Have questions about Isola PCB materials? @michael.gay and @michael.mcmaster will reply on December 13th!
Got it. Thanks Lucy
We are starting to include Bluetooth and Cellular modules in our products and the antenna connection between the module and the antenna connector runs on what is currently an FR-4 board. Is this going to be ok? Or do we need to make the whole board out of a better material for one signal?
Hi Michael, how would you recommend selecting materials for SI and high-speed applications? What are the material parameters to look for?
Could you provide an estimate of how prices may vary for various foil profiles?
When it comes to high-frequency materials, I’m curious about the foil profiles used by Isola. Could you provide a list of all foil profiles along with their roughness values in microns?
Can you provide information on the alternative materials Isola offers as substitutes for materials like Ventec, Megtron, EMC, and Nelco?
What is the correlation between peel strength and the foil profile in PCB materials?
Can you recommend the best materials for different frequency ranges, like up to 5Hz, up to 10 GHz, upto 20 GHz and up to 30 GHz?
What materials are suitable for high-voltage applications?
Hi all, a few questions:
- Is the vertical centering of the fiberglass weave an important consideration in the manufacture of prepreg?
- What kind of tolerances on the offset from vertical center have you measured?
- Are there signal integrity implications from large offsets from vertical center for the fiberglass weave (i.e. if the fiberglass weave is closer to the top or closer to the bottom of the prepreg), or is the effective Dk basically an average of material composition in a given cross-section?
Thank you!
Sounds like you are describing a mixed signal PCB or what is called a hybrid stack up using two different grades of material to allow for a specific Dk or lower loss in some layers and low cost for the balance of the PCB. It is common to mix FR4 materials with other material grades and we see this type of design on applications seeking to lower BOM cost yet attain higher performance. The FR4 carries the power and low speed signals and the top two or three layers on one side of the board is used as the RF portion of the stack up.
A cap lamination stack-up using a core for layer 1-2 is desired because the thickness will be better controlled compared to a prepreg layer due to use of some of the resin to fill features on layer 2 of the design and re-lamination thickness variation. Some designs require the same material as layer 1-2 be used between layer 2 and 3. If this is needed, many of Isola materials can be laminated in the same press cycle so that using a subassembly is not required.
An FR-4 material like 370HR can be used in a hybrid stack up with I-Speed, I-Tera MT40, Tachyon 100G and Astra MT77. The choice would be frequency and cost dependent. Your FAE will be able to guide you on the best choice of the best combination based on their fabrication experience and the UL recognition required for the hybrid stack up.
Great but difficult question. The foil offerings have grown significantly over the last five years. Our foil suppliers have worked on developing new types of foils for high-speed applications with smoother laminate side profiles. The initial offering for low profile copper foil was what is known in the industry as HVLP (VLP2 Isola naming) foil. We use the 2 to indicate the approximate surface profile. Smooth copper foils are not used for standard FR-4 products today, but it may be that extending the capability of 370HR, for example, using a lower profile foil might be something that we consider offering.
The cost of a given resin grade is going to determine the percentage of increase by going from a standard RTF type foil to an HVLP (VLP2) type foil. For example, I-Speed is offered with RTF and HVLP (VLP2, our standard for this grade), but the resin cost is significantly lower than the cost of our I-Tera MT40 and Tachyon 100G resin systems. Therefore, the increase in cost of using RTF versus HVLP(VLP2) copper foil is a bit difficult to nail down.
You also need to consider the PCB design and PCB processing costs which will effect the BOM cost adder for the higher performance copper foil. HDI structures with multiple relamination cycles increase the cost of the PCB. It is also important to note that the foil is provided by the fabricator and may not be a smooth foil. This will reduce the BOM cost impact of the foil used for the laminate as a portion of the overall BOM cost. So, the impact on PCB cost is PCB design dependent as well.
There are some new copper foils available today - HVLP3 (VLP1) and Advanced RTF grades of foil. The HVLP3 (VLP1) foil is generally the smoothest foil the industry offers in volume and performance is as good or better than Rolled Annealed foils that have been the standard for smooth foils for many years.
The laminator only control the roughness on the laminate side of the foil. The smooth side of a low profile copper is bonded to the laminate. When cost is a consideration, you need to know which bonding treatment is used and what the impact to signal integrity will be because the other three surfaces of the transmission line will be processed using a bond treatment. These treatments modify the metal surface and increase the foil roughness on those other three sides.
Advanced RTF foils are now available and are priced in the same range as an HVLP (VLP2) foil yet they provide improved SI performance. These foils are similar to the commonly available RTF foils, but have a smooth drum side treated surface that is bonded to the laminate.
Isola has been testing these foils for several years and we select the best performance to cost copper foil grades available. We have found that not all foils with the same data sheet Rz roughness perform the same.
