Introduction

The high frequency material the PCB fabrication process is using can affect the circuit performance for the end user. That means there must be interaction between the fabricator and the designer, while the end user should understand the concerns of the PCB fabricator for these materials. This is important for achieving good manufacturing yields along with a highly reliable and quality finished product, as each type of high frequency material has its own unique fabrication concerns.

The most common high frequency materials are PTFE (Teflon), PTFE with ceramic fillers, and non-PTFE thermoset resin systems with ceramic loading. Less commonly used are LCP or Liquid Crystalline Polymer materials.

Characteristics of PTFE Materials

The PCB manufacturing industry has long been using PTFE materials for high frequency circuits. Apart from pure PTFE, some of the substrates may also consist of a small amount of micro-fiber glass impregnated into it. Others can be PTFE with woven glass reinforcements. Still others may possibly be PTFE with ceramic filling. However, compared to all other types of materials used for high frequency circuits, the nearly pure PTFE is the most challenging type of circuit material faced by PCB fabricators. The reasons for this are:

• PTFE has a high CTE
• PTFE does not allow other materials to adhere to it easily
• PTFE is a soft substance, easily able to distort

However, from the perspective of electrical performance, PTFE substances are the best to use. PCB fabricators find the ceramic filled PTFE substrates the easiest to handle.

Working with PTFE Materials

When working with PTFE materials, PCB fabricators must be careful in not creating smears when drilling, not altering the substrate with scrubbing or other mechanical processes, fine tuning dimensional stability issues, and using best practices for minimizing handing damage to the soft substrate. PTFE substrates need a special through-hole preparation process to allow copper plating to adhere to the wall of the hole, and another special process for laminating PTFE materials with other bonding materials.

As there are no known methods or processes for de-smearing PTFE, it is very important to minimizing heating during the drilling process, since heating is responsible for causing smearing. The cleanest possible drilling for pure PTFE requires a new drill tool to ensure no smearing, but ceramic filled PTFE substrates can tolerate a re-sharpened tool.

Pure PTFE substrates need to undergo a wet-chemistry process prior to the copper plating process, to ensure good adherence. Typically, sodium naphthalene or a derivative can remove a fluorine atom to make the PTFE substrate accept the copper plating. Ceramic filled PTFE must undergo an additional process of baking to remove moisture the substrate has absorbed during the wet-chemistry phase. An alternative is to use a special plasma cycle using helium, which avoids the baking process.

Lamination on the PTFE surface requires a bonding medium, and most bonding materials that the PCB industry conventionally uses are applicable for PTFE as well. However, fabricators must be careful to not alter the exposed substrate surface after the copper etching process, as any scrubbing will polish the soft PTFE surface, and hinder further bonding.

Conclusion

The application of PTFE is unique due to the common use in high frequency application. If you are looking to improve the efficiency of your PCB with the use of Teflon, please contact the team at sales@pcbglobal.comfor more information and recommendation for your project and design. 

The printed circuit industry offers copper clad circuits in four different classes, with individual standards defined by IPC. These are rigid, flexible, high-speed, high frequency, and High Density Interconnect (HDI). Each of these families has more standards within them for defining the base material, acceptance, and design they use. However, users may utilize all four classes of boards in a single assembly for their application. This poses a problem of inter-connectivity, as the design rules vary from one type of board to another. This has led to improvements in flexible circuit technology to make them more suitable for high frequency and high-speed applications.

Difference between Flex and Rigid Materials

One significant difference with flex materials is their base material generally does not contain glass reinforcement as is usual with rigid boards. Both mechanical integrity and flexibility of flex circuits comes from the dielectric material, which comprises various grades of polyimide. Manufacturers usually have their trademark composition of polyimides, with emphasis on specific functional aspects. 

Another significant difference is the brittle solder mask is replaced in flexible circuits with coverlay, a thin coating of conformable, elastic layer, and is processed differently. Unlike rigid boards, manufacturers prepare flex dielectrics as large rolls of coated film and laminate them to the copper layer in a separate step. This makes the thickness of the cast films very consistent, and this has the major advantage of keeping a tight control over impedance, an important factor for high-speed applications.

Unlike electrodeposited (ED) copper commonly used in rigid circuits, flex circuits use rolled-annealed (RA) copper. The rolling process ensures the copper is smoother, ductile, and less likely to crack when bending. 

Flex Materials and High Frequencies

Applications meant for high frequencies and high speeds require consistent dielectric thickness, low dielectric constant, and low dielectric loss. However, as operating frequencies cross 1 GHz, the term dielectric constant loses its consistency. This is because the polymers used in flex circuits absorb energy from the RF and this is called the loss tangent of the material. As the operating frequency increases, materials with high loss cause greater changes to occur in the relative permittivity, which means, to work at high frequencies, materials used for flex circuits must be chosen carefully. Most popular materials for use at high frequencies are Advanced Kapton and Teflon, as they maximize signal integrity for high-speed flexible circuits.

