Printed circuit boards (PCBs) require an extra finish that would enhance their characteristics. There are multiple methods available, but immersion silver is the prevalent one. Still, even experts may have difficulty performing the process or comparing it to other options available.
Let’s deepen into the immersion silver process, discover its advantages and disadvantages, review common problems, and compare it to ENIG.
What Is Immersion Silver PCB?
Also known as IM silver, silver plating, and ENIAg, it is one of the common finishes that can be applied to a conventional PCB. Approximately one-tenth of all the PCBs is made with this technique involved. And that’s a lot considering that nearly half of affordable components tend not to have a surface finish at all.
The integrity of a PCB’s surface is crucial. It is so because most of the external impact is applied to it. Immersion silver finish protects PCB, making it less vulnerable to oxidation, temperature fluctuations, mechanical stresses, etc. Mostly, the process is intended to make a PCB as long-lasting as possible.
Immersion Silver Process and Principle
Let’s split the method into two parts – the working principle and the application process and review them separately.
Silver Plating: Basics
So, it is the deposition of a thin layer of silver on the PCB’s copper foil through a chemical reaction. The formula is the following: 2Ag+ + Cu ═ Ag + Cu2+;
In simple words, copper is partly dissolved from the surface, releasing two electrons into the solution of silver. In its turn, silver, which was previously dissolved, takes electrons and returns to its metallic state, depositing on a PCB’s copper surface.
In case you wonder how much copper is replaced by silver. The thickness of the coating is only about 0.1-0.4μm. Still, it is more than enough for silver’s properties to be fully manifested.
Silver Plating: Application Process
Let’s get down to business and review the complete step-by-step PCB immersion silver process:
- Pre-treatment: including pickling, scrubbing, washing, and drying with hot air at 80°C.
- Loading PCBs in an acid-resistant bath.
- Acid degreasing with a 10% solution of acid degreaser at 30°C.
- Remove oxide.
- Clean fingerprints and pollutants left.
- Water washing.
- Pure water washing.
- Clean liquid remains.
- Micro-etching with 80 grams of sulfuric acid, 200 ml/L of 35% Hydrogen peroxide, and 250 ml/L of WCD-126 at 30°C for 1-2 minutes.
- Pure water washing.
- Clean and dry PCBs.
- Perform preliminary dip with 300ml/L of MT-IMG 500A and 30ml/L of MT-IMG 500B at 40°C for 1-2 minutes.
- Perform sterling immersion silver with 300ml/L of MT-IMG 500A, 70ml/L of MT0IMG 500B, and 10ml/L of MT-IMG 500S at 50°C for 2-4 minutes.
- Deionized water washing.
- Drying with hot 80°C hot air.
Once you are finished, your PCBs are prepared for being inspected and packed. However, you may face a problem with the silver layer. For example, it is too thick or insufficiently applied. Here are some of the points to pay attention to next time and fix.
- Insufficient temperature.
- Wrong nitrate concentration.
- Duration of emerging.
- Stirring of the liquid.
- Presence of contaminants.
- Measuring method.
- Sulfate pollution.
- Poor cleaning.
- Outdated reagents.
Advantages and disadvantages of Immersion Silver PCB
Immersion silver PCBs are popular for essential reasons, so let’s review ones:
- They have excellent electronic properties ensuring high reliability and great efficiency of electronic devices.
- Have flat/planar surfaces that make them suitable for microelectronics.
- Can be used for lead-free assembly.
- Are environment-friendly components on condition of proper recycling.
- Are comparatively simple in production.
- Have a great shelf-life up to 12 months.
- Can be reworked and repaired.
However, they still are not deprived of some limitations, essential of which are the following:
- Are not the most affordable PCBs.
- Are difficult to handle and require to be put in use within 24 hours after production.
- Peelable masks cannot be used.
- Short operating window limits supply chain options.
- Are vulnerable to inorganic acids, including sulfuric and chloride.
Common Problems and Solutions During Immersion Silver
Besides the mentioned difficulties, you may also face two other common issues while finishing your PCBs with silver.
This issue is somewhat similar to the “crevice” corrosion mechanism. Typically, silver oxidation(dissolving) of copper and reduction of silver ions and their depositing are performed simultaneously during immersion. The copper layer is both anode and cathode. Which forms a uniform solver coating on the PCB’s copper surface.
However, in case there is a gap in a copper layer caused by corrosion or mechanical stress, the supply of silver deteriorates. And the copper in the gap becomes an anode providing electrons for the reactions with silver. It causes deposition of the silver on the copper pads, which actually should have been unexposed. It makes a PCB start malfunctioning.
Galvanic Effect Prevention
To reduce the risk of the Galvanic Effect occurring, you should consider the following advice:
- Choose an immersion silver process with lower pH, which will make it less corrosive, but avoid increasing silver thickness.
- Control the precision of the micro-etching, so it is performed in the required time with the required amount of solution.
- Revise the PCB design, so large copper surfaces are not connected with small copper lines.
- Optimize pretreatment, curing, and imaging processes and use a chemical-resistant solder mask, so it is less vulnerable to reagents used in immersion silver plating.
Solderability (IC Hole/PAD)
Immersion silver technology can still cause defects during its implementation. One of such is the solderability issue. The main reason for IC Hole/PAD problems is improper hole wall quality. It typically includes thick holes or insufficient copper thickness. It results in poor solderability of components overall. In a way, this difficulty is closely connected to the Galvanic Effect.
Both copper etching and silver plating should be performed in a proper way. So, they can fully prevent solderability difficulties and ensure that a solder mask is fully developed and completely cured. Besides the outlined recommendations regarding the Galvanic effect prevention, you should consider the following advice.
IC Hole/PAD Prevention
A PCB’s surface must be 100% copper to receive a good immersion silver layer. Each tank must have an appropriate capacity to effectively exchange electrons between saturated silver liquid and the copper layer.
It is also vital to control the micro-etching rate. It prevents excessive oxidation of the copper layer, which conceives low-quality holes. You should avoid connecting large copper surfaces with thin lines for the consistency of micro-etching.
And here are several additional tips to incorporate into the immersion silver PCBs production processes to prevent solderability issues:
- Store PCBs at 30°C and relative humidity of 40%.
- Immersion silver PCBs should be packed within 24 hours after being produced, or they will deteriorate quickly.
- Immersion silver PCBs should not be in contact with items/substances containing sulfur and chloride.
- Immersion silver PCBs should be clear of stains and fingerprints. You should wear gloves while handling them.
- Use non-sulfur/craft paper as primary packaging.
- Do not use desiccants to avoid PCBs’ surfaces yellowing.
Immersion Silver vs ENIG
ENIG stands for Electroless Nickel/Immersion Gold finishing. It is sometimes considered a more expensive but also a more technologically advanced version of silver plating.
ENIG plating is simply the protection of PCBs’ copper pads with nickel and then covering the copper surface with a thin Gold layer. Technically, the process is not that different from immersion silver. But it enhances PCBs with greater oxidation resistance, good solderability, and excellent surface planarity.
Let’s review which one of the two popular finishing methods is better.