Posts Tagged ‘PCB Trends’

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Manufacturing Process of Printed Circuit Boards

Written by Rush PCB Inc on . Posted in PCB, PCB Manufacturing

For anyone involved in the electronics industry, the process followed for manufacturing Printed Circuit Boards (PCBs) is very important. This is because PCBs are the very basis of all electronic circuits, being used to provide the mechanical basis on which circuits are built. PCBs come in different forms—rigid, flexible, rigid flex, and High Density Interconnect (HDI), the difference being mainly in the materials used to fabricate them, and give them their ability to flex or remain rigid. To some extent, the PCB manufacturing process depends on the material being used, but the main stages in the PCB manufacturing process remain the same, irrespective of the nature of the PCB. However, before coming to the manufacturing stage, the designer has to make a few choices depending on the application. These are:

  • Selecting the type of PCB required by the application
  • Deciding whether the board will have single, double, or multiple layers
  • Deciding the mechanical layout, the stackup, and the routing of tracks on different layers
  • Producing relevant documents and files for the manufacturing process.

The above steps are important for the success of the final product, and its reliability when used in the application. For instance, if the application demands a component moves back and forth during operation, such as the head of a printer does, a flexible circuit must supply it. Most wearables are shrinking in size, and HDI technology is most suitable for the rigid-flex boards they use. At this stage, the designer selects the material for the PCB design. The complexity of the electrical circuit that goes into the design decides the number of layers on the PCB. With the area available on gadgets shrinking as they tend towards miniaturization, the density of PCBs also increases proportionately. The only option for the designer is to have multiple layers on the PCB to contain the design within the specified mechanical boundary. Depending on the nature of the application, the designer has to decide on the stackup, or the design of consecutive layers. For instance, if the application has high frequency circuits, the designer has to define the impedance and minimize crosstalk. For this, he may have to use power and ground layers alternately, with traces carrying signals in between. While doing this, the designer has also to decide on the width, spacing, and routing of traces, placing of vias and test pads, and more. Once he/she completes the design process, the designer produces an output in the form of standard documentation, which helps the manufacturer fabricate the specified design. This documentation can follow either the Gerber X2 format or the IPC-2581 format.

Selecting Suitable Material for Printed Boards

Material for Substrate

The most widely used material for rigid boards is based on glass fiber known as FR-4. Being reasonably priced, FR-4 also has an appropriate degree of stability under temperature variations, and does not break down easily. Other cheaper materials such as paper phenolic are also available and used for low cost commercial products. On the other end of the spectrum are substances such as Teflon or PTFE used for substrates, offering very low losses and a stable dielectric constant for high performance high frequency designs. Flexible, rigid-flex, and HDI PCBs typically use Polyimide based substrates, although Teflon and Kevlar are also used for high frequency and high performance applications.

Material for Cladding

A PCB requires copper traces and planes, and these originate in the form of copper cladding on the substrate—a thin sheet of copper bonded to the substrate. The designer can specify the thickness of the copper cladding, as the manufacturers make them in a few standard thicknesses. Selecting the copper on a circuit board is also critical to reaching and maintaining desired performance levels, especially for adhesion to the substrate and performance at high frequencies. Commercially, manufacturers use two types of copper foils for PCBs—Electrodeposited (ED) copper and Rolled-Annealed (RA) copper. The two types of copper foils have different forming processes, and undergo different treatments for improving and preserving adhesion to various substrate materials. For instance, formation of ED copper follows electrical deposition of copper on a slowly rotating polished stainless-steel drum, placed in a solution of copper sulfate. The copper foil is removed in a continuous roll, with the side against the drum providing the smoother finish. On the other hand, ingots of solid copper successively passing through a rolling mill produce RA copper foils. The two types of copper possess different qualities. ED copper used on PCB substrates are useful for applications where mechanical stress may be critical, while RA copper is suitable for applications involving thermal shock. When subjected to thermal cycling conditions, PCBs with ED copper may develop cracks in narrow conductors. The grain structure of PCB copper varies according to its manufacturing process. For instance, manufacturers offer ED copper foils in coarse, moderate, and fine grain structures, and is linked to the finish on the copper surface. Although a copper foil with a coarse structure also has a coarse surface finish, and enables a stronger bond between the copper foil and the dielectric laminate, it typically exhibits a greater insertion-loss performance at higher frequencies. Additionally, skin effect at higher frequencies worsens the insertion-loss performance of the rough surface of copper traces on a PCB.

