Secrets of High Speed Printed Circuit Boards

Design for Assembly in Fast PCB Prototyping

In our fast-paced world, OEMs need to churn out new electronic devices very quickly to remain in the forefront of the market. For this, they require rapid PCB prototype services, which allow them to test their new designs thoroughly. Once they are ready to enter the market, OEMs need to tie up with a fast PCB production partner to fulfill the marketing demands. If the design requires high-speed printed circuit boards, the design house cannot afford the time to make trial and error, but must optimize the design on the first try. This ensures a smooth quickturn PCB production process. Therefore, the designer must start designing the board with assembly in mind.

Design for Assembly

Whatever be the type of PCB involved—rigid, flex, rigid flex, high density interconnect (HDI), or conventional—the bare boards will require assembly with additional components, before they are useful. Usually, the assembled PCB fits within a product or application, and overlooking this aspect of the assembly during design may ultimately lead to significant complications.

High Speed operation of PCBs requires the designer achieve the following:

  • Minimizing noise generation from the on-board power network
  • Minimizing cross-talk between traces
  • Reducing simultaneous switching noise
  • Proper impedance matching
  • Proper signal line termination
  • Reducing the effects of ground bounce

Board Material and Transmission Line Design

The dielectric construction material of the PCB is a major contributor to the amount of noise and cross talk the fast switching signals generate. A high frequency signal traveling along a long trace on the PCB could be affected seriously if the loss tangent of the dielectric material is high, resulting in high absorption and attenuation at high frequencies.

The modeling and effect of transmission lines also affects the signal performance and its noise separation. In general, any circuit trace on the PCB will have its characteristic impedance. This depends on the trace width, thickness, the dielectric constant of the PCB and the separation between the trace and its reference plane. Designers can route circuit traces on a PCB in two ways—in a microstrip transmission line layout or a stripline transmission line layout.

In a microstrip layout, the designer routes the circuit traces on an outside layer with a reference plane below it. The characteristic impedance of a circuit trace in a microstrip layout is inversely proportional to the trace width, and is directly proportional to the separation from the reference plane.

In a stripline layout, the designer routes the circuit traces on an inside layer of a multi-layer PCB, with two reference planes on either side. Here again, the characteristic impedance is inversely proportional to the width of the trace, and directly proportional to the separation from the reference planes. However, the rate of change with trace separation from the reference planes is much slower in a stripline layout as compared to that with s microstrip layout.

Designers for rapid PCB prototype services must be able to predict the characteristic impedance of their design if they are to get their design right the first time. Understanding the nuances of transmission lines helps with fast PCB production.


Minimizing Cross-Talk between Traces

While designing a high speed PCB, designers must take steps to reduce cross talk between neighboring signal lines, even when following either the microstrip or the stripline layout. Designers follow certain thumb rules to minimize the cross talk:

  • Utilize as much space between signal lines as the routing restrictions allow
  • Place the transmission line as close as possible to the ground reference plane
  • Use differential routing techniques for critical nets—match the length to the gyrations of each trace
  • Route single-ended signals on different layers to be orthogonal to each other

Routing two or more single-ended traces in parallel with not enough spacing will increase the cross talk between them. Therefore, designers prefer to minimize the parallel run, often routing them with short parallel sections, minimizing long, coupled sections between various nets.

Maintaining Signal Integrity

For high-speed boards, it is very important that the signal maintains its integrity, that is, it is able to keep its amplitude, and shape as it travels from its source to its destination. Signals may be single-ended, such as clocks, or may be differential, which are very important for high-speed design. For traces carrying single-ended signals, designers follow design rules such as:

  • Keeping traces straight as far as possible, and using arc shaped bends rather than right-angled bends where necessary
  • Not using multiple signal layers
  • Not using vias in the traces—they cause reflections and impedance change
  • Using the microstrip or the stripline transmission line layout
  • Minimizing reflection by terminating the signal properly

Designers follow additional rules for differential signals:

  • Minimize crosstalk between two differential pairs with properly spacing them
  • Maintain proper spacing to minimize reflection noise
  • Maintaining constant spacing for the entire length of the traces
  • Maintaining the same length of the traces as this minimizes phase and skew differences
  • Not using vias in the traces—they cause reflections and impedance change

Effective Filtering and Grounding

Conducted noise from the power supply can hinder the functioning of a high speed Printed Circuit Board. Since a power supply may deliver noise of high as well as low frequencies, designers minimize this problem by effectively filtering the noise at the points where the power lines enter the PCB,

An electrolytic capacitor across the power lines can filter the ripple and low frequency noise, while a non-resonant surface ferrite bead will block most of the high frequencies. Since the ferrite bead will be in series with the supply lines, its rating needs to be adequate to handle the current entering the PCB. Designers also keep provision for a decoupling capacitor very close to each IC on the board, to smoothen out very short duration current surges.

