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…

Use A Chassis Ground

Similar to grounding yourself and a computer tower case before you start pulling things out, you can ground the external casing of your PCB board. By allowing your board and chassis to share a ground, you can improve the grounding of the entire system. One of the easiest ways to implement a chassis grounding is to include a “chassis screw” that connects the ground plane to the chassis. However, you need to make sure you’re using adequate standoffs so other components aren’t crushed or shorted to the casing when the PCB is screwed in. 

Additionally, grounding with a chassis screw makes ESD protection circuits more effective when you’re using transient voltage suppression at inputs. Remember, you’ll want to separate the chassis ground from the digital and analog ground by using inductive components. That way, a discharge into ground won’t accidentally be shared with all your other components.

If you are designing high-speed circuits, you know that they are always more difficult to optimize for performance. This is especially true if you’re routing across multiple ground planes, like a chassis and a PCB layer. The best case scenario is that you can connect the chassis ground directly to an earth ground. If that’s not an option, you should keep all of the ground planes tightly coupled to each other. That will help to minimize any “ground shifts” around key components.

Lead Free HASL

Although lead free HASL (LFH) has some issues to overcome to modify its tarnished reputation urgently, this not so popular surface finish is consistently gaining popularity amongst customers making manufacturers give it a second look as a main staple up against immersion silver, OSP, and immersion tin. But why? In this article, we’ll say something about LFH.

Actually, LFH needs to be applied at a hotter temperature. On the first pass the surface is left grainy and dull. Once a second pass is added the surface and appearance improves to a shiny, much flatter, and smoother even coat. 

After a lot of changes to chemistries and processes lead free HASL is now a stable surface application to use on PCBs. This longtime reliable surface finish is still used today in military, aerospace, medical, and other applications.

ENIG, OSP and even immersion tin and silver all took the lead in manufacturing while lead free HASL was still a process headache. However, as technology develops and continues to do so, real estate on surface becomes tighter and foot prints decrease, it is clear LFH still can prove itself to be a winner.