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What is Microvia PCB?
Micro vias are not easy to process. Especially vias with a diameter of less than 0.1 mm, the difficulty is very big. But a lot of electronic products have this kind of microvia structure design, like cellphones, wearable electronics, IoT electronics, oil pumps, nozzles, water jets, mold, etc. will use micro vias.
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The Main Types of Micro Vias in Microvia PCB
No microvia is needed on most PCBs. But for HDI PCBs (high-density interconnect PCBs), microvia is a must because of small and high-density wire connectors. The following are the main types of micro via in PCB:
Micro PTH via
Blind via and buried via
Buried via means that all the vias are buried in the top and bottom layers. We could not see any buried vias from the outside. We take 8-layer buried via PCB for example. Buried vias can be in layers 2-3, layers 2-4, layers 2-5, layers 2-6, 2-7, layers 3-4, layers 3-5, layers 3-6, layers 3-7, layers 4-5, layers 4-6, layers 4-7, layers 5-6, layers 5-7 and 6-7.
Buried via can be achieved by laser or by mechanical, and the PCB with buried vias is called buried via PCB.
Blind via PCB and buried via PCB are widely used in wearable products to achieve light and small packages, such as earphones, cellphones, portable cameras, and virtual reality electronic products.
Stacked and staggered vias
Staggered via also means the vias from 2 layers are connected after copper plating, but the 2 vias are not exactly at the same place. Staggered microvias are also very common in HDI PCBs, this kind of PCBs are called staggered via PCB.
Any layer via is the ultimate type of blind and buried vias. It was realized by stacked microvias. In some PCB designs, there are many types of blind and buried vias that are impossible to produce by the normal production process, so we have to use the any-layer drill. It means the drill will be done in every 2 layers. For an 8-layer any layer PCB, the process should be: drills on core material: layers 4-5 → laminate layers 3-6 and drill blind vias between layers 3-4 and 5-6 → laminate layers 2-7 and drill blind vias between layer 2-3 and 6-7 → laminate layer 1-8 and drill blind vias between layer 1-2 and 7-8. To produce good quality any-layer PCB, PCB manufacturers should have very advanced equipment and experienced staff.
What is PCB Assembly?
The 4 Types of Technology Used to Drill Microvias on PCBs
Now, the commonly used in micro vias PCBs are mechanical drilling, laser drilling, Plasma etching hole Chemical etching hole, etc.
Mechanical drilling is produced by high-speed machines. The most important part is the drilling bit. The drilling bit is normally made with tungsten cobalt alloys. This kind of alloy uses tungsten carbide powder as the matrix, uses cobalt as a binder, and then sintered under high temperature and high pressure so will have very high hardness and high wear resistance and can drill the holes needed successfully in the PCB.
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With the development of science and technology, electronic products become more and more miniaturized, so the size of PCB is decreasing. In this situation, the aperture of the microvia used for connecting and locating in the PCB is gradually decreased, which brings certain challenges for microvia drilling.
The reason why Laser drilling technology for microvia can be widely used in many fields is that its technology has higher peak power and faster processing speed. But in PCB production, microvia processing quality will have an important impact on the quality of PCB. So using laser drilling technology for the microvia process when producing the PCB, will get higher quality PCB. And it can provide convenience for PCB assembly and use. Therefore, we should strengthen the research on the application of laser microvia processing technology in printed circuit board production, so that can better promote the development of the PCB production industry.
Laser microvia process technology uses the laser to drill holes process technology to process microvia that a diameter of less than 50 um, this is a mature microvia process technology. For now, Laser microvia process technology almost can use in the process of various materials, the size of the microvia has direct relative to the energy density of the laser, types, wavelength, and PCB board thickness. Because different materials have different absorption coefficients for laser wavelength, so we should also use the specific wavelength of the laser to process specific PCB material.
See from the principle, laser microvia process technology mainly use photothermal burn etching and chemical burn etching to make microvia for PCB. The photothermal burn etching makes the material complete the absorption of high-energy laser in a very short time, then the material will be heated till the state of melt and evaporation, then achieve the purpose of microvia processing. Using this technology, it can make the PCB form holes under high energy, but the wall of the hole will remain blackened carbonization residue, so will need to finish the clean before the material is perforated.
And use chemical burn etching technology, actually using a laser wavelength of less than 400 nm to destroy the long molecular chain of organic materials, then make the molecules become tiny particles. And when the energy of the molecules is bigger than the original molecule, it will escape from the material. So under strong external force adsorption, the material will be removed quickly, and then become microvia. Using this technology, the surface of the material will not have a carbonization phenomenon, so just need a simple cleaning for the wall of the hole.
For now, there are mainly three kinds of laser microvia processing machines that have been applied in the production of PCB. They are Molecular laser, Carbon dioxide laser, and Ultraviolet Nd: YAG laser.
