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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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

Background

It is becoming increasingly difficult to define a frequency at which it becomes necessary to transition from FR-4 types of material to high-frequency materials for circuits. This is because different technologies now accept various performances from a laminate. For instance, with enhanced signal processing one can obtain performance at higher speeds and frequencies from FR-4 material that was earlier thought possible. As a rule of thumb, high-speed digital applications at 10 Gbps will certainly need to use a high-frequency laminate.

In exceptional cases, some applications may demand a high-frequency laminate even if working at a lower data rate, as they require a very well controlled dielectric constant and low losses. As a general rule, insertion loss concerns preclude the use of FR-4 materials above 3 GHz in RF applications, especially where dielectric constant control is a critical concern, equally as important as controlling the substrate thickness.

Applications

Mixing High-Frequency Materials with FR-4

Contrary to popular belief, the microwave industry often mixes high-frequency circuit material such as PTFE (Teflon) with FR-4 material, as they have fewer compatibility issues. Such hybrid builds result in lower cost as the more expensive laminates are used in only those layers that need better electrical performance.

In addition, hybrid materials often improve the reliability of a circuit. With circuits built with several layers of PTFE, the high CTE or coefficient of thermal expansion of PTFE can be a matter of concern. The thermally stable FR-4 has a low CTE, and when used on non-critical layers, helps to offset the higher CTE of the PTFE layers. On the other hand, there are PTFE laminates with ceramic fillers that exhibit extremely good stability and low CTE values.

Fabricating High-frequency Circuit Materials

Fabricating with high-frequency circuit materials such as pure PTFE is indeed difficult, as the material is soft and does not readily accept bonding with copper. However, PTFE materials such as those with ceramic and hydrocarbon fillers make it easier for fabricators to handle these alternative high-frequency circuit materials. PTFE laminates are also available reinforced with woven glass. This negates several issues such as softness, dimensional stability, and handling.

In addition, the filled PTFE substrates show much better CTE, and fabricators find the PTH preparation process more forgiving. The most fabrication-friendly and recommended material for the microwave industry is the woven glass-reinforced ceramic-filled PTFE laminate. Laminates made of this material offer extremely good electrical performance as well.

Option of Different Dielectric Constants

RF applications need a larger variety of dielectric constants than digital applications do. Since the microwave industry works with frequencies varying from 300 MHz to 30 GHz, they often use the PCB circuit patterns as the microwave circuit component. For instance, the conductor pattern on the PCB may as well perform as a band pass filter rather than an extra component soldered onto the PCB.

The physical size of such simulated components depends on the wavelength at the frequency of interest. If the designer selects the dielectric constant of the circuit material to be very high, the size of the circuit would shrink to maintain the same properties at that wavelength. Since wavelength is related to dielectric constant, a higher dielectric constant is necessary when working with shorter wavelengths.

Conclusion

We at PCB Global can assist with any of your requirements, including the application of Teflon PCB’s. For more information on the application of Teflon PCB’s or to arrange a quote, please email sales@pcbglobal.com

Introduction to Circuit Boards

At virtually all times of the day, we are almost surrounded by technology, and most of it helps in making our daily routines easier. In fact, most of us would probably feel completely lost without our electronic devices.

Almost everyone knows the pieces of technology are essential to them, but what many do not know is these devices need flexible printed circuit boards to keep them running. According to industrial reports, the revenue for printed circuit boards in the US in 2014 alone amounted to nearly $44 billion.

Flexible PCB’s and their Application

Flexible printed circuit boards are a patterned arrangement of components and printed circuitry utilizing flexible base materials. The main advantage over regular printed circuit boards is their flexibility, which allows the flexible printed boards to conform to the three-dimensional space available within devices.

Printed circuit boards are almost always unique, each being designed to exact specifications to meet the requirements of individual devices. The thickness of flexible circuit boards can vary widely, ranging from 1.6 mm to 0.08 mm. Among common household items, you will most likely find flexible printed circuit boards being used in microwaves, clocks, cell phones—their touch-type interface and LCD requires flex circuits.

When are they used?

As long as televisions and computer monitors were using Cathode-Ray Tubes (CRT), there was ample space inside for a rigid printed circuit board. Moving over to LCDs has limited the space inside these devices and forced the use of flexible circuit boards. Desktop printers, computer keyboards, and memory devices all use flexible circuit boards now.

Apart from the everyday technology and electronics, flexible circuit boards are also used extensively in almost all industrial applications involving transportation, military, aerospace & avionics, medical, automotive, defense, consumer electronics, communications, satellites, semiconductors, and many more.

