Tips for Flexible PCB Board Design

Before designing printed circuit boards, I almost unheard of flexible PCB board. But after several years’ experience, I have concluded sound design practice that is specific to flexible PCB is required to eliminate problems in fabrication and assembly. Here are some of them:

Avoid vias in bend areas-- Via should be avoided in bend as it can cause breakage on the flexible PCB when the stress applied goes beyond a threshold.

Clearance between components and flex area-- It is important to maintain a gap of at least 100mil for components and bend area. When placed on a bend area, the solder on the component’s pin became the weakest point as it is not meant to be bent.

Copper to edge clearance-- Like rigid PCB, you’re bound to have power and ground planes on Flexible PCB. It is important to have a decent clearance between the copper area and the edge of the flexible PCB. Neglecting this important guide could result in the copper shorting at the edges when the PCB is cut.

Ground plane design-- Cross hatch polygon is preferred for ground planes as it increased flexibility and reduce weight.Minimum hole size--Bear in mind that it is harder to drill on a flexible PCB than on a regular rigid PCB. Be sure to keep the hole size at a minimum of 10 mils for your design.

The IPC-2581 File Format

Eminent PCB manufacturers eliminate the problems by adopting design transfer standards that addresses all aspects of the fabrication and assembly process. Two new open standards are available, and these enable efficient and accurate data exchange from the PCB designer to the manufacturing fabricators and assemblers. Ucamco administers one of these standards, theGerber X2, while the IPC Consortium administers the other, the IPC-2581. Both are open standards, free from any proprietary restrictions.

Contributors from a wide range of PCB industry segment initiated, developed, and drove the IPC-2581 standard. The IPC-2581 is a single data format and within a single file, contains all aspects of the PCB design, such as layer stackup, materials, assembly, and test details.

With the IPC-2581 standard, the designer can include details of layer stack and information on materials to ensure proper layer order. The standard is suitable for stackups of complex board design such as related to rigid-flex boards, and is capable of handling special materials. It can also include drill and mill data for blind, buried, and filled via types. It also supports information on back drilling, V-grooves, slots, and cavities. For bare board testing, designers can include the net-list as well.

In addition to a complete set of fabrication data, the IPC-2581 can also hold assembly data. Therefore, it can contain not only the pick-and-place information, but also the information on polarity and rotation of a component, enabling support for both stacked and embedded components.

In addition to assembly drawings, the IPC-2581 standard has the capability to generate the documentation for bill of materials and purchasing. Therefore, the standard can tie up with PLM/ERP system data to create links between design and supply chain facilities. The greatest advantage of the IPC-2581 is one single file containing the entire data related to fabrication and assembly.

How does ESD Affect Your PCB Board?

Electrostatic discharge (ESD) occurs when two objects with different charges get close enough, or charged enough, to break down the dielectric between them. 

Any PCB may be subject to an ESD if it’s touched or comes close enough to people, packaging, cables, furry pets, or any other object that might contain an opposite charge. When they do touch, that voltage discharges and creates a comparatively massive voltage spike. As the voltage spike dissipates, the discharge current generates electromagnetic fields across the PCB. The goal of ESD protection is to minimize any impact or effects from the discharge.

In particular, many modern chipsets are made using such small lithography features that they have little or no tolerance for high voltage, even direct current values above their operating voltage of 3.3V. The result of an ESD event directly reaching one of these components is usually disastrous, completely ruining the integrated circuit.Nearly every element of your PCB design (traces, routing, layers, component placement, and spacing) can affect the ESD protection on your board. That means you need to consider ESD early in your design process; otherwise, you’re likely to require major PCB redesign to fix routing and component placement issues.

Why HASL and LFH are Concerned as a Hassle?

Hot air solder leveling, also known as HASL, has always been the major part of PCB surface finishes. As an excellent solderable finish for printed circuit boards, it has kept this surface finish alive in the market since late 80s. Given that PCBs are in everything from appliances to toys, it was almost instinct to find an alternative. Exposure of lead to children and overall health in general being a concern, getting lead out of products was the focal point of the electronic manufacturer.