Using Molecular laser, you can use a 248 nm short UV laser to produce the PCB, and PCB can better complete the absorption of this kind of light, at the same time, the laser pulse generated by the device can reach a peak power of several megawatts as long as it lasts for 20ns. After homogenization, the beam can produce a flat top waveform, then it is projected through the formed mask, thus, the bottom of the PCB material is corroded. Using this equipment, one can get clean and smooth holes, but because the speed of corrosion is a little slow, so will need hundreds of laser pulses.
Carbon dioxide laser
Using a carbon dioxide laser, the wavelength can be 10.6 μm infrared wave output. Compare with the UV solid-state laser. Using this laser can obtain greater processing depth. In specific applications, it is necessary to complete the selection of the central part of the beam according to the metal aperture diaphragm, focus the beam on the lens with a focal length of 4 inches, and then irradiate the surface of the circuit board.
Ultraviolet Nd: YAG laser
Using Ultraviolet Nd: YAG laser, it can output 260-1060 nm laser. Compare with the Molecular laser, this laser pulse has lower energy, but the peak power obtained is the same. Because using this equipment can get Gaussian-shaped laser beam, so can obtain much small facula. Therefore, this equipment can finish small-size microvia processing.
Comparatively speaking, using molecular laser processing, can obtain a higher laser resolution ratio and can finish clean-hole processing at the same time. Now, this technology is mainly applied in Polyimide material processing, it can use a large light beam mask to complete the simultaneous processing of holes. Use Carbon dioxide laser processing, then you can complete the processing for material containing FR4, and get microvia with a smooth hole wall and uniform hole taper. But the disadvantage for use this equipment is that the via’s quality you get is not so good, and need to use standard cleaning technology to clean the finished via. Use YAG laser, then can get smooth and straight hole wall, also can finish the processing of the holes less than 50 um. Now, this type of equipment is mainly used in processing the Alumina PCB material.
During the production of PCB, because the UV solid-state laser can better dissolve copper material, also can better couple with glass, etc. polymers, and obtain smaller light spot size, therefore, it has been better applied. But in the actual production, use laser processing technology for microvia when producing the PCB, you can use three processing technology, they are laser punching holes, laser sleeve holes, and laser spiral type processing.
Laser punching holes technology
Laser punching holes technology, need to keep the laser beam remains stationary when required, then use a high repetition rate pulse to process the material. So the size of the holes is decided by the size of the light spot. Using this technology can process the small diameter via.
Laser sleeve holes technology
Laser sleeve holes technology, that is to make the focused laser spot walk around the via. With each turn, the depth of the hole will be deepened. Using this technology need to calculate the processing depth for each turn. But if this technology is used, it will leave the undissolved material in the middle of holes, so can’t use this technology for blind via PCB processing.
Laser spiral-type processing
Laser spiral type processing, when processing the big-size via, you can use laser spiral-type processing. Specifically speaking, it’s the technology that makes the focused laser beam move from the center of the hole outward. The move shape is spiral type, thus. The PCB material can corrode layer by layer. According to the size of holes, material characteristics, and the layer parameter, then you can determine the parameter of the pitch of the screw, several walking cycles, etc. Through NC programming, then you can finish the microvia processing of the PCB.
After analysis, you find the laser microvia processing technology can be better applied during PCB production. But in the actual production, still need to combine different technology equipment and technology characteristics to choose the suitable processing technology, to better complete the process for different PCB materials.
Via Plug Technology in Microvia PCB
Wearable and IOT electronics are developing rapidly nowadays, and these products need to be” small, light and thin”, so PCBs are also developing towards high density and high difficulty to meet the customer’s requirement. Under the premise that electronic products tend to be multifunctional and complex, the PCB trace and gap are getting smaller and smaller. The speed of signal transmission is relatively increased. Many people would be shocked at the miniaturization of end-product designs because high-density integration (HDI) technology is gaining ground. Compared with the routine PCB technology, HDI PCB has a smaller trace/gap (≤ 2/2 mil ), smaller via ( <0.15 mm ) and pad ( <0.4 mm ), and also higher pad density ( >20 pad / cm2 ).
And HDI PCB will use via in pad frequently, a lot of HDI PCB with BGA ( Ball Grid Array ), and the plug hole is required to be full for the via in pad, so the requirements for plug holes are becoming higher and higher. Not accepting any solder mask in holes, will cause the solder ball to hide in holes, if so, when PCB in wave soldering, tin penetrates the component surface from the through holes and causes a short circuit; Not accepting oil explosion, will cause Pseudo Soldering when doing the SMT, etc. So the customer will require a plug-hole when soldering components. The plug-via will have a few ways as below:
Solder mask plug via use in normal via of PCB, after plug the via, the surface is solder oil, will not conduct electricity. Generally, the requirements after the plug-hole are:
A, The plug must be full.
