PCBs are called “laminated sandwiches” simply because they comprise multiple layers bonded together with high temperatures and pressure. It is an accurate definition of “lamination” and also a vital step of PCB fabrication.
If you want to advance in your knowledge of printed circuit board fabrication – manage this post to the end. Let’s deepen in PCB lamination techniques, review a conventional process, discover lamination materials and factors of their choosing, and learn how to calculate laminate thickness.
TYPES OF PCB LAMINATION TECHNIQUES
Let’s start by reviewing each of the four existing lamination types in detail.
Multiple-Layer PCB lamination
PCBs that comprise more than one layer of prepreg, i.e., PCB epoxy reinforced substrate, are called multi or multiple-layer ones. Regardless of if these prepregs are etched or trace boards, they are to be bonded with lamination.
In this case, layers are arranged in proper order and heated to 375°F and a pressure of 375-400 psi. This way, PCB can be laminated with a dry film photoresist/photographic film. The pressure lowers gradually as the PCB slowly cools down.
Double-Sided PCB lamination
A double-sided PCB is no different from a conventional single-sided PCB. But, it has traces on both the top and bottom layers. The lamination process here is similar to the multiple-layer PCB one. Pressure can be slightly lowered since there is no need to bond together many internal layers, but just the prepreg, copper foil, and a dry film photoresist.
Teflon (PTFE) Microwave Lamination
This type of lamination is intended for PCBs that suit high radio frequency and broadcast purposes. They have a low electrical loss and tight thickness tolerance.
What is different in this technique compared to others is that it requires CTFE chlorotrifluoroethylene (CTFE) thermoplastic film instead of a conventional photo resistant one. Lamination conditions are basically the same with a consideration of a number of PCB layers being laminated together. Recommended settings are 400°F and pressure of 100 to 200 psi.
In case you have a PCB with two or more subsets – rigid-flex circuits, you will definitely need to use sequential lamination. Given that subsets are created in a separate process, and they have insulation material in-between them, they can be laminated in a conventional way.
You may face a situation where your sub-composites do not have insulation – these components have not been laminated yet. This way, you will need to laminate them with dry film photoresist and copper layers. Only then can they be laminated with each other once again.
Sequential lamination makes more complex PCB design possible. For example, etching routes onto internal layers of copper foil can be created or drilling buried vias integrated.
STEP-BY-STEP PCB LAMINATION PROCESS
Here is a complete procedure for PCB fabrication with a particular emphasis on the lamination phase:
- Preparation. It includes washing panels/rinsing with purified water. Removing corroded copper surfaces and fingerprints as well as dry film, carbonate, and anti-foam residues.
- Micro-etching. It is the process of oxidizing a previously plated copper foil with acid. It makes the copper layer homogeneous and leaves pads and routes for electrical signals unexposed.
- Black oxide treatment. It is simply a black oxide bath in which PCBs are treated to ensure better adhesion for epoxy resin. As a bonus, it prevents delamination.
- Inner-layer stack-up. At this stage, inner layers and prepregs are arranged in the proper order by an operator and bonded together on the glue machine. Stack-up also involves a riveting process that strengthens inner later and prepregs together.
- Stack up. Then, copper foil is placed between a stainless-steel patch and a prepared prepreg. This way, stack-up is reliably fixated and has all the inner layers and copper plating included.
- Stack up is placed in a PCB laminator.
- After a launch, the equipment unit gradually increases temperature and pressure.
- Stack-up is held at the required conditions for up to 2 hours.
- Once the process is finished, a newly formed board is moved to a cold press.
- Finally, the cooled-down panels are unmolded.
- Hole registration. After the lamination stage is finished, PCBs should pass the registration procedure with the help of an X-Ray machine. Holes are deburred, chamfered, and rounded at the corners at this stage. This step is necessary because the film covers the previously prepared holes and is required to be eliminated.
FACTORS TO CONSIDER WHEN CHOOSING PCB LAMINATES
Here is the list of aspects of PCB fabrication to pay attention to and some boards specificities to consider:
- Quality. It is a crucial concern while purchasing low-cost materials. It is essential that specific laminates were fabricated using particular materials and comply with the desired specification. Request corresponding documentation from the supplier.
- Cost. It is typically impractical to select the most expensive material, like ones with gold tabs. So choose high-quality ones among the most rational offers.
- Power and energy efficiency. Make sure that the purchased laminates comply with your requirements. Such as the ability to withstand high currencies or heat dissipation capabilities.
- Glass Transition Temperature (TG). Substrates chosen for a laminate may be vulnerable to specific temperature ranges, above which it starts to soften. It impacts manufacturing methods you can use, so choose wisely.
- Coefficient of Thermal Expansion (CTE). It is another vital indicator that shows how significantly laminates expand once exposed to high temperatures. It is especially vital if you consider the fabrication of PCBs for microelectronics.
- Thermal conductivity (k). It is simply the heat transfer capability of the board. If this indicator has a low value – heat transfer remains stable even when the temperature tends to increase.
