Do you know that a via needs to be filled? After it has been used to establish a connection between the circuit’s layers, you have to get them filled. In this article, we help you understand the two major ways to get this done.
What is a Via?
It is the copper plated through-hole primarily used for connecting multiple (two or more) layers in a Printed Circuit Board (PCB).
What is a Via Fill?
It simply refers to the filling or closure of the via. It is important to mention that the filling or covering process is important, as it improves the reliability of the connections.
Thus, you need to use any of the common via fill processes to close the via’s holes.
What to Use for Via Fill?
It is common to use epoxy for covering the via holes. The choice of epoxy for the process is borne out of the fact that the material supports the retention of the solder at the assembly level. This allows for the reduced risk of compromising the solder joints and to keep away the bath solution from the surface.
Via-in-Pad vs. Through-Hole Technology
Today, you can choose between the through-hole technology and via-in-pad for filling the via in a PCB.
It is best to stick to using the via-in-pad process as compared to the through-hole process. The benefits to using it include:
Routing Density Improvement
The routing density (when measured in terms of the higher density per layer), is improved.
Streamlined Signal Transfer Process
The via-in-pad process also streamlines the process of transferring the signals. It is evident in the streamlined method of filling, planarizing and plating the via over copper. This proves to be a better method when compared to the traditional process of transferring the signal from the Ball Grid Array (BGA), pass it through the via and then have it passed on to the inner layers.
Additional benefits of using the via filling process are:
- Reduction of the circuit board’s size, as well as the number of layers used therein.
- The filled vis process also does away with the high-speed design constraints, especially low inductances.
- It strengthens the pad’s attachments.
- Filled vias also reduction the EMIs.
- Increased thermal and electrical conductivity.
- Through the use of filled vias, the high-frequency designs will have shorter possible routes, for bypassing the capacitors.
What is Conductive Via Filling?
It is one of the via fill processes/options for circuit boards. It refers to the process of initiating a transfer of electrical signals from one part of the PCB to the other. At the same time, the via’s thermal transfer properties are improved, while enabling the electrical signal transfer.
Conductive Via Filling Materials
You can choose between copper and silver epoxy for this purpose. The major advantage to the two is the excellent thermal conducting property. For example, you can rely on these materials to do the following:
- The improved heat/thermal conductivity is accentuated by the metallic design of the via fill. This sees to the excellent transfer of heat from the Integrated Circuit (IC) to the other side of the PCB.
- Once the heat is dissipated to the PCB’s side, it is further moved away via the heatsink.
The major materials used here are the DuPont CB100 and the Tatsuto AE3030 epoxy fill. With the two making up the “silver coated copper particle” and filled with the epoxy matrix; they are ideal for the improved thermal and electrical conductivities, even in a cured state.
Here are some of the additional points to note about these materials:
- CTE Differences: there is a difference between the Tatsuto AE3030 and DuPont CB100, based on the Coefficient of Thermal Expansion (CTE). This difference is based primarily on the CB100’s longer usage and the fact that it comprises both a higher CTE and a larger particle size.
- The silver epoxy is used mostly and quite affordable, when compared to the superior heat conductivity capabilities of the copper conductive epoxy.
- The use of a conductive polymer layer deposition method significantly the flow of current between the internal layers and vias of the circuit board. This is derived via the deposition method’s placement on the PCB’s via-hole.
- It is also possible to save up on costs via the conductive polymer layer deposition method. This can be enhanced via the direct metallization of the via hole’s inner walls.
What is Non-Conductive Via Fill?
This process is not entirely an opposite of the conductive via fill option, because it is processed the same way as the latter.
However, the difference between the non-conductive via fill and the conductive via fill is based on the purpose. While the non-conductive is used for preventing the contamination of the via by solder and other contaminants; the conductive option allows for the effecting both signal and heat transfer.
Here are some of the additional benefits of the non-conductive via fill process:
- The plating of the vias also supports the conduction of electrical signals and heat.
- Non-conductive via fills also see to the creation of a structural support for the copper pad that covers the holes.
Non-Conductive Via Fill Materials
Both Peters PP2795 and San-Ei Kagaku PHP-900 epoxy are the common materials used for non-conductive via fills. The former has been enjoying the many years of usage, but now has a fierce competition in San-Ei, which has become a popular choice in recent years.
Conductive vs. Non-Conductive Via Fill Differences
The first difference between the two is that the conductive via fill facilitates the transfer of electrical signals and heat, while the non-conductive via fill primarily prevents contaminants from getting into the via.
Depending on the PCB designer and targeted application, using the non-conductive via fill option might be better. These reasons support the claim:
- Solder Bridging Prevention: the risks of solder bridging are greatly reduced due to the stoppage of the solder mask on the filled vias a few mils short of the pad. When used in the medium-density circuit boards, it helps to prevent solder bridging between the nearby pad and the via.
- The fill materials for the non-conductive vias help to fill the gap that contained air before.
