Essential Factors Affecting the Cost of Printed Circuit Boards Manufactured by Rush PCB Inc.

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

At Rush PCB Inc., the production of printed circuit boards (PCBs) involves so many diverse and varied possibilities that they complicate the process of estimating the factors affecting the cost of manufacturing them. Essentially, the considerations for the cost factor depend majorly on the various production strategies manufacturers use, different production equipment they employ, and the multitude of technologies available to them for creating the final product.

Irrespective of the factors responsible for the cost build up, it is essential to consider control of costs in the early phases of the PCB manufacturing process. This is because the cost of a PCB is designed into it, and it is impossible to reduce it later without re-design. Although additional process steps do add to the associated cost in terms of materials, consumables, process times, waste treatment, and energy, the process cost impacts the PCB price regardless of the manufacturer.

At Rush PCB Inc., costing in PCB fabrication depends on several variables that contribute to it through different percentage factors. The variables include the complexity of the board, the raw materials used for the fabrication, the equipment used, and the overall efficiency of the process. For ease of classification, dividing the cost factors into three categories leads to:

  • Significant cost factors — Category I
  • Important cost factors — Category II
  • Minor cost factors — Category III

It is important to note that important and minor cost factors related to Categories II and III depend on the equipment used for fabrication and are therefore, specific to the manufacturer.

Significant Cost Factors — Category I

Major factors contributing to the cost in category I include:

  • PCB size
  • Yield or effective utilization of material
  • Number of layers or layer count
  • PCB complexity
  • Materials used

PCB Size

This is a simple linear equation with the cost increasing linearly with the size of the PCB.

Yield or Effective Utilization of Material

Unless the unit PCB is of an excessively large size, manufacturers fit several units into a panel. Usually, panels are vendor specific and available in standard sizes such as 18x24”, 18x21”, or 21x24” and so on. Utilization depends on the area of the panel used for the PCBs, with 77% and above qualifying as good utilization. For high volume production of PCBs, panel utilization becomes one of the most critical aspects with respect to PCB cost.

Also Read:  Key Elements of an Ideal PWB Material from Rush PCB Inc.

Number of Layers or Layer Count

This is a simple equation with the cost increasing with the number of layers. Usually, more layers translate into additional costs because of more production steps, more material, and additional production time. However, this is not a linear increase, as converting a single layer PCB to two layers may increase the cost of the PCB by 30-40%, whereas adding two additional layers to a 10-layer PCB might increase the cost only by about 20-30%. This cost factor also depends on the complexity of the PCB design and differs from manufacturer to manufacturer.

The reverse also does not work out linearly. Reducing the layer count does not always reduce the overall cost, as reduction of layer count mostly involves more complex technology and aggressive design practices, both of which affect the yield. However, if the complexity and design remain constant, the change in cost of the PCB depends linearly to the number of its layers.

PCB Complexity

PCB complexity depends on the number of layers and the number of vias on each layer, as this defines the variations of layers where the vias start and stop on, requiring so much more lamination and drilling steps in the PCB manufacturing process. Manufacturers define the lamination process as pressing two copper layers and dielectrics in between adjacent copper layers using heat and pressure to form a multilayer PCB laminate.

As via structures majorly affect the manufacturing process, eminent fabricators such as Rush PCB Inc. optimize costs by using micro-vias and High Density Integration (HDI) technology. Not only does the HDI technology reduce the overall costs by decreasing the number of layers, it also makes the PCBs smaller, lighter, and thinner, while at the same time providing much superior electrical performance.

Material Used

The selection criteria for the materials used for fabricating Printed Circuit Boards depends on several application-based factors, governed mainly by frequency and speed of operation, and the maximum operating temperature.

These factors comprise thermal stability, temperature related reliability, temperature cycle reliability, heat transfer rate, time to delamination, and many others. In fact, the higher the frequency of operation or speed of the signals, more important is the choice of materials for fabrication of the PCB. For instance, compared to the regular Phenolic FR-4 material, use of Polyimide increases the cost by about 3-5 times, but use of PTFE based microwave materials may inflate the costs by nearly 10-50 times.