Effective power distribution throughout the PCB is extremely important for printed circuit boards operating at high speeds. For doing this, designers often use power planes or a power bus network. Power planes on a multi-layer PCB comprise two or more copper layers carrying power to the devices—typically, the VCC and GND lines. By making the power planes as large as the entire board, the designers ensure the DC resistance is as low as possible.

This offers multiple advantages to high speed boards—high current source and sink capability, shielding, and noise protection to the signals. For two-layer PCBs, designers often use the power bus network, which has two or more wide copper traces for carrying power to the devices. Although the DC resistance of the power bus is high compared to power planes, they are less expensive.

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As high-speed digital devices operate simultaneously, their fast switching times may cause a board-level phenomenon known as ground bounce. This is a very difficult condition to predict, as several factors may influence to the occurrence, such as number of switching outputs, socket inductance, and load capacitance. Designers follow a number of broad guidelines to reduce the effects of ground bounce:

  • Placing vias adjacent to a capacitor pad, and connecting them with wide, short traces
  • Using wide, short traces to connect power pins to power planes or decoupling capacitors
  • Using individual links to connect each ground pin to the ground plane, no daisy-chaining
  • Adding decoupling capacitors for each IC and each power pin
  • Placing decoupling capacitors very close to the IC
  • Properly terminating the outputs to prevent reflections
  • Buffering loads to limit the load capacitance
  • Eliminating sockets as far as possible
  • Distributing switching outputs evenly throughout the board
  • Placing ground plane next to switching pins
  • Using pull down resistors rather than using pull up resistors
  • Using multi-layer PCBs with separate VCC and GND planes
  • Placing power and ground planes next to each other to reduce the total inductance
  • Minimizing the lead capacitance by using surface mount devices
  • Using capacitors with low effective series resistance

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Rush PCB UK recommends designers follow the above design guidelines for delivering rapid PCB prototype services to satisfy customers. However, please note that all other general guidelines for PCB design are also important and designers should follow them meticulously for fast PCB production.


Best Practices for Assembly and Fabrication of PCBs

Best Practices for Assembly and Fabrication of PCBs

As an expert manufacturer and PCB assembly company, Rush PCB Inc. uses several best practices when working with industries including aerospace, consumer electronics, automotive, and many more. The best practices involve PCB design, technical aspects, and assembly issues.

Best Practices Start Early

Efficient fabrication of PCBs that high-speed circuitry utilizes is essential to ensuring results for the end user. Often, the design of the PCB layout is not thought of as a proactive step in the process. However, it requires advanced thinking and adherence to important factors to provide designs where the results lead to successful fabrication of PCBs to achieve the desired functionality. Designers need to address the practice of DFM or designing for manufacturing and including extra considerations for demands of high speed circuits early in the design stages of board layout rather than taking them on as an afterthought.

The results of poor layout show up during fabrication, assembly, and later on, as issues related to performance when putting fabricated PCBs to testing or production use. However, at that point of time, redesign or rework can be exponentially more expensive and time-consuming, requiring evaluation of circuit failures and reconfiguration of layouts of prototypes.

Material Handling

Best practices in the assembly process of any PCB assembly company start with material handling of PCBs, solder paste, and SMD components.

PCB Handling

Resin coated foils, prepreg, and core materials are susceptible to damage while handling. They need handling by their edges by operators using clean latex or nitrite gloves. Prepreg needs storing on a flat surface in a cool dry environment, preferably at less than 23°C and lower than 50% relative humidity.