Benefits of Flex PCB’s

With consumers and applications demanding a reduction in the size of electronic devices, the importance of flexible printed boards is projected to rise. Their popularity among manufacturers is gradually increasing as many original equipment manufacturers are gradually becoming aware of the significant benefits of flexible circuits.

Although designed and manufactured in much the same way as their rigid counterparts, there are specific differences between the two processes, which make flex circuits typically more cost-effective. Flex printed circuit boards:

• Reduce production costs by simplifying system design
• Solve internal packaging problems, especially for smaller devices with compact designs
• Offer considerable weight reduction when replacing wire harnesses
• Offer enhanced flexibility, which helps absorb and reduce the effect of vibrations
• Increase the reliability of the device, as reduction in wiring lowers the risk of connection failures
• Can be used in harsh environments, as they are resistant to corrosion, high temperatures, shock, moisture, and water

Properties of Flex PCB’s

Engineers designing equipment look for greater flexibility and stretch, which can only be met by flexible circuits. In contrast to the regular rigid printed circuit boards, flexible circuits can bend and wrap around small or unconventional spaces. This exclusive feature makes flex circuits so attractive to design engineers. All this is making product design engineers and manufacturers recognize and take advantages of flexible printed circuit boards.

Conclusion

At PCB Global, we are aim to assist all our customers unique and varying requirements. We work with a variety of factories that specialise in flex PCB’s which ensures quality and efficiency with regards to supplying you flex PCB requirements. For more information or for a quote please feel free to contact us at sales@pcbglobal.com

Modern electronic modules demand increasingly high levels of density. The miniature sizes of most surface mount devices (SMDs) that have now reached unbelievably small dimensions are primarily fueling this demand. Other advancements such as improvements in packaging technology has led to fine-pitch components such as the Ball Grid Array (BGA) sporting more than 1500 pins and a 0.8 mm pitch on average, covering entire bottom surface of the package.

Microvias, Buried Vias and Sequential Build-up

 All the above has led to an unprecedented reduction in the widths of structures on the PCB, that is, the track widths and the spacing between them. To improve the density further, fabricators are using microvias, buried vias, and a sequential buildup of multi-layer boards to improve the integration. Use of these technologies offers designers more space on the outer layers for placing components. For instance, use of buried vias precludes the use of through-holes, which would have to go through all the layers.

Use of microvias, buried vias and fine structures results in boards with very high-density interconnection (HDI), and Sequential Build Up (SBU) of multiple layers supplements this. For instance, an HDI-SBU multi-layer board will have at least two layers or a multi-layer core and one or more external layers with microvias.

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Advantages of HDI-SBU Technology

PCBs with standard lamination and through vias are less expensive to manufacture, as they have simple via models and use ordinary dielectrics such as FR-4. The manufacturing process is mature, and fabricators can produce high-reliability boards.

As the layer count increases, fabricators start facing problems with boards using standard lamination and through vias. The costs skyrocket, and few fabricators can reach good yields. They face difficulties such as delamination at high temperatures, increase in layer count as large vias reduce the ability to route, implementing BGAs with pin pitch below 1 mm, capacitive coupling from through-hole vias, long via stubs creating impedance mismatches, and many more.

HDI-SBU boards solve most of the above problems. Microvia technology makes vias much simpler to fabricate, with potentially shorter via stubs. As microvias are dimensionally much smaller than through-hole vias are, designers have much more area for routing traces, thereby offering them the only practical way of implementing multiple fine pitch components such as the BGA.

Advantages in relation to the Manufacturing Process

With smaller feature sizes for traces and vias, HDI-SBU boards enable designers achieve much higher route densities than before, resulting in fewer number of layers. Therefore, although the HDI-SBU manufacturing processes are more expensive compared to the cost of fabricating standard boards, the high-density boards can be made in much smaller sizes and lower number of layers, thus helping to offset the cost. In fact, multi-layer HDI-SBU boards with larger number of layers are cheaper to manufacture compared to standard boards with the same number of layers. Moreover, newer materials are now available, making HDI-SBU boards compatible with RoHS processes, offering better performance at high temperatures, and improved signal and power integrity at high frequencies.

Applicable Standards for HDI-SBU

Japan Printed Circuit Association publishes IPC/JPCA-2315, jointly with IPC, and these standards provide easy-to-follow tutorials on selecting HDI and microvia design rules and structures.

Conclusion

The team at PCB Global are able to assist with any queries you may have in relation to if HDI-SBU PCB’s are necessary or beneficial to your PCB specifications, requirements and outcomes. Please feel free to contact the team at sales@pcbglobal.comfor any more information on this area of PCB manufacture.