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Let PNC Simplify Your Printed Circuit Board Design With, CPLDs

New product designs continue to get more compact, while the performance and the number of features that customers expect continue to increase. To the engineer, this means higher PCB circuit densities and less room on the PCB for just-in-case design, such as unallocated I/O, or 0 ohm resistor networks to allow for reconfiguration of the PCBs at PCB assembly.

Meanwhile, new product prototype cycles are also getting faster. 3D printed mechanical parts are available within hours, putting pressure on electrical engineers to work faster and get their PCB designs right the first time. Even the fastest PCB fabrication, such as PNC’s 24-hour fabrication turn-time can’t help if the PCB has to be redesigned to fix errors.

The answer to both problems may be the CPLD. PNC’s CPLD programmers can help engineers reduce PCB size and allow on the fly circuit reconfiguration. Most people know that PNC specializes in fast PCB prototyping, but PNC is more than aPCB Manufacturer, PNC can speed prototyping by designing PCBs that replace inflexible circuit designs with PCBS that can be reconfigured to remap I/Os or change the order that circuit elements power up. A CPLD design developed by PNC can also allow the same PCB to be reconfigured to be used for the next generation product.

When it comes to programmable circuit elements, FPGA and microprocessors get all the good press. They are powerful, versatile, and generate more revenue for the manufacturers than workhorses such as CPLDS. Even though CPLD capability has improved dramatically over years, while both cost and power consumption have dropped, they are still often considered only for low level tasks such as “Glue Logic.” PNC designers can tell you that even a CPLD used for “low level” glue logic is appreciated when a late breaking design change means that two outputs now need to be two inputs, and one input needs to be inverted. All in a day’s work for PNC.

A PC Board Manufacturer, such as PNC can help you use these new, more capable CPLDs in places that can solve tough problems, replacing more expensive, complex and power-hungry solutions. Here are four examples.

I/O expansion

One of the most common CPLD applications is to expand the number of available microprocessor I/O ports. The CPLD I/O can either be multiplexed to the microprocessor or controlled via a serial interface. The advantage of a serial bus interface is that it allows you to locate this extra I/O anywhere, even on another Printed Circuit Board through a compact two or three pin connector.

The CPLD combinational logic architecture allows the creation of either a big fan-in or fan-out (over a hundred ports in some cases), and the outputs have enough current to drive small LEDS, a great way to create an array of circuit status LEDS.

When the CPLD output is used in conjunction with a CPLD’s internal clock the CPLD can also drive multiple PWM outputs allowing it to control things such as LED brightness, cooling fan speed, and simple sound producing devices.

The CPLD’s architecture gives it another useful capability for I/O expansion, the ability to accept inputs and drive outputs at different voltages. This multi voltage capability is often utilized for another common application; the communication bridge.


CPLDs are often used as a bridge between one or more bus protocols, potentially at different voltages. They can support

  • serial to serial
  • serial to parallel,
  • parallel to parallel

They can even be used to drive an LCD. Because of their simple architecture, they have a low pin delay, making high speed synchronization possible.

Power Management

Another one of CPLD’s features is that they retain their programming and will boot within 500 µs. This means that the CPLD is the first programmable element to wake up on power up, so that it is awake and ready to manage the power up of power supplies and programmable devices ensuring they start in the right order.

Safety Systems

Because of the CPLDs simple architecture and 100% deterministic behavior CPLDs are often used in safety critical systems. One example application is to monitor interlocks, ensuring that the system is in a safe condition before the system can begin operation.

CPLDs pack a lot of capabilities into a compact package, they can reduce PCB complexity and allow reconfiguration on the fly. If you have never considered a CPLD in your design, the designers at PNC can help you with the CPLD circuit design, CPLD programming and Circuit board fabrication. Talk to PNC today.

Printed Circuit Board thickness considerations and requirements

How Do You Select a Printed Circuit Board Thickness?