Influence of Copper at High Frequencies

Flex circuits are usually very thin, which means the copper layer is thin as well, and this has an exponentially increasing impact on signal loss as compared to that from the dielectric. This is because of the phenomenon known as skin effect, wherein high frequency currents tend to concentrate around the periphery of the copper conductor rather than flow uniformly across its cross-section. This increases the resistance offered by the copper, and hence increases the loss. Therefore, manufacturers use special types of RA copper known as Meg4 and Meg6, as these present a lower loss at higher frequencies.

Conclusion

Flex circuits work very well at high speed and high frequency applications, provided suitable materials are used for the dielectric and copper traces. For more information of flex circuits or to speak to a PCB Global Team member to see if utilising flex in your PCB project is right for your design and application, please don’t hesitate to contact us at sales@pcbglobal.com

Introduction

Regular Metal Core Printed Circuit Boards (MCPCBs) use copper or aluminum as their metal base. The use of metal as base helps to conduct heat away from the components on the PCB, as copper and aluminum are good thermal conductors. However,such MCPCBs have to operate well below 260°C to prevent solder joints on the components from melting. For operating temperatures up to 650°C, the industry uses Thick Films withSteel Alloy as the base.

Applications of Steel Alloy Base Thick Films

Unlike MCPCBs that strive to keep components cool by conducting heat away from them, thick films with steel alloy as base are popular for providing heat in confined spaces. The steel alloy base offers superior structural properties along with a thin profile, which is suitable for a fast ramp up and down for high temperature applications.

Thick films with steel alloy base are common in applications requiring heating such as in medical and life sciences for dialysis, blood/fluid warming, temperature therapy, instrument warming, and sterilization. The aviation and transportation industry uses them for de-icing, freezer protection, battery and oil heating, and providing personal comfort. The food service industry uses thick films with steel alloy base for warming cabinets, grilling platters, heated dishware, and fryer systems. The printing industry uses them in laser print heads, 3-D printing, thermal printing, and in printer heads in commercial and industrial printing. The semiconductor industry uses thick films with steel alloy base for water heating, as wafer chuck heaters, and high temperature burn-in boards.

Advantages of Steel Alloy Base Thick Films

The major advantage of the steel alloy base is its ability to allow heating up to 650°C. Apart from this, heaters based on thick films with steel alloy base offer a low profile, are compact and lightweight, which results in fast and reliable operation. Their low mass allows fast temperature ramp up and cool down, along with power densities greater than 31 W/cm²or 200 W/in².

With a low coefficient of thermal expansion due to the presence of the steel alloy base, thick films do not gas when operating with inorganic substances. The steel alloy base allows machining the film into complex forms and shapes.

Manufacturing Thick Films with Steel Alloy Base

Manufacturers screen-print the insulation dielectric material onto the steel alloy base, firing it at 850°C. This produces a robust substrate with high resistance to thermal shock. With normal features of porcelain-enameled steel, these substrates offer the advantages of higher processing and operating temperatures. The insulation offers an ideal area for screen-printing the thick film resistive element.

Manufacturers often incorporate temperature-sensing elements within the heating device using resistive thick film materials with positive temperature coefficient.

Manufacturers use different types of stainless steel for the substrate. For instance, EC regulations mandate the use of at least 12% Cr for the stainless steel for use in the food industry. The heating elements use both austenitic and ferritic steels, with 304 austenitic steel offering higher temperature coefficient of expansion in comparison to the TCE of 430 ferritic stainless steel types.

Conclusion

Utilising a steel alloy base has many advantages in due to its thermal profile and conductivity. To find out if your PCB design and application would benefit from the use of steel alloy as its base, feel free to contact our team at sales@pcbglobal.comto discuss.

Very often, small PCB modules such as Wi-Fi modules that solder onto a larger printed board much the same way as ICs do, have plated half-holes or castellated holes along the edges. Although it looks like a PTH cut through, PCB manufacturers use a high quality bespoke process to plate the castellation holes, and do not cut out the holes. This makes the holes clean, smooth and with stronger edges that have no sharp burrs or deformation.

Castellation features are useful for mounting a PCB on to another and soldering, or for inserting into specially designed edge connectors. Several features contribute to the complexity of a castellated hole. For instance, the critical design attributes are the hole size, the number of holes per board, single or multiple hole design, surface finish, and overlay pad design.

Accordingly, manufacturers support four types of castellated holes: 

Type I:Large drilled plated holes cut in half 

Type II:Smaller lead in hole cut in half, placed on the periphery of the larger drilled plated hole

Type III:Primary large drilled plated hole with a smaller feature hole, with a post processing tool cutting into the smaller hole. This feature accommodates a special connector.