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The voltage for automotive applications has been increasing with the ramp up of EV production and the super fact charging stations. The systems are now requiring the material to be capable of handling voltages of 1500 VDC and higher. We are seeing applications with voltages of up to 15K VDC now. These very high voltages require resin systems that are very robust that do not allow CAF to form through the resin. Proper spacing of biased features is also required depending on the voltage of the design.
IS550H is an example of a halogen free resin system that was specifically designed for the automotive industry to manage high temperature and high voltage. The material has been CAF tested to 1500 VDC and 3000 VDC and has passed. The material also has excellent thermal reliability and toughness. This allows for use of heavy copper coins/inserts for heat removal without fracturing.
Although the material does not have the very high Tg of Polyimides, the heat handling is excellent and has an attractive cost to performance ratio. The material is well suited for standard FR-4 PCB processing and does not require high temperature press temperatures like a polyimide materials. We have materials ready for pressing in our Chandler facility.
In the past, the foil profile was strongly correlated with the roughness of the material. However, with the introduction of smooth foils, the peel strength has been reduced and the peel strength is dependent on the chemical treatment as well as the roughness. Most HVLP (VLP2) copper foil products have a lower peel strength around 3.5 pli. This is a value we use at Isola to down select various copper foil grades. Any foil grade that cannot meet this number is not qualified until the supplier can make adjustments and meet this requirement.
Smooth is not always good when the treatment is not well matched with the resin system. Even if the roughness is within the expected range, the wrong treatment may not meet the peel strength expectation.
the interface between the bare laminate and prepreg after the copper has been etched off. Some of the treatments have been found to interfere with the bond of the bare, shiny laminate surface after inner layer etching. We work with the copper suppliers to ensure that the treatment can be removed, and we can bond to the laminate without a bond line failure.
Finally, we have seen a lot of different foil materials including a relatively new entry into the offering. Advanced RTF foil. Because it is the drum side treated foil, the laminate side surface is very smooth and yields a better electrical performance than the available HVLP (VLP2) copper foil. There are at least two suppliers that are supplying very good products that have been adopted for our high-speed digital product line.
The choice pf the material is going to be dependent on the type of design you are working on. To keep the response shorter, let us focus on the high-end materials performance. There are various levels of performance that have been used in the industry. Intel has ratings of categories of materials using PCIe design criteria for example - PCIe performance ranges. We also see performance tiers based on electrical testing at various frequencies. These tiers (I have seen up to 8) range for standard FR-4 materials to the high-end materials with specialty resin systems with very ow loss.
When developing materials, we look at the properties of the resin, glass and copper to create products that have the lowest cost to performance using a combination of these raw materials to tune the product to meet the requirement for these categories of materials.
At the top end, for high-speed digital applications, there are a groups of products available in the market that have similar performance properties, The most commonly used group today is PPE resin systems with different glass and copper offerings. The product groups would look like this.
PPE resin system, e-glass, HVLP(VLP2) 2 um copper foil
PPE Resin system, Low Dk glass, HVLP(VLP2) 2 um copper foil
PPE resin system, Low Dk Glass, HVLP3 (VLP1) 1 um copper foil
PPE blend resin system, e-glass, HVLP(VLP2) 2 um copper foil
PPE blend resin system, Low Dk glass, HVLP(VLP2) 2 um copper foil
PPE blend resin system, Ultra Low Dk glass, HVLP3(VLP1) 1 um copper foil
These materials would be candidates for PCIe 5 or 6 or Tier 6-8 applications.
This last two types of products being the current mainstream offering for AI applications on the ultra-high-speed layers of the design. This leads me to suggest the use of hybrid designs. Many of these materials can easily be matched with lower cost layers to reduce BOM cost. We see stack ups that have products like 370HR matched with Tachyon 100G for example. You would need to work with your FAE to determine the best solution and the BOM cost reduction potential versus manufacturing impact if any.
With that said, what is the frequency and loss budget for the design. Laminators have incorporated SI labs into their capabilities. We understand the loss performance of our materials and have SI experts that can assist with modelling parameters and simulations using the most common tools. If you need coper roughness parameters to model with or would like to see loss performance based on a certain structure, laminators like Isola can assist in determining the best choice without over specifying the material.
Isola has been studying copper foil for many years now and we have just added a new type of copper foil called Advanced RTF foil to several of our product lines. We try to find the best foil performance for each product type to give us the best cost to performance ratio.
The industry standard foil has been HTE for many year. Reverse Treated Foil, RTF, followed which offered better fabrication performance for finer line applications. That has been followed by a series of new foil type that are based on better foil technology that which is the very low profile foils used for high-speed digital and RF applications because of the very smooth laminate side treatment.
Below is a list of foils that we offer. These are material dependent. Some of the high-end copper foils we offer would not make sense to put on a standard FR-4 like 370HR because it would potentially double the cost.
The Rz roughness values shown are nominal and should not be used for design as the methods to determine them is not standardized within the industry at this time. a 2um value on one foil does not guarantee similar conductor loss performance. We have tested many foil claiming to have a 1 um roughness profile where we find conductor losses are significantly different.
