Posts Tagged ‘pcb manufacturing’

How does Conductor Surface Roughness matter?

Written by Admin on . Posted in PCB, PCB Manufacturing

In RF circuit design, it is necessary to select the proper Printed Circuit Board (PCB) material for the application. In this context, modern simulation tools such as Computer Aided Engineering (CAE) help engineers by predicting the electrical behavior that circuits exhibit on various types of PCB materials. The tools use material parameters in their calculations, with dielectric constant (relative) being one of the important parameters. However, most such tools overlook an equally important material parameter of PCBs during the process of design—the roughness of the conductor’s surface. Contrary to popular belief, conductor surface is never perfectly smooth, and this has consequences in high-frequency PCB design.

Conductor Surface Roughness

Eminent PCB manufacturers such as Rush PCB Inc. understand the influence an imperfect conductor surface has on the performance of a PCB. In fact, scientists have since long studied the effect of grooves present on the conductor’s surface on a PCB, having noted the increase in losses caused by the grooves. Under worst-case scenarios, the grooves caused losses that were twice the original. The explanation the researchers extended was electromagnetic (EM) waves travel mostly along the conductor’s surface, such as along the copper signal trace on a PCB. The grooves actually cause the signal paths to become longer, as the EM waves, while traveling along the surface, have to enter into, and then exit out of the grooves.

Skin Effect

The surface roughness of the conductor thereby causes the EM waves taking a path with a longer mean resulting in the increase in losses. Effectively, higher the degree of surface roughness of the conductor, higher is the resistance from skin effects. When an EM wave propagates in a conductor, skin effect tends to change the EM wave’s current distribution to accumulate more towards the conductor’s surface rather than remaining deep inside the conductive material.

When using EM simulators or other commercial CAE tools, designers overcome the surface roughness of a conductor by relying on the traditional Morgan Correlation. They do this by involving a numerical factor for correcting the surface roughness, depending on the ratio of a smooth surface to a rough one. While calculating the loss of high-frequency microstrip lines, using Kr mostly does a good job of matching closely the results of measurement of conductor losses. However, there are cases where the measurements fail to match the computer predictions so closely.

Furthermore, such deviations between the calculated and the measured values can be expensive at the design stage, especially as achieving the desired performance requirements can lead to additional design iterations. Avoiding such delays in design might mean considering carefully the choice of an RF PCB laminate based on its conductor’s surface roughness.

Types of Copper Cladding

Manufacturers have to use some form of a cladding of copper conductor on the PCB substrate. Three types are most common—electrodeposited (ED) copper, rolled-annealed (RA) copper, and reverse-treated (RT) copper.

Forming RA copper foils involves rolling the copper ingot through a rolling mill, where subsequent passes through the rollers of the mill results in a copper foil consistently thin.

ED copper formation requires depositing copper onto a slowly rotating, highly polished drum made of stainless steel, within a bath containing a solution of copper sulfate. While the roughness of the copper surface where it meets the stainless-steel drum is analogous to that of RA copper, the copper surface of the deposition side facing the solution is much rougher.

RT foil production starts by plating the ED copper foil on the drum side, when the foil on the bath side is still low profile.

As the copper foil has to adhere to the dielectric material, which may range from FR-4 to polytetrafluoroethylene (PTFE) substrates, the copper surface has to be treated to increase its adhesion. The reason being a reasonably smooth copper surface does not adhere ideally to the dielectric. Whether formed by the RA or ED processes, an untreated copper film has a surface typically covered with tiny teeth-like imperfections, and the jagged surface is perfect for forming a powerful bond between the dielectric material and the copper.

However, this is in direct contrast with the requirements of a good transmission line, as the rough surface is then not suitable for transmission of high-frequency EM waves. On the other hand, a surface with a mirror-like finish on an utterly smooth copper foil is inadequate for foil-to-dielectric adhesion.

That means fabricating PCBs with low-loss copper traces while keeping good adhesion between the dielectric material and copper depends on accepting a compromise in the surface roughness of the copper foil.

Effect on Dielectric Constant

Another important factor involving the design and manufacture of PCBs is the relative permittivity of the dielectric material—commonly referred to as its dielectric constant Dk. In reality, Dk, rather than being a constant, varies with frequency.

The value of Dk, as the dielectric manufacturer’s data sheets report, is often assumed as the intrinsic property of the material. However, manufacturers generate the effective dielectric constant using a specific test method, sandwiching the dielectric material between two copper plates. When comparing simulation against measurements, this often causes a discrepancy in insertion loss—caused by increased phase delay resulting from surface roughness.