Important Cost Factors — Category II

Although the cost factors involving category II are greatly dependent on the involved manufacturer, important cost factors can be listed as:

  • Track and gap geometry—thinner is more expensive
  • Control of impedance—additional process steps increase costs
  • Size and count of holes—more holes and smaller diameters drives costs upwards
  • Plugged or filled vias and whether they are copper covered—additional process steps increase costs
  • Copper thickness in the layers—higher thickness means higher costs
  • Surface finish, use of gold and its thickness—Additional material and process steps increases costs
  • Tolerances—tighter tolerances are expensive.

Minor cost factors — Category III

These minor cost factors involving category III are dependent on both, the fabricator and the application of the PCB. They mainly involve:

  • PCB thickness
  • Various surface treatments
  • Solder masking
  • Legend printing
  • PCB performance class (IPC Class II/III etc.)
  • PCB contour—specifically for z-axis routing
  • Side or edge plating, if any

Conclusion

For estimating costs accurately, experts at Rush PCB Inc. recommend taking an existing design scope and making adjustments on the requirements based on estimated technologies. Such estimates offer more relative data points for making any cost per technology decisions. The estimates also prevent later surprises in the process once resources are committed to a design.

Key Elements of an Ideal PWB Material from Rush PCB Inc.

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

Printed Wiring Materials (PWBs) from Rush PCB Inc. are available in huge diversity and wide range of applications. This has made the concept of any ideal laminate very fuzzy, to say the least. For instance, a material suitable for a high temperature application may be impractical for high-speed digital design such as in a microwave. Therefore, although it is possible to list potentially important properties, each design team will need to establish a prioritization among them since no one material will present the optimal value for all properties that might be considered to be important. Essentially, Rush PCB Inc. presents some common ingredients or properties of PWB materials listed below, from which designers can make their choices during the design phase.

Tg—Glass Transition Temperature

Polyimides with Tg of 250°C or above are suitable for the highest temperature systems. Designers consider Tg as a rough indicator for total Z-axis expansion and hence, a proxy for reliability indication for plated through holes. For applications where either in-use temperature or process temperatures (or both) are less demanding, manufacturers offer a wide variety of epoxy systems with Tg in the range of 170°C.

Tg offers a good frame of reference for polyimide and traditional epoxy materials. However, it is not reliable for characterizing the reliability of non-traditional resin systems and highly filled systems used in high frequency and low loss applications where the PWB is a composite of various components.

Td—Thermal Decomposition Temperature

Depending on the chemical composition of PWB materials, this property may vary greatly form the mid 300°C range for several epoxy systems to over 400°C for some polyimides. When a PWB material reaches its Td temperature, it begins to degrade thermally. In general, data sheets list Td temperature as one at which the material loses 5% of the original weight due to decomposition.

However, the onset temperature at which significant weight loss of PWB material begins to occur, presents a better indicator of performance. The reason being by the time the material has lost 5% of its weight from decomposition, it might be totally unsuitable for any application.

Loss Tangent and Dielectric Constant

A signal passing along a transmission line on a dielectric material loses its power. Loss tangent is a measure of how much power the signal has lost. Dielectric constant is a measure of the speed of an electric signal as it travels in a dielectric material, relative to the speed of light in vacuum—the dielectric constant of space (vacuum) being defined as 1.00. Therefore, a higher dielectric constant for the PWB material implies a slower propagation speed through it.

Dimensional Stability

Etching causes shrinking in all laminate materials to some degree, while consistency of both process and product depends on the consistency in registration. A suitable PWB material would be one that shrinks minimally when etched, has consistent and reproducible shrinkage that allows predictable factors for artwork compensation. Ideally, the PWB material should not require any artwork compensation, and should always register properly, without requiring any compensation while drilling. The IPC test of dimensional stability is at best a measure of the actual registration of a specific board design, and registration continues to be a major fabrication concern for HDI and high layer count designs.

