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Posted on 25/08/2017

Introduction

While designing multi-layer Printed Circuit Boards (PCB’s), one of the most basic elements that the engineer must include is the requirement for interconnected traces/planes on one layer to traces/planes on another. The most efficient technique of achieving this is to use vias. These are small holes drilled into the layers of the PCB, and fitted with a copper tube connecting to pads on either end. The pads in turn connect to the required traces on respective layers.

Why use Vias?

With increasing use of high-density boards, and engineers must reduce trace widths and spacing to accommodate for applications. Vias are another technique of achieving higher density boards by making them in multiple layers. In turn, the design of vias has also been evolving, with designers and engineers trying out different types of vias such as ‘landless’ and ‘swing types’. One of the very effective methods of achieving increased layer density is by using ‘via-in-pad’ designs.

Example

Consider the plight of an engineer in the process of breaking out the connections from an FPGA or BGA package of, for example, 1760 pins with a 1mm pin-pitch. According to the application data sheet of such a package, 6 signal layers are necessary to breakout the connections from all the pins. However, with advanced via techniques, engineers can now accomplish this with only 2 signal layers, resulting in better interconnection as well as being a substantial cost saving option.

Fabricators using the high-density interconnect (HDI) techniques use advanced technology such as buried vias, blind vias, via-in-pad, and micro-via techniques to improve the density of their boards spectacularly. Micro-via techniques involve using lasers to drill holes of very small diameter. Together with the via technologies above, the use of micro-vias results in 24% increased routing density per layer over conventional design processes.

How do they work?

As the name suggests, via-in-pad is a via deliberately placed within the area of a solderable pad. Normally, conventional design practices prevent the use of a via very close to or within a solder pad. Most manufacturers also recommend not using a via this way. The main reason being the via often acts as a wick does during the reflow process, allowing all the solder paste to melt and flow into its hole, leaving the solder pad starved of solder and resulting in an unsoldered joint. This problem is solved by filling and capping the hole of the via-in-pad.

Therefore, just as with any other tool, via-in-pad technology can lead to spectacular results if used properly, or to disastrous consequences if misused. For instance, inadvertently leaving a via-in-pad uncapped under a BGA solder ball can result in the solder paste flowing down into the hole of the micro-via, leaving the joint open. Therefore, it is essential to have every via-in-pad filled and plated over. To be on the safe side, all vias on the board are filled and plated over, and this effectively takes the via out of consideration.

While filling and capping the via-in-hole does solve a major problem, it creates another one—lack of coplanarity. Unless care has been taken to achieve good planarity, there can be a tiny bump or an indent over the via. This can lead to a less reliable assembly, especially with chip scale and BGA packages.

Conclusion

To discuss whether integrating the use of via-in-pad technologies for your PCB specifications, please feel free to contact the team at PCB Global for the most efficient advice followed by a rapid quote.  Please email you design file to sales@pcbglobal.comfor a rapid and competitive quote. 

Posted on 28/07/2017

Introduction

Manufacturing a multi-layer flex circuit board starts with a base material of copper clad flexible laminate. This is typically an unreinforced film coated with adhesive on both sides, with the outside surfaces covered with a thin sheet of copper. A lamination process bonds these elements together under heat and pressure.

The Materials

Such copper clad flexible laminates come in large sheets such as 24x36 inches—one of the standard sizes. The laminate material must be of the correct size and thickness, and be free from imperfections in the copper surface, such as pits and dents. After certification of the size and quality, a shearing machine cuts the full-size sheets down to more usable panel sizes, such as and typically 18 x 24 inches. To remove moisture and balance any internal stresses evenly, the panels are baked in ovens and then cooled before use.

The Process

1.    Using the computer-generated drilling information provided, the computerized drilling machines will drill holes in the panel as per specification. At the same time, the machines drill two or more ‘locating’ or ‘registration’ holes on the periphery of the panel to enable lining up the different films for further processing.

2.    The panel now undergoes a preparation process for the application of the conductive pattern. Usually, the chemical process starts with the panel being dipped in an acid bath, followed by the application of an anti-tarnish agent, ‘micro-etching’, or ‘chemical cleaning’. The next step involves creating the conductive pattern on the copper surfaces of the panel according to the circuit image.

3.    Following these steps, the image is transferred using a dry film. This is achieved through thedry film laminator using heated rollers to press a layer of photoresist onto each copper surface.Next, a sheet of film containing the negative image of the desired conductor pattern is placed on this layer of photoresist, and the sandwich is exposed to UV light inside an exposure chamber. UV light passing through the clear areas of the image sets up a chemical reaction in that area of the photoresist layer it touches. The unexposed areas remain relatively soft and a developing process in the followingstep removes this soft and unexposed resist, exposing the unwanted copper.