Type IV: Primary drilled plated hole, tangent to the board profile, with a post processing tool cutting into the hole.

The size of the holes and their numbers per individual PCB allows using the plate break process at the final fabrication operation. This post plate processing allows efficient removal of burring at the hole profile interface. Type II holes may need an additional burr removal process depending on the dimensions at the interface of the two holes. Although castellation allows use of any surface finish, a decrease in hole size with an HASL finish is more difficult to process and affects the quality of the castellation.

General rules for designers to follow when putting in castellation holes are:

• Use the largest hole size possible.
• Use overlay pads on both sides, top and bottom, and make them as large as possible.
• If possible, place inner-layer pads for anchoring the barrel of the hole. This also helps reducing burrs.
• When not using the castellation for a connector device, allow a larger tolerance for the dimensions of the opening.

The plating process requires the plated edge to be on an outer edge of the panel. This is important during panelizing the design. For instance, a board with castellation holes on both the left and right edges can be panelized only on the top and bottom edges. This also means one cannot panelize a rectangular board with castellation holes on all its four sides.

Conclusion

It is necessary for designers to consult their fabricators to ensure they are capable of producing castellated boards. Some fabricators have a limitation on the type and size of holes they allow for castellation. For instance, some limit the minimum diameter of plated half-holes to 0.5 mm. Designers need to add a plated hole or via on the border of the board where they require the half hole. Half the via should be on the board and the rest outside the outline. For more information regarding if the application of castellated holes is suitable for your PCB design and function, please don’t hesitate to contact our team at sales@pcbglobal.com

Advantages of Buried Resistors

The electronic industry is increasingly demanding faster processing times and improved performances. This has led to the generation of new challenges in both RF and digital circuit board designs. Replacing discrete and SMD resistors on the surface of electronic boards with embedded or buried resistors offer several advantages:

• Reduced number of solder joints—improves reliability
• Reduced crosstalk and noise—reduces EMI
• Elimination of inductive reactance
• Reduced series inductance
• Shorter signal paths
• Improved signal routing—lower number of vias
• Increased active component density
• Reduced form factors
• Reduced cycle times in PCB assembly—shorter time to market
• Laser trimming capabilities

Types of Resistors

The industry currently uses two types of buried resistances—thin film and thick film.

Thin Film Buried Resistances

Manufacturers use a thin-film vapor deposition technique to deposit a uniform resistor material layer on the PCB. The deposited resistive material is usually around 0.1 µm thick and an alloy of Nickel, Chromium, and/or Aluminum and Silicon. Different layer thicknesses produce a range of resistance values. As the layer is uniform and dense, fabricators use a subtractive process of laser trimming to increase the resistive path and achieve a specific resistance value.

Benefits of Thin Film

Thin film technology produces resistors generally known as metal film resistors, thin enough to bury within the inner layers of a multilayer PCB. With low thermal coefficient of resistivity, thin film foil resistors adapt well to lamination. Precision vacuum metallization techniques ensure better resistor tolerances. Good thermal dissipation properties offer excellent thermal stability, exhibiting excellent ductility at elevated temperatures to with stand stresses without cracking.

Thick Film Buried Resistances

This is a somewhat older technology for incorporating reliable carbon resistors printed on to PCBs using standard PCB processing steps. Fabricators use a novel hydrophobic polyimide resin as a printed resistor material to serve as a polymeric thick film resistor.

After printing the thick film resistor, the fabricator has to cure the film in a standard convection oven, a thermal belt furnace, or an infrared furnace. Once cured, the film is ready for laser trimming.

Benefits of Thick Film

In comparison to thin film embedded resistors, thick film buried resistors are cheaper, have lower TCR values, are less stable, and do not reach high tolerances. On the other hand, thick film resistors are able to handle higher power, provide a wider range of resistance values, and are capable of withstanding high voltage surge conditions.

Although the process burying resistors in PCB layers is more complex, it is more versatile than using SMDs. Buried resistors free up the surface space and allow designers to place additional active components, thereby increasing the functionality.

Application and Properties

Embedded passives, both resistors and capacitors, yield a more reliable PCB as they reduce the number of solder joints. This reduces rework on assemblies, while lowering the total system cost. As laser trimming produces specific values with close tolerances, designers can achieve higher performance and low signal losses, by placing the embedded passives more optimally in the circuits.

Embedding components within PCB layers achieves an additionally important function, that of preserving the intellectual property of the OEM and preventing it from being easily duplicated.

Conclusion

The team at PCB Global have the skills and expertise to assess and advise if buried resistors are more suitable or beneficial to your PCB design and applications. For more information or to speak to our team about the benefits of buried resistors, please don’t hesitate to contact us at sales@pcbglobal.com

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