- Dielectric constant (Er). Impedance and signal integrity depends on the stability of the dielectric constant, which typically ranges between 3.5 and 5.5 Er. It is not essential what the value is, but rather how minimal its fluctuations are.
- Surface resistivity (pS). Since laminate material is some sort of dielectric, its surface’s insulation has electrical resistance. Ideally, this value falls in the range of 103 to 109 megaohms per square meter. Remember that moisture and high temperature affect the surface resistivity.
- Flammability. It is simply an indicator of how prone a laminate is to open fire and other such sources of extremely high temperatures. The material should not ignite for more than 10 seconds once the source of flame is removed.
- Moisture absorption. The ability to withstand moisture exposures is crucial for laminates. The common values range from 0.01% to 0.2%. The lower value is, the better.
- Chemical resistance. Also known as methylene chloride resistance because of the test’s name. It shows the level of absorption of chemicals, which should not exceed 0.2%.
- Density. This measure is calculated in grams/cubic centimeter. It shows how heavy the material is.
- Time to delaminate. Finally, this indicator shows how long it takes for a material to delaminate under exposure to high temperatures (thermal shock) or excessive moisture.
Types of PCB Laminate Materials
Once you know exactly what are the properties of your desired laminate, you may choose from many offers on the market. Here are the main manufacturers and types of laminate materials they offer.
- ISOLA. It offers FR-4, FR-4/Phenolic, FR-4 High Temperature, Polymide, and Modified Epoxy clad laminates.
- NELCO. It offers FR-4 High temperature and Modified Epoxy clad materials.
- Arlon. It offers Ceramic/Glass clad laminates (85N series).
- ROGERS. It offers PTFE/Glass, PTFE/Microfiber, multiple Ceramic/PTFE, and several Hydrocarbon/Ceramic clad laminates.
- TACONIC. It offers PTFE/Glass clad laminates only (TLC, TLE, TLT, TLX series).
- EMC. It offers FR-4 laminates only (EM-827 series).
The options are not numerous to describe each in detail. Their characteristics also tend to fluctuate over time. So, research on ones that you need on the supplier’s official webpage to learn more about their properties.
But in case you also need to know more about the general properties of such laminate materials, here is a brief overview:
- FR-4. Good but not superior performance overall. It is a reliable material offering a good strength-to-weight ratio.
- High-performance FR-4. Its Tg is increased for better reliability in high-temperature environments. It suits some high-frequency applications.
- High Tg epoxy. It is great for multilayer PCBs mostly. It withstands moisture, excessive heat, and exposure to chemicals.
- BT epoxy. Nearly the only laminate perfectly suitable for a lead-free manufacturing process. It is also good for multilayer PCBs. BT epoxy has superior thermal, mechanical, and electrical characteristics.
- Polyimide. It is prevalent for technologically advanced and specific applications. It offers excellent environmental stability and superior thermal, mechanical, and electrical characteristics. Suitable for nearly any board type, such as high-density, flexible, multilayer, etc.
- Copper clad. This one is intended for high-voltage PCBs and offers a good balance between specific requirements, including appearance, size, and performance level.
- Teflon. Such laminate is absolutely irreplaceable when it comes to high-frequency and low-loss communication applications.
HOW TO CALCULATE LAMINATE THICKNESS?
Laminate stack-up is a composition of multiple layers, the thickness of which is typically known. PCB core, for example, FR-4, is 0.2 to 3.2 mm. in thickness. Copper foil’s thickness is about 254 mm. Lamination will bond them together with prepregs and slightly increase the overall thickness of the dry film. And that’s it. But it is not that simple at the second sight.
Bond-Ply Thickness Calculation
- Resin system used. Each of such epoxy materials has a particular constant – cured resin density (g/cm³). For example, for Toray Ceterx TC1000 Premium it is 1.27 g/cm³. Alternatively, you may input resin density yourself.
- Fiber used. Same with reinforcement fiber systems. That has a specific fiber density constant (g/cm³). For Toray M30S, for example, this value equals 1.73.
- Fiber Areal Weight. This indicator stands for the amount of fiber or weight of fiber per measurement unit (gsm). For example, your square meter of fiber may weigh 80, 400, or even 650 grams.
- Prepreg resin content. It is simply the proportions of your finished prepregs. In case resin content is 10%, which means that fiber content is 90%. Resin content 40% = fiber content 60% and so on. It is a sort of resin saturation of your prepreg.
Once you input all these values, you are given multiple indicators, the most vital of which is nominal cured ply thickness and nominal laminate density. These estimations tell you how thick one you prepreg will be and how much it will weigh.
But here’s the interesting thing, None of the formulas and none of the calculators respectively can provide you with accurate estimation. It is simply because these materials are really not something that shows constant performance. It will fluctuate significantly from the batch of fiber and resin to batch.
What are you supposed to do? Do several estimations. Choose different resin saturation, and define worst and best cases. The real situation will be somewhere in-between. This way you will know how much resin you should apply to your fiber, so the nominal ply thickness is satisfactory.
And the best advice – give a try multiple fiber/resin/areal weight combinations. And do estimation for each new batch of fiber and resin even if they are purchased from the same supplier.
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