If the room temperature of the PCB assembly services is significantly higher than the storage temperature, the prepreg needs to be acclimatized to the ambient temperature, prior to starting assembly. During acclimatization, the prepreg must remain in its sealed package for the stabilization period to prevent any moisture condensing on it. Any unused prepreg must be returned to their package bags and resealed. Therefore, it is best to package PCBs in brick counts that closely emulate run quantities. Prepregs must not be folded.

Some moisture is inevitably absorbed into the PCB material during the time the fabrication process is completed and start of exposure to the assembly soldering. Removal of this absorbed residual moisture may need baking the PCBs at 105-125°C for 4-6 hours.

Also read:  History of Circuit Boards

Solder Paste Handling

As solder paste is a shelf-life dependent item, it should be put directly into a storage refrigerator of the PCB assembly services on delivery, and stored as FIFO or first in first out manner, preferably with refrigerator temperature below 10°C. Preferably store solder past in lots, and ensure older lots are used first for optimal material management.

Manufacturers usually print the manufacturing date on each label and include a use by date for best performance of PCB assembly in UK. This must be strictly followed. Prior to use, equilibrate the solder paste to the environmental conditions where it will be used. For a jar or cartridge of solder paste, it is best to remove from refrigeration one day prior to use. This allows the solder paste plenty of time to equilibrate in the environment. However, this is not recommended for syringes.

Never expose solder paste to heat greater than 25°C for bringing it up to temperature fast. However, temperature-controlled water bath at around 25°C may be used. Whenever removing a container from refrigeration, label it with the date of removal for monitoring exposure.

Although homogenizing solder paste prior to use may not always be required, if necessary, stirring with a plastic spatula is recommended. Solder paste removed from the stencil must always be stored in a separate jar, rather than reintroducing it into fresh paste, as this can result in process inconsistency. Do not return solder paste to the refrigerator after opening the container, as this can cause condensation and compromise performance.

SMD Components Handling

While storing SMD components, it is essential to ensure they are kept in conditions that prevent moisture ingress and avoid electrostatic charge build up to prevent any damage.

While storing incoming SMD material, PCB assembly in UK such as Rush PCB Inc., use an ERP system help to keep track of information such as delivery date, order number, and material data. If unused material is returned to the stores, the ERP system can keep track of the used components, rejections, and damaged SMDs.


Best Practices for Screen Printing

Consistent stencil printing requires proper board support, typically provided bch as y vacuum tooling. Adequate paste must be used to enable a generous bead to roll freely when the squeegee moves. The squeegee pressure must be adequate to ensure a clean sweep without leaving paste on the stencil after a pass.

Enable proper gasketing to align the apertures with the pads properly. Ensure levelness of the board surface, and solder mask definition must not detract from contact between the stencil and the surface of the board.

Occasionally wipe the underside of the stencil to remove any excess paste. Although wipe frequency is recommended with the product data sheet, it also depends on the process optimization and proper gasketing.

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Best Practices for Reflow Soldering

For best results, the reflow soldering profile should be broken down into four phases—preheat, pre-reflow, reflow, and cooling.

The preheat phase allows preconditioning the PCB assembly prior to the actual reflow. It removes flux volatiles while reducing thermal shock to the assembly.

The pre-reflow phase uses the flux activator to remove any existing surface oxide from the PCB pad finishes, component leads, and any oxides on the powder particles within the solder paste. Basically, it prepares the surfaces to be joined during reflow. This phase also involves a temperature soak, allowing the thermal gradient across the PCB assembly to equilibrate prior to reflow.

The actual reflow of the solder alloy allows the creation of a suitable electrical and mechanical bond. Formation of an optimum bond involves two critical parameters—the peak temperature, generally 20-30°C above the liquidus temperature of the alloy, and the time-above-liquidus, typically 30-90 seconds to form the effective intermetallics.

To form a reliable mechanical bond, the grain structure should be fine, which can be formed via the cooling phase. A rapid cooling rate while transitioning from liquidus to solidus can stress the joint; therefore, a cooling rate of 4°C/second is preferable.

Best Practices for Handling PCB Assemblies

ESD is one of the major causes of failure of assembled PCBs. Therefor proper electrical grounding of worktables and operators is necessary. Worktables must have electrically conducting mats and workers must wear anti-static clothing, while being grounded with discharge straps on the wrists or ankles.

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