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.
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.
Let’s split the method into two parts – the working principle and the application process and review them separately.
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.
Let’s get down to business and review the complete step-by-step PCB immersion silver process:
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.
Immersion silver PCBs are popular for essential reasons, so let’s review ones:
However, they still are not deprived of some limitations, essential of which are the following:
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.
To reduce the risk of the Galvanic Effect occurring, you should consider the following advice:
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.
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:
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.
|Shelf life||Up to 12 months||Up to 12 months|
|Sensitivity to handling||Require gloves to be used, or fingerprints and minor scratches may be left.||It is recommended to use gloves, but careful handling does not deteriorate components|
|Packageless sustainability||Comparatively poor, require to be used for the assembly nearly immediately after being unpacked||Comparatively average, but still require to be put in use fast|
|Safety||Safe for the environment and labor||Safe for the environment and labor|
|Suitability for lead-free assembly techniques||Yes||Yes|
|Chemical properties/resistance||Is vulnerable to some inorganic acids||Is not vulnerable to most acids|
|Plating||Good plating, but highly depends on the conditions of the process/can be insufficient||Superior plating over both the copper pads and via holes|
|RoHS Directive compliance||Fully compliant||Fully compliant|
|Magnetic properties||Do not show undesirable magnetic properties||Has undesirable magnetic properties because of Nickel|
|Rework||Can be relatively easily reworked multiple times and repaired||Is expensive and difficult to rework. Typically, is not repaired/is difficult to repair|
|Cost||An affordable method||A most expensive method|
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