CTE—Coefficient of Thermal Expansion

Designers must match the expansion requirements of PWB materials to the expansion requirements of devices to be mounted on the surface, claddings, and the thermal planes buried in the interior. For instance, a CTE of 6 ppm/°C is ideal for leadless ceramic chip carrier attachments, and laminates such as Arlon’s 45NK woven and reinforced with Kevlar with low resin contents are suitable. Rush PCB Inc. tend to use other material also, such as nonwoven aramid reinforcements, copper-invar-copper distributed constraining planes, and quartz reinforcements for achieving values as low as 9-11 ppm/°C. This represents a substantial improvement over conventional polyimide or epoxy laminates and such PWB materials have proven consistent and acceptable in a variety of SMT designs.

Also Read;    Manufacturing Process of Printed Circuit Boards

Tc—Thermal Conductivity

With increasing density of components on a board, required for meeting the demands of improved functionality, as against a steady decrease of the overall surface area of PWBs, the watt-density of power the PWB generates also increases. As critical devices could fail at rates doubling for every 10°C increase of temperature, this pushes designers to use PWB materials with high thermal conductivity to remove heat directly from devices placed on the surface of the board.

However, designers must do this without allowing the board to suffer in terms of dielectric and electrical properties. While traditional polyimide or epoxy systems have thermal conductivity values between 0.25 and 0.3 W/m-K, Rush PCB Inc. targets thermal conductivity figures in the 1.0 to 3.0 W/m-K range, to achieve significant reduction in the board surface temperatures, especially near active devices.

Expansion in the Z-Direction

For the ideal PWB material, expansion in the Z-direction must match that of copper within the PTH to avoid damaging the plating inside the holes during thermal excursions in processes such as solder reflow. Typically, standard materials exhibit CTEs of 50-60 ppm/°C, when the operating temperature is below Tg, while this increases to roughly four times higher when the temperature crosses Tg. PWB materials with high Tg value, such as Polyimides, show a lower overall Z-direction expansion than typical epoxy systems do.

Compatibility with Lead-Free Processes

PWB materials compatible with lead-free processes need to withstand higher soldering and reflow temperatures associated with lead-free solder systems—typically 30 to 50°C higher than traditional lead-tin systems. Manufacturers usually characterize PWB material suitable for lead-free systems in terms of Tg>155°C, Td>330°C (for 5% decomposition), and overall CTE<3.5%.

This classification includes materials such as Polyimides as being lead-free compliant, while Rush PCB Inc. is using newer generations of epoxy systems to meet additional requirements. Lead-free systems are inherently more complex, as several current laminate materials can survive its applications, but some devices mounted on the boards may not even have been tested at the highest lead-free temperatures. Therefore, the trend is to use materials that have a margin of safety in the range of higher temperatures as seen during lead-free soldering.

               

Also Read;   PCB Testing: Why is it Important?

 

Green Prepreg and Laminate

Green PWB materials are typically compliant to UL-94 V0 flammability ratings specifically achieved without the use of brominated flame-retardants. Although most manufacturers use the brominated bisphenol-A for FR-4 systems, there is a movement towards non-brominated systems wherever possible, as these are more environmentally friendly.

Meeting Environmental Regulations

Several countries are now complying with regulations such as those modeled on RoHS and WEEE from the European Union, or are in the process of developing their own. The cost of compliance to multiple independent and different regulations becomes a significant part of the cost of a finished PWB.

Simple Processing

Ideally, manufacturers should be able to process the laminate and prepreg in simple ways using regular methods of photo-imaging, etching, processing through wet chemicals, and traditional techniques of lamination. High process yields and design flexibility demands the ideal material be used in multiple designs with high probability of success. Although most manufacturers still define normal processing as being suitable for the conventional FR-4 material, Rush PCB Inc. uses advanced materials in high performance PWBs that require more than the normal ideal processing.

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Manufacturing Process of Printed Circuit Boards

Written by Rush PCB 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.