B, No redness or false copper exposure.
C, Not allowed to have too full a plug cause protuberance higher than the pad to be soldering nearby ( will affect the SMT mounting ).
The difference between a resin plug via and a solder mask plug via
Before the popularity of resin plug via technology, PCB manufacturers commonly use the easier process of solder mask plug via, but the green solder ink will shrink after solidification, will easy to blow air in the hole, unable to meet the customer’s requirements for high plumpness. Resin plug via technology uses resin to fill the blind vias before lamination, perfectly solving the malpractice that the green solder mask plug will bring, and balancing the contradiction between the thickness control of the dielectric layer that can be pressed in balance and the design of HDI embedded hole filling glue in the inner layer. Although resin plug via technology has relatively complex processes, and costs are higher, plumpness, plug quality, etc are more advantages than solder mask plug vias.
The advantages of resin plug via
The advantages of via plug in microvia PCB
Methods to Plug Microvia PCB Vias
Firstly we talk about via plug for micro vias PCBs after the surface finishes. The procedures are solder mask → surface finish → plug via →solidification. Produce the PCB according to the procedure of not plugging via, after HAL, use aluminum plate screen or hinder ink screen to finish all the vias should be a plug of customer’s requests. You can use photosensitive ink or thermosetting ink to plug via, in the condition of the wet film color being the same, it is best to use the same brand/type ink as the board solder mask to plug via to avoid different colors in one PCB. This technology procedure can make sure the via is plugged after the surface finish to avoid solder links in the PCB assembly process. But the problem is via the plug by solder mask ink is not flat which may cause bad soldering, especially in the BGA position. So a lot of customers do not accept this method to plug via.
Plug PTH vias and copper plate over
Pretreatment→ plug via ( must make sure the through hole is plugged very full, the plug ink can use thermosetting ink, its characteristic is high hardness)→ grinding Board → Imaging transfer → etching → solder mask.
Tips: Generally speaking, via plug by resin is better, since resin has a small shrinkage rate, and also good adhesion with the hole wall.
1) This method of plug-via can make sure the through holes after the plug is smooth..
2) here will not have quality issues like ink explosion, hole edge drop ink, etc. after the surface finishes.
The normal via-hole copper thickness is 20um, to meet this stand, we need to add thick copper at one time, so the requirements for whole board copper plating are very high.
The requirements for the performance of a plate grinder are also very high, need to make sure that the resin and other substances on the copper surface are removed completely, and that the copper surface is clean and not polluted after the grinding Board.
A lot of PCB factories do not have to add thick copper at one-time technology, and the performance of the grinder equipment can’t reach the requirements, so this technology is rarely used in PCB factories.
Plug PTH via hole and directly print the solder mask
Pretreatment → plug via ( stop for no more than 30 minutes after completing the plug hole) → print solder mask (use 36T silk screen print the solder mask directly)→ Pre-bake → exposure → develop → solidification.
1) Using this technology can ensure that the solder oil cover on the through hole is good.
2) Plug via are flat.
3) Consistent color for wet film color and the plug ink. ( must make sure the through hole does not have tin after HAL, and the holes do not hide in the tin ball)
1) Solder ink may be on the pad after solidification if it is not strictly controlled in production, if so, will cause poor solderability, the through hole edge drop ink after HAL.
2) It is difficult to control production with this process, the process engineer needs to use special procedures and parameters to make sure the high quality of plug holes.
Plug PTH via hole and be solidification first, then print solder mask
Pretreatment → plug via ( plug must be very full, it is better to protrude on both sides ) → Pre-bake → develop → Pre solidification → print solder mask.
1) This technology use plug-hole solidification which can make sure the through holes do not drop solder ink.
2) There will not have an oil explosion after HAL.
It’s difficult to completely solve the through-hole hide tin ball and tin on-hole problems after HAL, so a lot of customers do not accept that.
Plug via at the same time when printing the solder mask
Pretreatment → print solder mask and plug via (plug all the through via at the same time when finishing the solder mask printing)→ Pre bake→ exposure → develop → solidification.
This technology takes a short time, high utilization of equipment, and can make sure through holes do not drop solder ink, the tin will not be on the through holes after hot air leveling.
Because it use solder mask printing to plug holes, there still have a lot of air inside the holes, so in the solidification process, air expansion and breakthrough solder mask, cause cavity and not flat, so there still have a few tins on the through holes after HAL.
Viasion’s Microvia PCB – The High-Tech PCB For Your Business
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Frequently Asked Questions
Microvia PCB refers to the PCB where there are microvias. Micro via size is normally less than 0.15 mm. The microvia size in HDI PCB is normally 0.1 mm which done by laser drilling.