The explanation for the above is that surface roughness decreases the effective separation between the parallel plates, thereby increasing the electric field strength leading to an increase in capacitance, and that accounts for the increase in effective dielectric constant.

Laminate suppliers commonly use a method called the clamped stripline resonator test method, described by IPC-TM-650, to measure the effective dielectric constant of their material. As the measurement is highly dependent on the test apparatus and the measuring conditions, it does not guarantee the values are accurate for design applications. This is mainly due to reason that the copper foils used for the test are not physically bonded to the laminate, leaving small air gaps in between the layers. This affects the measurement results.

Designers have to get around this mismatch during simulations by using a multiplication factor for the dielectric constant for their impedance calculations, rather than using the Dk directly, as published in the data sheets.

Commercial PCB Laminates

Recognizing the effect of surface roughness on PCB performance at high frequencies, suppliers offer commercial laminates and copper foils in numerous profiles. They produce these laminates with copper treatment at different levels. For instance, they offer PCB materials with low profile (LP) copper conductors that provide excellent adhesion between the dielectric material and copper, while the smooth conductor surface improves etch definition and reduces conductor losses.

Other suppliers offer materials with reverse-treated copper foils of low profile, which are suitable for high-frequency analog and digital circuits. They come in a variety of panel sizes and dielectric thicknesses, with 1- or 0.5-oz. cladding of reverse-treated ED copper in low profile. Two popular models of laminates have dielectric constants of 3.38 & 3.48, while their dissipation factors at 10 GHz in the z-direction are 0.0027 & 0.0037. Both the materials are suitable for high-density circuits and are appropriate for low passive intermodulation distortion, low insertion loss, and superior signal integrity.

Conclusion

Although special material can help overcome the effects of surface roughness of conductors at high frequencies, selecting a PCB material for minimizing the effects of surface roughness is not a simple task. When targeting to minimize the effects of surface roughness, PCB materials copper foils of lower profile will perform better at higher frequencies showing low conductor losses, rather than with materials using foils of higher profiles.

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

Written by Admin 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.

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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:

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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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Preventing Delays During Your PCB Fabrication

Written by Admin on . Posted in PCB Fabrication, PCB Manufacturing

pcb fabrication

Delays in your printed circuit board fabrication can be costly and time-consuming.  In today’s world, they are a commodity that there is never enough of.  Believe it or not, there are steps you can take to avoid those dreaded delays during your fabrication process.  We are going to discuss how you can take an active role in ensuring your printed circuit board fabrication runs smoothly and on schedule.

 

Communication

Follow up is an extremely important factor during the fabrication process.  Placing your project at the top of your “things to do list” is a must.  If you are communicating with your fabricator through email, or phone, you must be sure to answer any messages left in a timely manner.  Not answering them promptly can cause an interruption of the manufacturing, especially if a question is being asked or your guidance is needed.

 

Identify Long Lead-Time Parts

You may not realize it, however, some of the parts that are necessary for your project may take up to six months or longer to receive.  Working closely with your manufacturer as soon as you can will help them determine what they order and how quickly they can get it.  Printed circuit board manufacturing is not like shopping at your favorite grocery store, they will not always have the products in stock.

Also Read: Uses of Printed Circuit Board Components & Technology

 

Correct Documentation

You should always keep your product documentation up to date with the correct information.  Speaking with the manufacturer candidly to find out what their specific requirements are is the best way to understand what they need, this way there will not be any confusion.  This will also help eliminate any unnecessary mistakes that will inevitably cause delays in the manufacturing process.

Product Demand Information

Never wait until you are out of (or almost out of) your product.  This only increases stress, time, and cost.  The cost of shipping overnight is much higher than normal shipping methods.  Come up with a way to order your supplies before you run out to avoid additional cost, stress, and time.

Consider a Domestic Manufacturer

Although the cost may be less expensive, you may end up paying a higher cost in the end.  You want to consider the following;

  • Shipping prices will be higher with overseas manufacturers
  • Communication may become more difficult due to time differences
  • The difference in time may also result in the delay of delivery

Find a Manufacturer Who Can Handle the Entire

Manufacturing Process

A manufacturer who can provide their services from start to finish is the most beneficial option for your project.  This saves time because it cuts down on the amount of time it takes to get from one manufacturer to another, be worked on, and then shipped back to the original fabricator.  It makes smarter business sense to keep your product in one place from beginning to end so it can be shipped directly to you.  It also will stop the “blame game” if something is defective you know who is responsible.

Also Read: PCB Fabrication Making a World of Possibilities a Realty