Selecting the correct PC Board thickness for your product requires balancing of three often competing aspects of the design: manufacturability, electrical performance and mechanical constraints. Modern PCB fabrication techniques at PNC give the PCB designer great flexibility in specifying the PCB lamination stack-up and gives them the option to design a PCB in a thickness other than the typical choices of .031”, 062” or .093”.

Manufacturing considerations in selecting a PCB thickness

To understand how to specify PCB thickness, it is important to understand how PCBs are fabricated. Most multilayer PCBS from 2 layers to 40 layers are constructed of these three basic materials.

  • Core – a fully cured fiberglass panel usually with copper foil on both sides. It is essentially a two-sided Printed Circuit Board. PNC stocks cores in a variety of materials and thicknesses, down to 3 mils thick.
  • Prepregs – fiberglass sheets impregnated with uncured epoxy resin. This resin will cure and harden when subjected to heat and pressure during the lamination process. It is the functional equivalent of double-sided tape. PNC also stocks prepregs in a variety of materials and thicknesses.
  • Copper foil – used to create conductive layers. Copper foil thickness is chosen by the amount of current the traces in each layer of the board will need to carry.

Multilayer boards can be manufactured in a variety of thicknesses by mixing standard core thicknesses with standard prepreg thicknesses. The lower limit of PCB thickness is set by the number of layers and the minimum available core and prepreg thicknesses. Copper foil thickness also plays a small role in overall PC Board thickness.

Providing you want to stay with the standard thicknesses, I’ll give you an example of PNC’s standard core thicknesses based off the core copper thickness. Let’s take an .062 and compare the stack-ups. See Figure  below:

Stack-up for an .062 multilayer:
062 thick PCB (002)




Notice to achieve the .062 overall thickness, the core thickness needs to be compensated based on the copper weight of the design. When using 1 oz copper we would use .038 core and .035 core for 2 oz. Also, the amount of pre preg would need to be adjusted as well to get to the thickness required. This is just one example when trying to determine overall thickness of your PCB.

The maximum PCB thickness is usually governed by something called the drill aspect ratio, which is the ratio of a drilled holes depth vs its diameter. When a hole is drilled, the deeper the holes becomes the harder it is to guide the drill accurately (because the drill deflects as it is pushed through the material) until you eventually reach a limit where you can no longer guarantee that the resulting drill hole will be centered in all the pads through the PCB. This is also referred to as drill wander in the PC Board fabrication world.

The aspect ratio for a through hole is the (board thickness) / (drill hole diameter).

Typically, the ratio is limited to approximately 10:1 This means that for a .062” thick board the smallest through-hole drill size is .006” which is the minimum drill size available at PNC. For a .093” thick PCB the smallest drill hole size will be .009” As board thickness increases, more complex via creation techniques such as laser drilled microvias are required to maintain the required pad density under components such as BGAs.

PCB thickness considerations for high speed circuits

One of the assumptions an electrical engineer makes when designing a circuit is that the PCB itself does not contribute any impedance to the circuit. However, in high-speed PCB design the integrity of the signals is definitely affected by the physical characteristics of the PCB.

Unfortunately for the designer the primary sources of parasitic capacitance and inductance of a PCB are affected by board thickness in conflicting ways, forcing the designer to carefully consider the trade-offs.


PCB vias can introduce both inductance and capacitance to the high-speed circuit. Parasitic inductance and capacitance of a via both increase proportional to board thickness.

Capacitance between layers
The capacitance between traces on different board layers or between traces and the ground plane are inversely proportional to the thickness of the dielectric between them. More space between layers will reduce parasitic capacitance and ensuring that all high-speed signal traces are the same distance from the ground plane will help impedance matching between them.


Crosstalk between signal traces can be minimized by routing the PCB traces further apart and reducing the dielectric thickness between PCB trace and reference plane.

Mechanical Constraints

A PCB is ultimately a mechanical component of the product and is therefore subject to a variety of mechanical constraints. The PCB or PCBs must fit within the product envelope. This often requires that a PCB be as thin as possible. On the other hand, PCBs are subject to the torqueing forces from cables connected to the board edge and external connections. Preventing board failure during assembly or in use requires a circuit board fabrication thick enough to withstand these forces.