1) Microvia PCB with micro PTH vias
2) Microvia PCB with blind and buried vias
3) Microvia PCB with stacked and staggered vias
4) Microvia PCB with Any-layer vias
5) Microvia PCB with VIPPO
1) Consumer Electronics Industry
2) Communication equipment
3) Industrial automation
4) Medical equipment
5) Security devices
Microvia PCB helps save space and some weight of electronic products.
The working principle of microvia PCB is to use the board-base insulation material to isolate the conductive layer of the copper foil on the surface so that the current along the pre-designed route in the flow of various components to complete such functions as power, amplification, attenuation, modulation, demodulation, coding, etc.
In general, the basic process of PCB design is as follows: pre-preparation → designing the structure of PCB →the layout of PCB→ routing→ routing optimization and silkscreen → the DRC check and the structure check → outputting PDF file.
The fabrication process of microvia PCB is as follows:
Cutting → inner layer fabrication → press-fit → drilling → copper plating → outer layer fabrication → anti-solder mask → silkscreen → surface finish→ the processing of the shape→ testing→ inspection→ packaging.
Ensuring signal integrity and controlling the impedance better are challenges that will be encountered in manufacturing microvia PCB.
The following are Viasion’s surface finishes for microvia PCB.
1) Immersion Gold (ENIG)
2) Immersion Silver
3) Immersion Tin
5) Pb-Free HAL
7) Hard Gold Plating
8) Soft Gold
9) Selective Finishes
Stacked microvias are two or more layers of blind microvias overlap, and this kind of PCBs are called stacked vias PCB; staggered microvias are the two or more layers of blind microvias connecting with each other but without overlap, and this kind of circuit boards are called staggered vias PCB.
The amount of current that the microvia can withstand is not only related to the size of the via but also the thickness of the copper plating on the inside of the via. This is because the amount of current that can be withstood is related to the cross-sectional area of the conducting metal.
The microvia aspect ratio are different between PTH via and HDI microvia. The normal the best aspect ratio for PTH via is 15:1, and the aspect ratio for HDI microvia is around 1:1.
Microvia HDI PCBs ultimately lead to faster signal transmission as well as enhanced signal quality. Microvia HDI PCBs are widely used to reduce the weight and overall size of products, as well as to enhance the electrical performance of devices.
Actually, without being designed in the CAD software, microvia PCB can’t be manufactured.
1) Impedance control ultimately needs to be achieved through PCB design and combined with the use of EDA software to control the impedance of the routing under signal integrity requirements.
2) In the design of the PCB we must control the impedance of the routing, to avoid as far as possible the reflection of signals and other electromagnetic interference and signal integrity problems, to ensure the stability of the actual use of the PCB.
3) Adjust the width and spacing of the routing within the software according to the impedance value requirement.
4) Consider adjusting the press-fit structure.
1) Microvia PCB has a complex manufacturing process.
2) The manufacturing time of microvia PCB is relatively long.
3) The processing cost of microvia PCB is high.
1) DFM Check
2) 100% Electrical Test and AOI inspections
3) 100% Visual Defects Inspection(including warp & twist check)
4) Dimensional Inspection (finished hole sizes, PCB profiles, etc)
5) 100% AOI Dimensional Inspection if required, with +/-0.05mm tolerance
7) Solderability Test (245±5℃; 3-5 sec)
8) Thermal Test (288±5℃; 10 sec. inspect delamination, etc)
9) Tape Test (adhesion test)
10) Peel Test (conductor peel strength)
11) Impedance Test
12) Ionic Contamination Test
13) Inspect to IPC-A-600 Class 2 and IPC Class 3
The following factors are the cost considerations for microvia PCB.
1) Base material
2) The dimension of the inverter PCB
3) The number of layers of the inverter PCB
4) The number of microvias
5) Copper thickness
6) The thickness of the inverter PCB
7) Layup structure
8) The number of blind/buried vias
9) Surface finishes
10) Special technology or requirements
Are you still struggling with how to choose the right manufacturer of microvia PCBs? Viasion can be your best choice!
1) Viasion has a strong technical force, a high-quality technical management team, and advanced automatic production and testing equipment. That means we can provide a strong guarantee for the quality and the delivery of products.
2) Viasion has many years of experience in market operation and sales service as well as a perfect management system and has won a good reputation among customers.
3) The price of our products is advantageous and our raw material supply channel is guaranteed. To pursue the high quality of our products, we have passed the ISO9001 certification and all our products can meet the requirements of RoHS and REACH standards.
We will take every customer’s needs seriously, and we are committed to being a long-term supplier to our customers!
There is microvia in-pad some time, we always use via-in-pad plated over (VIPPO) to make sure good BGA soldering quality. Please refer to the details in this page: https://viasion.com/vippo-pcb/.