CTE Value Matching
You also need to match the value of the Coefficient of Thermal Expansion (CTE), as it pertains to the via filling to be applied. This matching is important because of the reduced risks of stress-related fractures. These fractures are commonly caused by the contraction or expansion triggered by the same circumstances.
Here are some of the steps involved in the CTE value matching:
CTE & Surrounding Laminate
The first value matching should be between the surrounding laminate material and the Coefficient of Thermal Expansion (CTE).
This comes in handy as the circuit board “lives off the life” in an on/off and heating/cooling state of expansion and contraction of the padstack structures.
Between Conductive and Non-Conductive Epoxies
Should you choose the conductive epoxy or go for the non-conductive variant? Generally, the non-conductive variants are a better bet for the CTE’s value matching. This is because they provide a more reliable overall structure for matching the CTE to the laminate.
On the other hand, despite the widespread adoption of the non-conductive epoxy; the conductive variant can also be used.
Considering the popularity of the DuPont CB100 as the best via fill, it is evident why the conductive epoxies are still popular in some quarters.
The following reasons also support the necessity of using the conductive epoxies for CTE’s value matching to laminate:
- Conductive epoxies are ideal for use with the high pin count Ball Grid Arrays (BGAs), especially the ones that “run hot.”
- Conductive epoxies are a better option for the thermal vias. These vias benefit from the epoxies’ capability of fast-tracking thermal energy out and away from the originating source.
- Legacy or older electronic products also benefit from conductive epoxies. The usage is due to the long-standing popularity and improved lifecycle properties.
Away from Conductive and Non-Conductive Via Fills
It is common for the non-conductive and the conductive via fills to be used in covering the vias in a PCB. However, there is a third option, which is not fully harnessed. The name is Copper-Plated Shut Filled Vias.
It is a filled vias process commonly used when the aspect ratio gets to 10:1. You can also use it when the diameter is less than 8 mils.
Considerations for the Copper-Plated Shut Filled Vias
It is imperative to mention that the copper-plated shut filled vias can also be prone to a couple of challenges. The following are some of the things you need to know:
- If you are using either the Teflon or PTFE materials, rest assured that they would distort in the process.
- Certain materials are better not used for the design. For example, thin materials or subs measuring less than 20 mils may not be ideal. This is because of the difficulty in processing through planarization. Besides, the material might be subject to tears or distortion.
- The making of fine spaces or lines might not be possible. This is the case when wrap plating is required. It is a plating method that requires the termination of surfaces so they (the surfaces) could have a buildup of copper.
How to Manufacture the Filled Vias
Whether you are using conductive or non-conductive via fills, the processes often range from cleaning and preparation.
Here is a breakdown of the major steps that go into it:
1. Vias Drilling
You can choose between the laser beam and mechanical drilling methods for the vias’ creation. You can determine the best vias’ drilling method based on the number of circuit boards you want to produce, the costs and the hole depth.
Talking about the holes, make sure that the right annular ring size is created.
2. Clean the Vias
The drilling is followed by the cleaning process. The different cleaning options you can use are:
- Brush and Solvent: this combination helps to get rid of debris, while paving the way for a smoother and finer drilling process.
- You can also use a combination of chemical and abrasive mechanical processes for the cleaning.
Irrespective of the cleaning method used, one point to note is that the debris must be removed at all costs. Also, put the circuit board under close scrutiny to ensure that there is an even plating of the copper within the via’s holes.
3. Fill the Vias
Depending on what you want, you can choose to fill the vias with either the conductive or non-conductive via fill.
The via filling processes for a PCB include the filling of the holes with pressure and vacuum. This enables the excellent hole filling without giving a way for gap formation in the resin. The other process is to clean the panel’s surface. This process serves a dual purpose of getting rid of the excessive resin and paving the way for planarization.
4. Planarize the Filled Vias
The planarization of filled vias refers to the mechanical-brushing process of getting rid of resin. It also requires the smoothening and flattening of the copper’s surface. This way, a uniform surface is achieved, while getting rid of the excessive resin.
5. Over-Plating
This process comes after the planarization process is completed. Over-plating the PCB with copper is an ideal way of soldering the electronic components, accurately.
Before Filling the Vias
Note that despite the disparities between the conductive and non-conductive filled vias’ processes; they also have some similarities and differences, which we have discussed in this article.
Generally, the processes of filling the via holes are fraught with some bottlenecks, which include:
- Structure-related issues tend to come up.
- There is a risk of filling up the via holes during the soldering process. If this happens, it tends to render the entire process useless.
- Another challenge is choosing between the conductive and non-conductive via filling processes, based on the benefits they offer.
Final Thoughts
For the best results, always contact a professional PCB designer to help you navigate the hole drilling, soldering and filling process. Rayming Technology helps you make a perfect choice on which of the conductive vs non-conductive via fill PCB process is best for your circuits.