It is obvious that lead may never be totally gone from all products, then the lead free version (LFH) existed. It became the most practice at surface finish next to immersion gold early on. So, why is lead free HASL still considered a hassle? 

The chemistry makeup of LFH has changed over the years as well as the applications. Vertical or horizontal applications both initially had the same issue as HASL, a pooling, non-flat finish with a somewhat foggy appearance in areas of the PCB.

Combinations of the LFH gave the finish a bad review. The combination of tin, silver, and copper alloy originally had poor results at the processing level, leaving behind a bumpy uneven coat that was dull and unattractive as well as having a poor performance in assembly. Removing the silver, changing the tin-copper, and tweaking the manufacturing process has allowed for a better smoother surface coating than originally found. 

Protect Your Ground Planes

For all the good a ground plane can do, it can also act as a direct path to your sensitive components if an ESD pulse discharges directly into it. To prevent this sort of damage, remember to use TVS circuits between the power and ground on sensitive components to divert the induced currents. When implemented correctly, the voltage differential experienced by components will be held to the clamping voltage of the TVS.

You can also use high-frequency bypass capacitors between power and ground on sensitive components. The capacitors will reduce the charge injection and voltage differences between power and ground. Keep those capacitors and your TVS close to the components you are worried about protecting.

Additionally, you should use a copper land when you are attaching connectors to your PCB. Make sure the land is separate from the PCB ground, or you just introduced a nice, low resistance path for ESD to reach all your components, even though you put all that other protection in place. And, in general, you should minimize path lengths whenever you can.

PCB Layout Tips

Hobbyists, students, even engineers tend to pay more attention to circuits, the latest components, and code as important parts of an electronics project, but sometimes neglect a critical component of electronics --- the PCB layout. As a Chinese saying goes, well begun is half done. Good PCB layout plays a great important role in your PCB projects, because poor PCB layout can cause function and reliability problems. 

Make Loops Small: Loops, especially high frequency loops, should be made as small as possible. Small loops have lower inductance and resistance. Placing them over a ground plane can further reduce inductance. Small loops also help reduce the amount of signals that are inductively coupled into the node from external sources, or are broadcast from the node. 

Decoupling Capacitor Placement: Place decoupling capacitors close enough to the power and ground pins of integrated circuits to maximize decoupling efficiency. Kelvin Connections: Kelvin connections are made at the exact points to reduce stray resistance and inductance. For example, Kelvin connections for a current sense resistor are placed exactly at the resistor pads. Placing the connections at the resistor pads may look the same on your layout, but real traces have inductance and resistance that could throw your measurements off if you don’t use Kelvin connections.

How to Choose Surface Finish

There are many surface finish types to choose when designing a printed circuit board. Do you know how to choose the best one for your electronics assemblies? EPCB lists some common but popular finishes with pros and cons for each.

Hot Air Solder Leveling    HASL has been around for many years. A thin layer of solder is mechanically applied to copper surfaces. A hot air knife is used to sheet the excess solder away to create a flat finish. It is cheap, and available from all PCB suppliers. What’s more, it has really long shelf life, and good resistance against oxidation and against handling.    However, its surface flatness is difficult to control. This could be an issue for components that are sensitive to surface flatness like BGA, fine pitch components, etc. The hole size may not be uniform which could be an issue where press fit connectors are used. And HASL is not suitable for RoHS application.

Electroless Nickel Immersion Gold   ENIG is a chemical plating process where a thin layer of nickel and gold is deposited onto the copper surfaces. The gold preserves the copper below and it remains solderable and conductive with very low contact resistance. ENIG’s surface flatness is excellent, suitable for BGA or other fine pitch components, and also available from all PCB suppliers. Like HASL, it has good shelf life and good resistance against oxidation and handling.    However, it requires a tightly controlled plating process to prevent excessive gold being plated, and not suit some high frequency or high-powered RF application. Besides, its processing cost is much higher because gold is used in plating solution.

To be continued…