HTE Grade 3 (6-10 um) industry standard “shiny foil” used commonly on FR-4 products. Example: 370HR
RTF(Reverse Treated) foil (4-6 um) is also a standard FR-4 copper foil but is also used for cost reasons on mid and low loss materials. Examples: 370HR FR408HR)
HVLP(VLP2) foil is commonly used on very low loss and ultra-low loss materials to reduce conductor losses. Examples: I-Speed, I-Tera MT40, Tachyon 100G
Advanced RTF foils are now offered. These foils are lower profile on the laminate side of the foil than standard RTF foils. RTF is around 4-5 um while the Advance RTF foils are 2-2.5 um
HVLP3(VLP1) (1 um) foils are becoming more mainstream for AI applications and the need to squeeze performance out of the resin system, but the cost can be significant depending on the number of layers it is used on.
There are some newer foils being called HVLP4 and HVLP5 that are in development. These foils have an even lower profile but are not yet ready for market. The smoothest foils use a chemical bond treatment to achieve acceptable bond strength. These foil present challenges to laminators to ensure the foil can be processed in the PCB shop and maintain reliability. Because these smooth foils leave a shiny laminate surface after etching the inner layer, we test the reliability of the foil to ensure that the fabricator and end user do not have downstream problems in fabricator, assembly or use.
The last thought is to keep in mind that if you invest in expensive copper foil, make sure to find out the roughness of the bond treatment used on inner layers. You may buy high end foil on your material, but if the oxide coating is fairly rough, you will lose all of the benefits of low profile copper on the laminate.
Hi Eleanor
Thanks for your inquiry. Isola, like Ventec, Megtron (Panasonic) , EMC and Nelco, makes a wide range of materials for manufacturing PCBs. Our offerings include high reliability FR-4 materials like 370HR; a full range of high speed digital materials from the mid- to extremely-low loss performance and non-PTFE based RF/MW products. We also offer specialty products for high temperature/high voltage halogen-free applications (IS550H) as well as polyimides (P95/P96) and no-flow prepregs (FR406N. P25N).
We would love to provide you with more information on our alternative materials, especially if you have specific materials and applications you’re interested in.
Regards
Mike McMaster
Stephen
Thanks for your question.
While having the fiberglass centered in the material is ideal, we don’t have specific requirements. The treating process does include an alignment process to keep the resin coating even on both sides of the glass. For laminate we want to have a minimum “butter” coat of resin on the top and bottom of the glass of 0.2 mils to ensure there is no weave texture or glass-to-copper contact. For prepregs this coating will usually be thicker.
With respect to signal integrity, glass-reinforced PCB materials have anisotropic properties due to the direction of the glass cloth. Ground coupling is typically in the Z-axis through the material (out-of-plane), while propagation and differential trace coupling is in the X/Y direction (in-plane). Characterizing the electrical properties in these two directions requires different test vehicles and test methods. Typically, the in-plane Dk/Df are higher than the out-of-plane though these differences are typically insignificant except for the most sensitive applications.
It’s also important to keep in mind that before curing, the resin in the prepreg melts and flows to fill in circuitry during the lamination process while the glass cloth can only deform slightly. This means that even if you started with prepreg with a perfectly centered fiberglass cloth, during lamination this will change depending on the thickness and density of the copper on the interfacing layers as the resin fills in around the circuits. Using multiple plies of prepreg can help mitigate this effect and for many critical high speed design, many customers intentionally use at least two plies of a thinner prepreg instead of a thicker single-ply for this reason.
Regards
Mike McMaster
Hi John
Thank you for your question. This is not necessarily a straightforward question to answer because it depends on many other factors than just the material.
These include:
- What is the performance budget for the system you are designing?
- PCB Design attributes such as conductor length, copper thickness and trace width? A 10 mil trace on 1 oz copper that is 3” long will work on a high loss materials while a 3 mil trace on 0.5 oz copper that is 10” long can require a lower loss material.
- Sensitivity of the product to market parameters such as cost versus time to market. Cost sensitive applications might be able to use a higher loss, less expensive material but take multiple design spins to optimize. For time to market critical applications, over specifying and using a lower loss, more expensive materials may be desired.
Taking all of these variables into account means we can see some higher loss materials such as FR408HR and 370HR being used in RF/MW applications up to 77GHz but with wide short traces and design optimization. But in the high-speed digital world, higher performance materials are generally required at much lower frequencies. At 4 to 10 GHz FR408HR (mid-loss) and I-Speed (low-loss) may be suitable but as you transition to 10 to 20 GHz you would need to use ultra-low loss materials like IS580G, I-tera MT40. Above that extremely low-loss materials such as Tachyon 100G and Terragreen 400G may be required.
Regards
Mike McMaster
Director High Speed Digital Products
Isola Group