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How to choose a good surface treatment for PCB?
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Re: How to choose a good surface treatment for PCB?

by adafruit_support_mike on Tue Oct 23, 2018 4:19 am

Specific answers need specific requirements. What kind of board do you want to make, what quantity do you want to make, what kind of environment will the board have to survive, etc?

Surface treatments generally do two things: make it easier to solder parts to the board, and keep the copper from corroding.

Tin spraying is the simplest, and usually cheapest. It basically means coating the copper with a thin layer of solder. The specific machine has a pool of molten solder and pumps that make fountains maybe 1/2" above the surface. A conveyor carries the board across the fountains so all the exposed copper gets covered, then there are compressed air jets that blow off the excess. The air is hot so the drips of tin don't just solidify on the surface of the board.

The next simplest is immersion tin plating, where a layer of tin forms on the surface of the copper chemically. The layer of tin is thinner and more uniform than a mechanically sprayed layer, but is a bit more fragile and has some long-term chemical issues.

All the other metal-based methods end up putting a layer of gold on the exposed copper, they just do it in different ways. The nickel, silver, and palladium processes put a thin (a few atoms thick) layer of some other metal between the copper and the gold, which is chemically desirable. Copper and gold in direct contact form an 'intermetallic' alloy layer that causes problems electrically. Silver and nickel plate onto copper easily and coexist with copper well, and both of them also work and play nicely with gold. Palladium is an exceptionally good base for metal plating, but is more expensive than the gold itself.

The major advantage of gold plating is shelf life. Tin oxidizes when exposed to air, so it's best to use tin-coated PCBs within about six months of fabrication. If you let them sit longer than that, it becomes hard to solder to them, and you need stronger flux to cut through the oxide layer. Gold doesn't oxidize in air (it doesn't like to form compounds with oxygen at all), so the pads remain easy to work indefinitely.

The organic coatings also exist to keep the exposed copper from corroding, and if you'll pardon some editorializing, are more hand-waving than useful. Europe has established a law to keep toxic chemicals like lead and cadmium out of electronics, called the Reduction of Hazardous Substances (RoHS) directive. Unfortunately, it bans certain chemicals without suggesting any useful replacement, and that vacuum has been filled by chemical labs with marketing departments.

The simplest organic coatings are thin layers of rosin (dried flux), wax, or shellac. Those do a reasonably good job of keeping copper from oxidizing, but don't last as long as tin. The new coatings are engineered to keep oxygen away from the copper longer, while still melting away with heat or dissolving into the flux when you start soldering. If you don't have a specific reason to use one of them (i.e.: a manager who got their hands on a trade journal somewhere), you can safely ignore them.

All the rest of the terms like 'microetching' and 'activation' are technical details of the electroplating process. Microetching means cutting microscopic pits in the surface of the exposed metal so the layer of stuff being applied has a large surface to hang onto. Activation means stripping away any compounds on the surface of a piece of metal so the raw metal atoms are exposed. Raw metal surfaces really like to form chemical bonds with something else, which creates good conditions for electroplating or chemical coating. You don't need to deal with any of those issues yourself if you're having boards made by a board house. You're paying them to know what to do.

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Re: How to choose a good surface treatment for PCB?

by Abby123 on Tue May 14, 2019 2:01 am

There are seven types of PCB surface treatments, each with its advantages and disadvantages:
1. Hot air solder leveling (HASL)
HASL is the most common surface treatment. The application process involves immersing the PCB in a molten solder solution and then using a hot air knife to blow off the residue. The molten solder solution is made of lead and a tin alloy.
The biggest advantage of HASL surface treatment is its economical benefits. • Suitable for all kinds of PCB
Disadvantage
This surface treatment involves immersing the plate in a molten solder solution consisting of lead and tin. This makes it unsuitable for boards that require RoHS compliance.
2. Lead-free HASL
Lead-free HASL is currently the most commonly used surface treatment on PCBs. It is applied by immersing the PCB in a molten alloy of lead or tin, which ensures that the entire surface is covered. The residue was then blown off using an air knife to obtain a uniform coating.
benefit
One of the biggest advantages of lead-free HASL is that it exposes potential delamination problems by exposing the PCB to temperatures of about 265 °C.
Disadvantage
• May generate thermal shock
• May cause surface irregularities, etc.

3. Immersion tin (ISN)
Immersion tin is the ideal surface treatment for fine pitch products, flat, backsheet and press fit. ISN is applied by initiating a chemical displacement reaction with the copper surface of the PCB.
benefit
• Unlike lead-free HASL, it produces a uniform surface
• It can be redone
• It can replace the reflow solder
• Very reliable
• Suitable for all kinds of PCB
Disadvantage
Tin and copper have a strong affinity for each other. This means that it is inevitable that one of the metals will diffuse into the other metal. The result is a shorter shelf life than most other PCB surface finishes.
4. Immersion Silver (IAG)
Because several immersion silver is mainly used for aluminum wire bonding, membrane switch and EMI shielding.
Benefit
One of the main advantages of immersion silver is that it contains OSP, which helps prevent tarnishing.
Disadvantage
The presence of OSP makes immersion silver easy to lose luster.

5. Electroless Nickel Immersion Gold (ENIG)

Electroless Nickel Immersion is rapidly gaining popularity in the PCB industry as it overcomes many of the major drawbacks associated with other surface treatments. ENIG has a two-part application process. First, a layer of nickel is applied to serve as a barrier to copper and a suitable surface for the assembly can be soldered. Another layer of gold is then applied to protect the nickel layer during storage of the board.
benefit
The biggest advantage of ENIG is that it is ideal for a new generation of complex surface components, including flip chip and BGA; other surface treatments have drawbacks that limit their use in these new boards. Other advantages include:
• It is very suitable for flat surfaces because the nickel and gold layers are thin and uniform
• It is lead free
• It is ideal for PTH
• Long shelf life
Disadvantage
Although ENIG has a long shelf life, it is also associated with "black mat syndrome", which is a common problem leading to phosphorus accumulation between nickel and gold layers, leading to breakage and board connection errors. The only other major drawback is that it is not suitable for reprocessing.
6. Nickel Palladium (ENEPIG)
Nickel palladium (ENEPIG) PCB surface treatment is an upgraded version of ENIG. In ENIG, immersion gold has been shown to destroy the nickel layer. ENEPIG introduces a layer of palladium between the gold and nickel layers.
benefit
ENEPIG is known as "universal surface treatment" because it is suitable for a variety of boards, including modern, highly advanced boards with multiple surface packages.
Disadvantage
• Black pads often appear
• Reduce solder joint reliability
• The palladium layer is too thick to support solderability
• Expensive compared to most other surface treatments, etc.
7.OSP (Organic Solderability Preservative)
OSP is an organic waterborne surface treatment agent. It selectively combines with copper and also allows the PCB to be soldered.
benefit
OSP surface treatment is known for its environmental friendliness because it is organic and water based. It also makes it easy to apply, and the process is quite simple compared to other surface treatments.
Disadvantage
The fact that OSP is organic and water-based makes it very sensitive and therefore susceptible to damage during handling.
And ENEPIG) has been shown to hinder solderability, while other surface treatments such as HASL are well suited.
As noted above, certain surface treatments (such as OSP) make the PCB fragile during processing, while other surface treatments increase durability. When considering storage and processing requirements, it should be considered in advance. Surface treatments that make the PCB more detailed can only be applied if the risk-free storage and processing requirements are met.
Last edited by adafruit_support_mike on Tue May 14, 2019 2:50 am, edited 1 time in total.
Reason: MODERATOR EDIT: REMOVED LINKS

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