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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.

Posted on 30/11/2018

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

Posted on 26/10/2018

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

Image Courtesy: http://ohmega.com/wp-content/uploads/IMS-2017-Workshop_r01.pdf

Heat Generation Issues

Heat generation in electronic circuits is equivalent to inefficiency, but then one cannot have a circuit that operates at 100% efficiency—the laws of Physics and Thermodynamics say so. Therefore, we must accept that any electronic circuit will generate some amount of heat when it is in operation. As most electronic components are now available as miniature, surface mount technology, removal of heat from the tiny devices poses a tough problem, especially when mounting them on a printed circuit board (PCB) that is a poor conductor of heat.

A typical PCB has an insulating core, which also does not conduct heat very well. Although there are copper traces present, the amount of heat conducted by conventional PCBs depends on their design—specifically, the surface area of the copper traces—broad traces conducting heat better than thin traces do. Engineers solve the problem of conducting heat away from PCBs by using metal core PCBs (MCPCBs), where they use a metal core as the base material in place of the regular FR4 or CEM3.

Why use Solid Copper?

As copper has high thermal conductivity, it is a natural choice for use as a heat spreader in MCPCBs. Circuit traces, necessary to interconnect various components on the PCB, remain electrically insulated from the base metal core as there is a thermally conductive dielectric layer separating them. This dielectric layer bonds the circuit traces to the base metal. In fact, the thermal performance of any MCPCB depends exclusively on this dielectric layer.

Examples of such MCPCBs with solid copper base are those engineers use for mounting LED lights. Although an SMD LED is a high-efficiency device, and it converts a major part of its input power into visible light, a minor part generates waste heat within the LED chip—with high power LED lights generating more waste heat. Unless removed, this waste heat buildup can be fatal to the LED.

Design and Application

Engineers remove the excess heat from the LEDs on the PCB in two ways—by using broad traces of copper to interconnect the LEDs with the rest of the circuit and by adding a solid copper base, insulating the two with a thermally conducting dielectric.

For medium power LEDs, the terminals conduct the heat generated from within the LED chip to the broad traces to which the terminals are soldered, dissipating the heat effectively. Part of the heat also travels through the thermally conducting dielectric to the solid copper base, which serves as the ultimate heat spreader.

For high power LEDs and circuits generating copious amounts of heat, additional channels are necessary to conduct more of the heat into the solid copper base. Engineers handle this with two additional mechanisms—a heat conducting metal tab under the LED or the IC attached to its internal die, and the use of metal filled thermal vias.

By incorporating a copper land immediately under the device, the IC or LED can transfer the heat from its die through the metal tab directly to the copper land. Furthermore, multiple metal filled thermal vias connecting the copper land to the solid copper base effectively transfer enough heat to the heat spreader to keep the LED or IC cool and operating safely.

Conclusion

At PCB Global, we consistently receive positive feedback about our customers use and application of copper based PCB’s. For more information on how copper can improve you PCB’s design and capabilities, please don’t hesitate to contact the team at sales@pcbglobal.com

 

Image: How heat is Dispersed

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