Multilayer PCB Fabrication Process

The actual process for PCB fabrication can begin on receipt of the necessary documentation from the designer regarding the proper choice of materials for the substrate and cladding, the number of layers and stackup, the mechanical layout and routing. The documentation must have individual details for each layer of the PCB.

Preparing the Central Panel

The fabrication process starts with obtaining the copper clad board, with the specified substrate material and copper cladding. For a multilayer board, the cladding will be on both sides of the substrate, and this forms the innermost or central layer. Usually, such copper clad boards come in large sizes of standard dimensions, and the necessary panel of a size matching the mechanical layout specified has to be cut out by shearing the board. Depending on the individual size and total number of PCB units to be made, the panel may have to be dimensioned to hold multiple PCB units. The copper cladding usually comes with a thin coating of protective layer to protect the surface from oxidation, and the protective layer must be removed by immersing the panel in a bath containing the solution of a weak acid.

Drilling and Etching the Central Panel

The panel is dried and is usually heated to remove excess moisture. Using details from the drilling files submitted by the designer, the fabricator proceeds to drill the necessary holes for the central layer. This starts with drilling the registration or locating holes on the periphery of the panel, and the CNC machine changes its drills to match the diameter for individual holes specified in the drill file. The registration holes are necessary for aligning subsequent layers. To obtain the correct pattern of tracks on the two sides of the panel, fabricators use a combination of a photographic process followed by a process of chemical etching. Typically, the copper surfaces on the drilled panel are covered with a thin layer of photoresist. Each side is then exposed to UV light through a photographic film or photo-mask detailing the optically negative pattern of tracks specified by the designer for that layer. UV light falling on the photoresist bonds the chemical to the copper surface, and the rest of the unexposed chemical is removed in a developing bath. This stage is usually supplemented with a visual inspection. When placed in an etching bath, the etchant removes the exposed copper from the panel. This leaves behind the copper traces hidden under the photoresist layer. During the etching process, concentration of the etchant and time of exposure are both critical parameters to obtain optimum results. Stronger than necessary concentration and a longer time of exposure can result in over-etching copper from under the photoresist, leading to tracks with widths thinner than specified by the designer. After successful etching, the photoresist is washed away to leave the necessary copper tracks on both sides of the central layer. Although the photographic process is very popular, other methods are also available. For low volumes or prototype PCBs, an etch-resistant ink may be transferred onto the copper surface using a silk screening process, with the silk screen representing the optically negative pattern of the tracks required. Another dry process uses a specialized, highly accurate milling machine to remove the unwanted copper from the surface of the panel. The machine uses inputs from the documents supplied by the designer to drive the automated milling head. However, as the process is time-consuming, it is suitable only for very small quantities of PCB.

Plated Through Holes

Holes in a PCB are necessary for connecting traces on one side to traces on the other. Sometimes, these holes may also be required to hold the leads of a leaded component, although this is becoming a rare requirement due to the availability of SMD components. Additionally, the PCB may require some holes to enable it to be mechanically mounted. To facilitate interconnection between the layers, PCB fabricators line the inner surface of the holes with a copper layer, using a plating process. On completion, these are called plated-through holes or PTH. A round of visual inspection and electrical continuity testing at this stage verifies the process. For single or double-layer type PCBs, the panel now goes for solder masking (coverlay for flexible PCBs) to cover those parts of the tracks that will not be soldered, and surface finish for the exposed tracks and pads. Multilayer boards will have further layers added on to the central or innermost layer.