4.    Next follows an etching process, where a chemical solution removes the unwanted exposed copper from the surface of the panel. The required copper circuitry remains on the panel underneath the photoresist. Another chemical strips the hardened photoresist to expose the remaining copper.

5.    After a thorough inspection of the copper pattern, a very thin film of electroless copper is chemically deposited over both the surfaces of the panel, including inside the holes, also called vias. The metalized vias now connect one side of the flexible circuit to the other. Subsequent copper layers are added to the core over a layer of insulation called prepreg, and bonded to it using heat and pressure.

6.    Depth-controlled drilling then creates the blind and buried vias, followed by the same process of image transfer, etching, and electroless copper deposition on all the added layers.

7.    Finally, all the areas that will not be coated with solder are masked off with a coverlay. The finished flexible circuit board then undergoes an inspection process.

Conclusion

For more information on this particular process or for any advice on if your PCB specification should incorporate a flexible circuit element, please don’t hesitate to contact the team at PCB Global.  Please email you design file to sales@pcbglobal.comfor a rapid and competitive quote.

Posted on 23/06/2017

Rigid-Flex Circuits

The identifying characteristic of a rigid-flex circuit is the presence of one or more sections of the circuit in the form of rigid construction, while the rest of the circuitry is of the flexible type. Conductors are present on both the rigid and the flex parts. Multi-layer rigid-flex boards will have plated through holes extending between the rigid and flexible sections and electrically connecting multiple conductor layers.

Although more expensive than alternative flex designs, rigid-flex circuits are ideal for applications requiring placing the board in three-dimensional space and components are often mounted on both sides of the rigid section. Defined by IPC 6013 as a Type 4 circuit, the rigid-flex circuit construction is different from a flexible circuit with a rigid stiffener attached. Being partly rigid and partly flexible, rigid-flex circuits combine the best of both types of circuits, leading to several advantages and benefits to the user.

Advantages and Properties of Rigid-Flex

Several small to large industrial/commercial/domestic electronic systems now regularly use rigid-flex circuits, because of specific advantages such as:

  • Flexibility to manufacture the board to fit into the device:Most modern electronic devices are compact, lightweight, and often flexible. It is easy to design a rigid-flex circuit to fit into the device, as against modifying the design of the device to fit the specifications of the board.
  • Flexibility to design the board to fit into confined spaces:With electronic design tending towards smaller dimensions, space inside the device is always at a premium. As rigid-flex circuits can easily bend and fold, the design can fit perfectly into small spaces, contributing largely towards miniaturization of products.
  • Reduction in overall package size:As rigid-flex circuits can be made lightweight and compact by design, this reduces the overall size of the product package.
  • Increase in integrity and reliability:By design, rigid-flex circuits can avoid using solder joints, connector, or contact crimps, leading to better integrity and reliability in the product.
  • Reduction in circuit failure:As rigid-flex circuits integrate both rigid and flexible circuitry, there are fewer number of interconnects, leading to substantial reduction in failures.
  • Ability to withstand temperature extremes:The excellent thermal stability of polyimide used in manufacturing rigid-flex circuits provides the ability to withstand temperature ranges beyond the working range of generic PCB materials.
  • Reduction of manufacturing and material procurement costs:Rigid-flex circuits require fewer materials for their assembly. For instance, rigid-flex circuits can replace wire-harnesses consisting of wires, connectors, and contact crimps, therefore reducing the overall cost of the assembly of the PCB.
  • Ability to resist radiation and UV exposures, oils, and harsh chemicals:The materials used for manufacturing rigid-flex circuit can withstand very harsh environmental conditions and substances.
  • Ability to accommodate SMD’s easily on both sides:By design, only the rigid part of the circuit can have SMDs mounted on both sides. However, it is not advisable to mount components on parts of the circuit that are likely to flex and bend during operation.
  • Possibility of designing with varied material options and layouts:Depending on application requirement, designers can select the substrate material for rigid-flex circuits as polyimide or polyester (PET), conductor as rolled copper or electro-deposited and adhesives as polyimide, polyester, acrylic, or epoxies.
  • Ability to withstand aggressive industrial conditions such as shock and vibration:With careful design, rigid-flex circuits can have a long working life even under adverse industrial conditions.

Conclusion

To discuss whether utilising a rigid-flex PCB is suitable for your unique design and specifications, please don’t hesitate to contact the team at PCB Global as we have combined experience of more than 30 years in the PCB manufacturing industry and will be able to assist you with any areas of PCB design and fabrication that you may require advice or recommendations. Please email you design file to sales@pcbglobal.com           for a rapid and competitive quote.

Why use a Ceramic Base?

Many applications need removal of heat from hot spots rather quickly and efficiently, while effectively dissipating it over the entire surface of the PCB. With epoxy-based PCB material such as FR-4, this is impossible, as epoxy has poor thermal conductivity. Therefore, using epoxy-based materials in such applications may form local hot spots, leading to a reduction in the life of semiconductor junctions.

