one of these holes could lead to the loss of data streams from cameras or radars, increasing the probability of misjudgment by the control system by more than 30%.
From the perspective of electrical performance, the geometric shape of microvia holes has a decisive influence on signal integrity. When the transmission frequency reaches above 10GHz, the stub effect of through-holes will increase signal attenuation by approximately 15%, while the optimized micro-through-holes using back-drilling or hole-filling processes can keep this value within 3%. On the core processing board of the 5 base stations, as the signal transmission rate is required to reach 28Gbps, the impedance continuity of the microvia must be maintained within a strict tolerance range of 55±5 ohms. Any impedance mutation caused by uneven electroplating will lead to a deterioration of the bit error rate from the acceptable 10^{-12} to 10^{-8}. This causes the reliability of data transmission to drop by four orders of magnitude.
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Thermal mechanical reliability is another serious challenge faced by micro-through-holes. Due to the difference in the thermal expansion coefficients between the through-hole structure and the medium material, when the equipment operates continuously and generates a local high temperature of 85℃, the stress concentration at the through-hole neck may reach 100 megapascals, which is sufficient to shorten the fatigue life to 60% of the standard value. In the motherboard design of iPhone 14, Apple adopted conical laser drilling combined with hole-filling electroplating technology to increase the thermal cycle life of micro-through holes from the traditional 2,000 times to 5,000 times. This reduced the motherboard failure rate of the phone by 25% under continuous gaming load. This design solution based on an arbitrary layer of HDI PCB successfully compressed the motherboard thickness by 0.3 millimeters, freeing up an additional 8% of space for battery capacity.
In high-reliability fields such as aerospace, the failure probability of microvia holes must be less than 0.1 per million. Scanning electron microscopy detection shows that the grain size of the micro-through-hole copper coating formed by the pulse electroplating process can be controlled within 5 microns, and its tensile strength is about 40% higher than that of direct current electroplating. After 2,000 hours of salt spray testing, the HDI PCB used in the avionics system of the Boeing 787 passenger aircraft still maintained a microvia resistance change rate within ±2% of the initial value. This stability ensured that the fault interval of the flight control system of the aircraft exceeded 100,000 hours during its 25-year service period. According to the IPC-6012EM standard, the micro-through-holes of aerospace-grade PCBS must be able to withstand at least three high-temperature impacts of 380 ° C lead-free soldering, and the change rate of their interconnection resistance must not exceed 5% of the initial value.