Adding on Subsequent Layers

Fabricators add subsequent copper layers to the central layer with a layer of insulation in between each. For rigid PCBs, this insulation is usually the prepreg, while for flexible PCBs this is an adhesive layer. Fabricators add insulation and copper layers on to each side of the central layer, using heat and pressure to bond them together. The copper surfaces on both sides now undergo the same treatment of protective layer removal followed by drilling. Only this time, the drilling depth is controlled so that the copper on the inner layers remains undamaged. Eminent PCB manufacturers such as Rush PCB Inc. use ultrasonic or laser drills in place of mechanical drills for achieving greater accuracy and reliability. The same process of photoresist and etching follows as above, and this leaves only the traces required by the designer for the specific layer. The drilled holes are then electroplated to provide the necessary connections. Usually, a round of visual inspection and electrical testing to verify the process follows. If no further layers are required, the panel now goes for the solder masking/coverlay process. For additional layers, the above process is repeated.

Solder Mask/Coverlay and Surface Finish

Fabricators protect areas of the PCB not to be soldered by covering them with a protective layer. For rigid PCBs, this is the usual green layer of solder mask. However, this is a brittle layer, and not suitable for flexible PCBs. Therefore, flexible and rigid-flex PCBs use a polymer adhesive layer called coverlay for the purpose. The solder mask/coverlay protects the board from other contaminants as well, as the PCB goes through the assembly process. To enable leaded or SMD components to be added to the board by soldering, the solder mask/coverlay has openings at appropriate places, exposing the copper surface. To prevent the exposed copper from oxidizing, fabricators tin or plate them with solder, plate them with gold, or use other combinations of different metals to achieve a surface finish, as specified by the designer.

Silk Screen

The last step in the PCB manufacturing process consists of printing text and other idents on the PCB. Usually, this helps in identifying the board, marking component locations, and fault finding instructions. After a final inspection, the PCB is ready for dispatch.

Ten-Layer High Density Interconnect Board from Rush PCB Inc.

Written by Rush PCB Inc on . Posted in PCB, PCB Assembly and component, PCB Manufacturing

The advent of revolutionary new products, driven by miniaturization of components and semiconductor packages supporting advanced features, is driving the Printed Circuit Board (PCB) industry to increase the functionality of their boards within the same or reduced areas. This includes products such as the hand-held touch-screen computers, 4G network communications, and industrial and military applications such as smart ammunitions and avionics. Eminent PCB manufacturers such as Rush PCB Inc. are providing solutions for the above with High Density Interconnect (HDI) boards.

HDI PCBs use high performance thin materials as prepregs, have fine copper lines, and use the Every Layer Interconnect (ELIC) technology to offer very thin flexible PCBs with very high functional density per unit area. Advanced HDI PCBs make use of multiple layers of copper filled stacked in-pad micro-vias that enable interconnections with even greater complexity.

Rush PCB Inc. is currently building a 10-layer board with an FR4 grade prepreg (TUC-872SLKSP), starting with all layers of 1/3 oz. copper. The outermost layers of copper will be coated with Electroless Nickel Immersion Gold (ENIG), and covered with a coverlay (green mask) of thickness 0.0249” ± 0.003”. Component placement will be aided by a printed white silk screen, while the width and spacing of copper traces in each layer has been carefully calculated to give precise control on the impedance. The board has an overall size of 6.857” x 5.287” and its model number is 104-032-01 Rev 10. Rush PCB Inc. will be manufacturing 100 Numbers of this HDI PCB with a lead-time of 9 days.

Stack-Up Design

Before finalizing the design of multi-layer PCB circuit boards, designers need to confirm the structure of the circuit board primarily based on the scale, physical size, and the requirements of electromagnetic compatibility (EMC). Considering the above, designers at Rush PCB Inc. have decided to use 10 layers of circuit boards. This also decided the placement of the inner layer and the manner of distribution of different signals in these layers—the stack-up design of the multi-layer PCB. This careful planning and rational selection of the stack-up design beforehand will be saving the user a huge effort in wiring and production later.

Two major factors need to be decided once the designers have determined the number of circuit board layers. These are the distribution of the special signal layers and the distribution of the power and ground layers. However, with multi-layer circuit boards such as the 104-032-01 Rev 10, designers at Rush PCB Inc. followed some general principles to obtain the best combination of signal, ground, and power layers:

  1. The signal layer was kept next to an internal power or ground layer, shielded by the copper film of the internal power layer.
  2. To keep a tight control over the impedance, the internal power layer was integrated tightly with the ground layer, so that the thickness of the prepreg between the internal power and ground layers was kept thin, of the order of 2.00 Mils.
  3. To minimize crosstalk, no two signal layers were kept adjacent each other. As far as possible, the designers placed a ground layer in between two signal layers to avoid crosstalk.
  4. To control the ground impedance, designers placed multiple grounded internal power layers.
  5. The layer structure was designed to be symmetrical.

