Laser Drilling Microvias and Stub Removal in PCBs
Anyone that has worked with a high power pulse laser knows the damage they can cause. In my optoelectronics days, destroying samples became routine as I ambitiously tried to coax a larger electrical response from my devices. But I was also able to use my trusty laser to fabricate interesting new devices by etching microscopic patterns in my samples.
Mechanical drilling has been the standard method for placing via holes in PCBs and is still a mainstay for most PCB applications. Mechanical punching is one other method for placing via holes thin, flexible PCB substrates. Via holes with diameters on the order of 0.002" can be placed on PCBs, but the process cannot be used on rigid PCBs lest they are damaged during the act.
Laser Drilling Your Microvias
Laser drilling has become more popular recently as it can overcome many of limitations placed on mechanical punching. Laser drilling can be used to form microvias in PCBs and is not limited by the same material constraints as mechanical punching. Lasers with emission wavelengths that span from deep infrared to ultraviolet can be used for laser drilling.
As newer devices require higher component densities, microvias are becoming more in PCB designs. Multilayer boards with smaller traces at high-density also require microvias to route your signals. Laser drilling of microvias is the preferred fabrication method due to their small size, and standard milling methods cannot accurately form vias with the high aspect ratios and density required in mobile, IoT, and wearable technologies.
The greatest benefit of microvias is their small footprint. Microvias are also important for routing signals through small traces in multilayer boards. Laser drilling allows the formation of microvias in a wide range of materials with profiles that meet IPC standards. This method will become more prominent as mobile and IoT devices become more complex and dominate the market.
Laser-drilled microvia hole
Choosing the Right Laser and Optics
The primary limiting factor that determines the best laser for drilling is reflection from the PCB, the microvia diameter, and its profile. If the substrate material is more reflective at the laser wavelength, less power is used to remove material from the board and machining time increases. This becomes important in high-volume manufacturing and can have a real impact on costs. If a smaller via is required, different focusing optics or a different laser should be used.
The chemical composition of your PCB substrate will need to be balanced with the microvia size you place in your layout. The microvia diameter you wish to place in your layout will be limited by the laser wavelength. The profile of your microvia will also be determined by the focusing system available to your manufacturer. These two parameters are interrelated, and one parameter cannot be modified without changing the other.
Laser cutting and drilling systems operate by focusing the laser down to a small point called the beam waist. In a perfect situation, where the focusing optics do not have any aberrations, the beam waist is proportional to the wavelength of the drilling laser. In reality, the smallest diameter will be larger than this. This still allows fabrication of vias less than a thousandth of an inch.
The focusing optics also determine the depth of focus. Focusing optics with a longer focal length result in a larger beam waist, but it also allows the microvia to have a more narrow profile. In contrast, optics with smaller focal length will result in a microvia with a wider area at the landing pad, but a smaller area at the capture pad.
If a microvia that penetrates multiple layers is required in your PCB design, you won’t be able to fabricate it by laser drilling through the stacked layers. Instead, blind microvias can be placed in each layer and stacked on top of each other. The buried or through-hole via is then formed naturally when the board layers are stacked.
Blind and buried vias in a multilayer PCB can cause signal integrity problems if the via stubs are left in between layers. The best way to remove leftover stubs is by back drilling. Larger leftover via stubs can be removed by mechanical back drilling, and a laser cutter is used to clean up the remaining material.
Microvias allow high connection density in PCBs
The most important signal degradation problem that can be solved by back drilling is known as deterministic jitter. The bit error rate in digital systems is strongly dependent on deterministic jitter, and any reduction in deterministic jitter with back drilling can result in a large improvement in the bit error rate.
In causing deterministic jitter, the leftover stub acts like an unterminated transmission line and signals can accumulate electronic noise at one of the resonant frequencies of the via. Smaller vias have higher low-order resonant frequencies, and these can create significant noise at higher frequencies. This is especially problematic in low-power/high-speed devices.
When leftover stubs are present in vias, mechanical back drilling may not be possible due to the small via diameter. A drill that is larger than the via diameter needs to be used in this process, and this leaves a large hole in the back side of the PCB. In this case, back drilling with a laser cutter may be the best option to remove the stub. Removing leftover stubs also reduces via-to-via crosstalk, increases bandwidth, and reduces EMI and radiation from the stub end.