PCB Tech - lead-free soldering surface treatment for immersion silver plating

PCB manufacturer, lead-free soldering surface treatment for immersion silver plating

There are many types of lead-free soldering in SMT chip processing. OSP was introduced above, and here is another common method of immersion silver plating.
In the arrangement of the Electromotives Series of various elements in the acid solution, the potential of silver is +0.80V, and the potential of copper is +0.52V; during this period, copper is more active than silver by 0.279V. Therefore, it is not difficult to prepare an immersion silver plating bath that dissolves copper and deposits silver. Moreover, at the moment of soldering, the silver layer will quickly dissolve into the high-temperature solder joints. 6 "When the tin and copper are in full contact in an instant, it will quickly form a benign IMC CL16Sn; so the tin dip time is very short. The solderability is also very good, and the subsequent solder joint strength is very reliable. It can be seen that the silver layer does not participate in the structure of the solder joint at all, but only protects the copper surface from rust.

1. Overview of Organic Silver
The new organic immersion silver plating with silver nitrate formula has about four main processes, namely: pre-cleaning of the copper surface, micro-etching, conditional treatment, and immersion silver plating, etc.; horizontal automatic conveying method can be adopted for continuous Sexual double-sided processing. There are many current product processes for immersion silver plating after green paint. Well-known brands in Taiwan such as MacDermid’s SterlingTM Silver, Cockson’s Alpha-Level, and Atotech’s Silver Finish ST, etc., Taiwan’s PCB industry will only conduct immersion under the special designation of the customer. The processing of silver plating is really due to various solderability issues that cannot be taken care of or solved! Of course the responsibility for the customer to specify is not with me.
Although silver is also listed in the group of noble metals, the surface of pure silver is very prone to vulcanization and oxidation in the normal temperature environment, which is a tenacious and poor quality film called Tarnish (staining or discoloration) or Passivation (passivation). . The powerlessness of the no-clean flux will immediately adversely affect the solderability. Therefore, various anti-fouling or anti-tarnish treatments for silver surface have been the goal of metal surface treatment industry for many years.
In the current commercial "immersion silver plating" baths, certain organic inhibitors (Inhibitor) must be added to co-deposit in the plating layer to help the ribs to prevent the discoloration of the silver surface. However, in actual operation, the first high-temperature welding is almost flawless, but the second welding or wave welding depends on the degree of discoloration. Once it turns brown, even if the water-soluble flux is very active, it is still indifferent to excessive discoloration. Therefore, when doing the first soldering, you must also be careful not to pollute it a little to avoid contamination and deterioration of its delicate soldering properties.

Generally, the thickness of immersion silver is about 0.2-0.3μm and the flatness is excellent. The organic inhibitors co-deposited in the film can be seen from the Auger depth profile. These organics will be uniformly distributed within a surface depth of 0.075 μm. The distribution ratio of the amount of pure silver, organic matter, and the amount of copper in the immersion silver layer is directly related to the depth of the film thickness. The figure below shows the relationship between the ratio of the three and their depth.
As for the immersion silver introduced by Alpha-Level, the average thickness is 4-5μin (0.125μm). When finished, a thin (5A) transparent organic protective film will appear on the surface, which is the same as the aforementioned OSP protective film on the copper surface. It can be said to have similar functions in the same door. Its imaginary composition can be understood from the following cross-sectional schematic diagram.

3. The solderability of organic silver
As for the solderability, it has been studied by many industry players, and it seems to be better than OSP, and it is not inferior to the solder plate, even the sample after steam aging (Steam Aging) still shows good solderability . Some people deliberately make it after three times of high-temperature welding (Reflow), and found from the Wetting Balance test that its Wetting Force is almost indistinguishable from the new sample. In addition, many companies have also confirmed that immersion silver is also very good in the results of various reliability tests.
In addition to soldering, the immersion silver can be used as the surface treatment of semiconductor wire bonding. Generally, the effect is quite good for the ultrasonic type aluminum wire with a diameter of 10 mils. Sometimes it can also be used as a contact connection, but this is directly related to the thickness. When the micro-etching of the bottom copper surface is roughened to a considerable extent, the immersion silver thickness often reaches 10μm and above, so it can be used to perform the contact task.
However, these excellent solderability and aging resistance capabilities are all written for the purest and cleanest baths and samples. Once the bath is slightly polluted or the assembly and welding environment is not ideal, The solderability of immersion silver plating will be greatly distorted. The solderability of the current immersion silver plating will be seriously affected by the trace amounts of ammonia, sulfur dioxide (SO2) and nitrogen dioxide (N02) in the water vapor. The water quality in the immersion plating process is also extremely important, and a small amount of chloride ion pollution will bring bad adverse effects. In addition, special "sulfur-free paper" must be used for the shipping packaging. This delicate paper is not only expensive but can only be used once. When it arrives at the client, the paper must be opened and sealed one by one, and it must be marked on the board immediately. "Parts" and welding, if the site environment is left for too long, the solderability may cause problems. When the second or third re-soldering was performed, the disaster scene of poor solder rejection was really horrible. At this time, the Sales Talk had to be speechless.

4. Process management of immersion silver plating
Commercialized immersion silver, the only well-known names are ATO's "Silver Finish ST" from German business, Sterling from American business Matter, and Alpha Level from Cockson.
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