Immersion Silver Flex-rigid PCB

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Oakley Mae

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Over the past few years, we have had a rise in the design and manufacturing of immersion silver flex-rigid PCB.

You might not know immersion silver is an environment-friendly finish on the circuit board’s copper surface that guarantees good component soldering. 

Surprisingly, the process involves depositing a thin silver layer by undergoing a chemical reaction between copper and silver ions.

And if you’re wondering about the cost, the process is more affordable and simpler than immersion gold. 

However, there are still many unanswered questions about immersion silver, including its advantages, disadvantages, storage, and why you must choose it.

This article focuses on all that to give you an idea of immersion silver flex-rigid PCBs. 

Table of Contents

What’s Immersion Silver?

Components on a PCB

(Components on a PCB)

An immersion silver treatment involves depositing a thin silver layer on the circuit board’s copper surface.

This treatment aims to guard the exposed copper pads against deterioration and oxidation.

Surprisingly, this process’s awareness relied on the potential difference between copper and silver to let them pass through some displacement reaction. 

Generally, the immersion silver treatment process is straightforward, easy, and consumes less time.

Moreover, you should add some organic matter to the treatment process to eliminate electromigration.

However, the quantity of this organic matter is mostly below 1%. Furthermore, flex-rigid circuit board fabrication and manufacturers employ immersion silver for aluminum wire bonding and EMI shielding.

In most cases, immersion silver has an average surface layer of 5 to 18 microinches. Also, they can last for as long as one year if you handle them properly.

Immersion silver vs. immersion gold (ENIG)

A flexible PCB on the surface

(A flexible PCB on the surface)

As we highlighted, immersion silver involves immersing the copper circuit board in silver ions. This is the best option for PCBs with EMI shielding and comes in handy for wire bonding and dome contacts.

Generally, the silver, in this case, has a 5 to 18 microinches thickness. 

Surprisingly, silver immersion is environmentally better than immersion gold. Additionally, it comes at a lesser cost. 

On the other hand, we have immersion gold, otherwise known as ENIG.

This option has two layers of metallic coating consisting of 2 to 8 microinches of gold over 120-240 microinches of nickel, which acts as a barrier to the copper surface. Also, gold offers low contact resistance and protects nickel during storage. 

Thanks to the implementation of the RoHS regulation, gold is now the most popular finish for circuit boards. Moreover, it’s more durable than immersion silver.

However, you’ll find it more expensive than immersion silver.

Common Problems and Solutions During Immersion Silver

There are common problems you’ll encounter during immersion silver, and we’ll discuss them below:

Normally, copper acts as both the cathode and anode.

Therefore, the reduction of silver ions and oxidation of copper happens simultaneously. Consequently, a silver-plated layer forms on the copper surface. 

However, any gap between the copper circuit and solder mask limits the silver ions supply in the gap.

This way, copper becomes a sacrificial anode, thus offering the electrons for the silver ions reduction reaction. 

Surprisingly, you can reduce and eliminate the galvanic effects risks by doing the following:

  • Ensure the micro-etching is within the required amount
  • Never connect small copper lines and large copper surfaces in the circuit board design
  • Use a less corrosive immersion silver process having the right pH. Also, avoid increasing the silver thickness. 
  • Employ a chemical-resistant solder mask and optimize imaging, curing, pretreatment, and developing processes to improve the bond between the copper surface and solder mask.

Solderability (IC Hole)

You might encounter IC hole solderability challenges thanks to hole wall quality. In most cases, the copper thickness is insufficient, or the hole is thick. 

Ensure the silver position is 100% copper to get the best immersion layer.

Also, during storage, ensure the PCBs are under a humidity and temperature of 40% and 30°C, respectively. And when you decide to use the circuit board, the one in-store should come out first. 

Why Choose Immersion Silver for Your Flex-rigid PCB?

A flex PCB

(A flex PCB)

Immersion silver is increasingly used to develop flex-rigid circuit boards. Check out some of the features below: 

  • One of the main functions of immersion silver is to allow the solder to spread evenly on the board. Therefore, it becomes easier to solder components. 
  • Designers can assemble fine components like pitch and BGA on the PCB with immersion silvers. 
  • Surprisingly, it’s environment-friendly thanks to RoHS-compliant technology and has no hazardous substances. 
  • With immersion, silver designers are able to implement multiple soldering reflows
  • Immersion silver is a superior technology with excellent electrical properties. Moreover, it has a flat surface and a low contact resistance. 
  • Immersion silver offers better conductivity. Therefore, you can comfortably use it where you need a high-frequency signal design. 
  • You’ll find immersion silver to be cost-effective, thus eliminating any cases of budget constraint. 

Disadvantages of Immersion Silver

  • You should solder components on the same day you remove the circuit board from storage.
  • It’s less durable than immersion gold, thanks to a lack of a nickel layer underneath.
  • If you handle it inappropriately, you can easily tarnish it. 

Handling and Storage of Immersion Silver Flex-rigid PCB

An engineer handling a PCB

(An engineer handling a PCB)

While handling immersion silver, we recommend using gloves to avoid contamination. Anything, including sweat, that drops on the circuit board can react with the silver and damage the board. 

Moreover, you should solder the circuit board within one day after removing them from storage.

You’ll know immersion silver PCBs are contaminated if they showcase black or yellow discoloration. Always keep in touch with a circuit board technician or expert any time you notice contamination. 

If you have stored your circuit board for a long time, you should conduct a solderability test before assembly.

This will require reflow and solder paste, and if you notice a complete wetting, the test board is alright. However, we have a solder paste specific to immersion silver surfaces. 

Finally, use only water to wash or clean immersion silver finishes. Also, store the surface boards in sealed containers to eliminate contamination. 

FAQ

Immersion silver vs. silver plating

Immersion silver involves depositing a thin layer on your circuit board’s surface. We restrict this process purely to circuit boards. However, silver plating involves covering a jewelry base metal like brass or copper using a thin layer.

This could happen on a jewelry metal material with a thickness of 1-10 microinches. 

How thick is immersion silver PCB?

Generally, immersion silver has a thickness of 5 to 18 microinches. Moreover, it comes with a shelf life of about 12 months. 

What materials are in rigid flex PCB?

All rigid-flex circuit boards have a substrate material, mostly polyester or polyimide. Also, we have a copper conductor material.

Sometimes, we add a thin zinc layer to improve copper longevity. Finally, adhesives create a secure connection between the copper and the substrate. 

Conclusion

As we’ve seen, immersion silver involves depositing a thin silver layer to protect the copper surface on your rigid-flex circuit board.

Consequently, your copper surface will experience less deterioration and oxidation. Surprisingly, the process is straightforward, easy, and consumes less time.

While implementing the immersion silver process, we recommend adding some organic matter to the treatment process to eliminate electromigration.