High speed Designs - Part 3

Transmission lines is the first term when you hear while starting to work on high speed designs. Transmission line In PCB terminology, transmission line is a trace that connects various chips on the board. The transmission circuit is generally visualized as RLC circuit. The frequency response of any circuit depends on the R-L-C elements and they become predominant when used at high frequencies. 

Traditionally, engineers used interfaces like SPI, I2C, UART which are low speed interfaces. These interfaces didn't have issues with transmission line effects of the PCB traces. Unless the signals are routed over permitted lengths, there is no issue with maintaining the integrity of the signal. As signal frequencies increased, beyond 100MHz, with interfaces like Gigabit Ethernet, DDR, PCIe, etc the transmission line effects have to be considered. We generally read in design guidelines that the signal can't be routed for example, more than 1 inch and also the signal should have a recommended routing in the PCB, etc in the case of high speed signals. So, for a high speed signal even if you route shortest but don't take care of the recommended PCB routing guideline, you might end up with signal having signal integrity issues. Some of the major issues that we come across while working with high speed designs are:

Impedance mismatch

Reflections - overshoot, undershoot, ringing

cross-talk

Radiation

The basis of signal transmission comes from Maximum power transfer theorem of electrical circuit. This theorem states that the maximum power will be transferred from source to load when the source impedance matches the load impedance. If there is a mismatch, then power transferred to load gets reduced which meant there is more loss. In high speed design, when such scenario comes up, the source tries to send the signal to load and load will not be able to absorb the complete signal. Some part of the signal is reflected back to the source. This is what causes the reflections on the PCB. the signal when reflected back travels to source is reflected as there is a mismatch again. The signal hence forth travels between source and load while degrading over time. When these occur, the reflected signal adds/negates with the original signal causing the actual signal amplitude to increase/decrease. This is what we call undershoot/overshoot in high speed domain. 

PCB Fabrication and SMT Assembly - 1

Listed are the few tips for your PCB fabrication and assembly.

CEM-1, CEM-3 as base material for PCB is low cost and not widely used now. this is not used any more. Following are the current material categories for the printed circuit board: 

• FR4, 
• High TG FR4, 
• Halogen Free Material, 
• Aluminum based material, 
• Copper based material, 
• High frequency material,
• PTFE material
• Heavy copper foil
• Paperphenolic plate
• BT
• PI
• Composite material
• PTFE + metal based
• Roger HF material

2. When finished copper thickness is equal to or more than 3 OZ, immersion gold as surface treatment is advised.

3. If the solder mask color is white, the PCBA boards may need washing.

Our PCB Tech. has good pre-sale design service and also good after-sale service. Please, do send your queries using below link:

https://ourpcb.wufoo.com/forms/instant-quote-form/

 ----- This post is contributed by Lillian from OurPCB Tech. Ltd

Conformal coating for PCB

Engineers who worked on hadware boards which have to operate very low temperatures might not have come across conformal coating requirement. But who work on marine, defence, military, aerospace domains and hardware board designers who worked on board designs which have to work up to high temperatures and humidity might have used conformal coating. Now a days we can see conformal coating being used very extensively across other domains also (Telecomm, industrial, automotive, instrumentation). We can say that where there is a need for reliability and high protection conformal coating of PCB is a must.

Conformal coating is a chemical coating on top of the assmebled PCB which helps electronics to get shielded against harsh environments. If you have seen some relays, transformers available in the market, they have a plastic enclosure for internal electronics with a gel compound filled for protection against shock, vibration, stress, thermal, corrosion, moisture. This is what we call "plotting". You can see the huge black ics in the below figure.

Plotting is an expensive technique and conformal coating helps to achieve the protection simpler. The also disadvantage is that specific temperatures are used to form the protective layer which may cause the electronics to loose bonding. Conformal coating also adds dielectric strength to the PCB. Also, conformal coating ensures board reliability. Conformal coating can protect electroincs boards against the following problems:

• Temperature (which we call thermal cycling in environmental tests)
• Humidity
• Moisture
• Mechanical stress
• Dust accumulation
• Fungus formation
• Corrosion because of salt spray
• Shock & Vibration
• Chemical contamination

There are specifications available forconformal coating guidelines especially in military and aerospace. For example, MIL-1-46058 is one such sytndard. Silicone is the most commonly used conformal coating material used for protection against high temperatures.

What are the various other conformal coating materials available?

Silicone, Epoxy, Acrylic, Urethane, Parylene. Urethane is the most widely used.

Once conformal coated, can we remove it?

Yes, it can be removed and there are several techniques available. Approach the manufacturer to get to know the type of material used and technique to remove coating. Acrylics are the easiest to remove.

What care need to be taken for resoldering a component on already conformally ciated PCB?

Once conformally coated, very high temperature must be applied by soldering machine to remove that coating. This may cause SMT pads to get lifted. In susch cases, careful attention must be provided while aplying temperature beyond certain range. Also, once rework is done take care that conformal coating residue is not present on the board which can give a problem in the long term. Take care of ESD which performing such procedures for removing conformal coating.

What can be the thickness of conformal coating?

Have seen a conformal coating of thickness up to 50um.

What are the advantages of conformal coating?

• Eliminates need for costly mechanical enclosures
• Less weight
• Easy removal

What are the techniques used for conformal coating?

• Brush
• Spray
• Dip
• Automated

What care need to be taken for conformal coating?

Eliminate any surace defects. This means you have to equally spray without any gaps

Chose the conforming material as per the application and requirement

Choose the conformal coating technique as per expertise available

The drying process after coating must be simple. Can be air dired, normal dried or UV dried.
Coating material complies to industrial standards and is environmental friendly

How can suraface defects be detected in conformal coating?

Using UV light inspection