The final stack-up is shown in Fig.1. The overall PCB thickness is only 27.20 Mils or 0.69 mm.

1

Every Layer Interconnect Technology

To achieve very high-density interconnection, designers at Rush PCB Inc. have used the Every Layer Interconnect (ELIC) technology. This is a method where each layer has its own copper filled laser-drilled micro-vias. When stacked up, it provides the opportunity for dynamic connections between any two layers in the PCB. Not only does this offer an increased level of flexibility but also maximizes the circuit density. The designers took up the additional complex challenges in routing with Via-In-Pad (VIP) and employing blind and buried vias. Laser drills were used for drilling the via holes, and they were filled up with conductive copper paste.

PCB 104-032-01 Rev 10 uses a total of 32 sets of blind and buried vias between the following layers, as shown in Fig.2:

2

Impedance Control

Designers at Rush PCB Inc. have referenced the signals on the top and bottom layer to the ground plane next to them. Likewise, signals on other layers are referenced to ground planes adjacent to them. High-speed signal routing on an inner layer is sandwiched between ground and power planes. Careful design of trace width, spacing, and prepreg thickness has led to a tight control over single-ended and differential impedance as shown by calculations in Fig.3, Fig.4, Fig.5, Fig.6, and Fig.7:

Fig.3

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PCB Trends 2016

Written by Rush PCB Inc on . Posted in PCB

pcb trend 2016

The first concept for a printed circuit board (PCB) was developed in the 1930s. The concept became commercially viable by the 1950s, and since then, PCBs have moved into just about every phase of our lives. They surround us, and have changed out lives in many ways. In the 2000s, trace was reduced to 3.5-4.5mil, and Flex and Rigid-Flex PCBs proved innovative. The ability of engineers and designers to use PCBs to infiltrate our lives continues on an ongoing basis.

Moore’s Law suggests that the number of transistors in a dense integrated circuit doubles approximately every two years. Moore made this claim in in1965. In the early 1970s, the RCA1802 was state-of-the-art, and able to deliver a whopping 2,500 transistors. Moore’s Law was shown to be validated when the Pentium chip was released in the mid-1990s, delivering five million transistors (5×106). 2010 saw the release of the Quad-core Z195, with 1,000,000,000 (1×109) transistors.

Based on Moore’s Law, we can anticipate chips having the capacity to deliver 6 x 1010 transistors by 2020. If that is to happen, then miniaturization will continue, unabated. The PCB industry will be challenged in basic process capability and in material properties to allow for ongoing developments that deliver improvements ininterconnection density and electrical performance. Moore’s Law has shown itself to be valid for the last 50 years, but if this is to continue to be the case, new concepts will need to be incorporated into PCB design.

Some current innovations in PCB design provide some insights into how those developments are happening. Some new developments that are facilitating this include Every Layer Interconnected (ELIC); High Density Interconnection (HDI).

ELIC design uses a method of stacking microvias on every layer. It provides the opportunity for dynamic connection between anytwo layers in a PCB. The level of flexibility provided by such a design maximizes the area available for use in dense component placement. It also provides increased circuit density when designers are faced with complex challenges in routing.

HDI is based on continually reducing in feature size the spacing and conductor width of trace, micro-via diameter and pitch. The aim inevitability is to incorporate more components and layers without compromising the size, weight or volume of the PCB. The electrical performance of the PCB will continue to be challenged by increased wireless bandwidth and increased processing speeds. However, there are concerns regarding the cost-benefit of incremental developments. One challenge is ensuring dimensional stability as individual isolation layers are reduced in thickness to 50 microns or less. Electrical performance is also affected, as signal performance and resistance to leakage are challenged at this level.

In addition to the use of HDI and ELIC, innovations include high aspect ratio products, high performance pulse plating copper, and the application of plasma technology. As each of these technologies are incorporated into PCB design, miniaturization will continue, and Moore’s Law may continue to be validated over the coming years.