Finally, PCBs in the field are subjected to both vibration and shock. For example, large PCB panels can vibrate in several different modes when subjected to vehicle vibration, causing fatigue and eventual failure of solder joints. The rule of thumb is to get the resonance modes of a PCB to be 10X the input vibration. This requires designing thicker, stiffer PCBS and by careful placement and design of the PCB mounts.

These are the three most important considerations in selecting a PCB thickness. To optimize a PCB design, the PCB designer needs to understand all of these requirements and constrains on the PCB lamination stack-up, and have the flexibility to choose a PCB thickness that balances these often conflicting constraints. The team at PNC can help you design a PCB that meets your needs at a competitive cost and lead time.


Ensuring a successful Turnkey PCB Assembly project

There are many detailed factors involved when pursuing the right company for your electronic or PCB assembly needs. These factors can be broken down into two distinct areas, customer communication and supplier contract review. Either the customer or the supplier cannot afford time lost if there is a misunderstanding or lack of data to efficiently and effectively produce a quality product on time. Time spent up front makes for a smooth and efficient transition through the quoting and manufacturing process.

Customer communication

A majority of communications for a request for quote, RFQ’s, in today’s industry are via email. The email needs to contain the required data files and be clear and concise in regard to quantities and delivery dates, along with any details that are not stated on the fabrication/assembly drawings. Since we are talking about Printed Circuit Board Assembly Turnkey projects, let’s break this down further with the required data files for PCB and PCBA.

PCB data files:

1- Fab drawing with build details such as material type, thickness, Copper weight, Tg rating, IPC-A-600 Class, Stack-up, Drill Chart, LPI & silk screen color, Serialization, Panelization array, MIL Spec, final finish and type(RoHs/Non RoHs) etc.
2- Complete set of gerber files.
3- Drill files.
4- IPC-356 Netlist for electrical testing.
5- Read me file for additional information not stated in fabrication drawing or email.

PCB Assembly data files:

1- BOM with manufacturers part number/description and alternates if applicable or DNP’s.
2- Assembly drawing with build details, Solder paste requirements, torque specs, IPC-A-610 Class, DNP’s, serialization, etc.
3- Pick & Place file.
4- ICT or Probe testing if applicable.
5- Functional test procedure if applicable.
6- Read me file additional information not stated in fabrication drawing or email.
If all the required information and data files are complete, we have successfully met the first half of the RFQ process. With this in mind, it’s up to us to compile this information in our contract review process. Let’s take a look at what is processed on our end to complete the RFQ cycle.

Supplier Contract review

All incoming turnkey projects are given an internal number for uniqueness especially for part numbers that has been revised. They are stored in a secure file folder based on two groups of data. ITAR data is stored separately than non-ITAR data. Once the customers data is stored and secure, engineering is notified to start to contract review process for the PCB and PCBA data sets.

Contract review for PCB:

1- Gerber files are imported and overlaid into correct layer structure.
2- Drill files are imported and overlaid against the gerbers.
3- If there is no IPC-356 Net list file, we extract the net from the gerber.
4- The gerbers are ran through a design rule check for manufacturability.
5- Fab drawing is reviewed by engineering for manufacturing capability.
6- If any discrepancies are determined, customer is notified immediately, If no discrepancies, engineering hands off the internal contract review check sheet to customer service.

Contract review for PCB Assembly:

1- BOM is scrubbed to ensure all parts are identified by manufacturer and P/N.
2- BOM parts stock research from approved vendor list.
3- Assembly drawing reviewed by engineering for assembly capability.
4- Pick & Place file review.
5- Review for testing if applicable.
6- If any discrepancies are determined, customer is notified immediately, If no discrepancies, engineering notifies customer service.
7- Quoting team is notified to officially create the quote and send to customer.
Customer communication and supplier contract review is a relatively simple step in order to achieve and ensure a successful assembly turnkey project. Adhering to the steps above can make for a great partnership.