Ceramic substrates such as beryllium oxide, aluminum nitride, and alumina each have very high thermal conductivity. Compared to the thermal conductivity of 0.8-1.1 W/m-K for FR-4, the thermal conductivity of beryllium oxide ranges from 170 to 200 W/m-K, while for aluminum nitride the range is from 140 to 180 W/m-K and for alumina the figures range from 28 to 35 W/m-K. Therefore, PCBs made of ceramic substrates are capable of distributing heat more evenly, thereby allowing components mounted thereon to function more efficiently.

Hybrids with Ceramic Base

Typical uses for PCB’s with a ceramic base are hybrids or microelectronic circuits. Hybrids may comprise multiple devices, very often in die form, which are hermetically sealed inside a metal enclosure or within an impervious conformal coating. This effectively insulates and isolates all components to a high level, protecting the constituent parts from ingress of moisture. Manufacturers fire the insulating layers and conducting tracks of hybrid substrates at 850°C for nearly 10 minutes, while curing the conductive adhesive device termination at 150°C for about two hours. In comparison, FR-4 PCBs can withstand maximum temperatures of about 80 to 200°C for short periods, with damage to PCB structure starting beyond this.

Hybrids also utilize components such as on-board resistors capable of long-term operation up to 250°C. Additionally, it is possible to trim these resistor values by adjusting them with laser trimming, while the circuitry is functionally operating. This allows dynamic tuning as a process in automated production.

Direct Copper Bonded Ceramic PCB’s

The technology involving Direct Copper Bonded (DCB) Ceramic PCB’s uses a special process that bonds the copper foil to one or both sides of the ceramic substrate at appropriately high temperatures.

Although the DCB substrate is super-thin, it has high thermal conductivity, excellent electrical isolation, high bonding strength, and fine solderability. It is possible to etch it like the regular FR-4 PCB, but it can carry much higher currents. This feature has made the DCB ceramic PCB popular as the base material for construction of interconnection technology of electronic circuits using high power semiconductors. Its capability of handling higher currents also makes it the basis of the Chip On Board (COB) technology, representing the future in packaging trend.

DCB Properties

The DCB ceramic PCB possesses some very desirable and excellent properties such as high corrosion resistance, good adhesion, excellent electrical isolation, high thermal conductivity, and high mechanical strength. Additionally, it has excellent thermal cycling capabilities along with high reliability.

As the coefficient of thermal expansion of the DCB substrate is nearly the same as that of a silicon chip, it is possible to solder the chips onto a DCB board directly, thus saving on the interface layer and labor while reducing the cost. This process reduces the solder layer, lowers the thermal resistance, and increases the yield of the finished products by reducing cavities.

Conclusion

For more information on Ceramic based PCB’s, please don’t hesitate to contact the team at PCB Global. Many of our online capabilities are displayed on our website, however, for specialised PCB requirements, please email you design files to sales@pcbglobal.comfor a rapid and competitive quote as well as advice for improving your board’s capabilities and efficiencies. 

Introduction

In the past few decades, there has been a tremendous expansion of the wireless industry, which transformed itself from being predominantly military-driven to a consumer-driven, cost-conscious commercial market. Simultaneously, wireless applications are venturing further into the frequency spectrum. For instance, personal communications systems that earlier operated on 1.9 GHz, are being replaced by newer designs operating on 5.8 GHz.

Applications

With applications moving up in the frequency scale, there is renewed interest in high frequency, high performance, but low-cost substrates. Earlier, only military applications considered polytetrafluoroethylene (PTFE) as a substrate that satisfied the above technical requirements. PTFE is also commonly known as Teflon.

Why Use PTFE?

With a melting point of 327°C, Teflon has high temperature resistance, negligible water absorption, good resistance to processing chemicals, and a very low loss tangent. That makes the PTFE substrate suitable for all the technical requirements of RF/Wireless designs. However, commercial applications also demand low costs along with high performance, but the conventional PTFE substrates were eight to ten times more expensive than regular FR-4 epoxy glass laminates are. In addition, processing of PTFE substrates was difficult because of the inert and soft nature of PTFE.

Ceramic Filled PTFE Substrates

New research has established ceramic filled PTFE substrate, such as RF-35, as not only satisfying the commercial price requirements, but also exceeding all electrical and mechanical property that manufacturers seek in a PCB substrate for high frequency applications. The dielectric constant of RF-35 has a value of 3.5±0.1, and the material is available in thicknesses of 10, 20, 30, and 60 mil.

Substrates such as RF-35 are unique as the variation in their thickness and dielectric constant within a sheet are minimal. For instance, a sheet of RF-35 shows a standard deviation for dielectric constant as 0.01, while that for thickness as only 0.00023” within the sheet. Within a range of 500 MHz to 11.2 

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