Today we want to discuss a topic related to printed circuit board finishes that significantly influence the results of a final assembly (PCBA).
Generally, the main factor in choosing a PCB finish is the final application.
Sometimes, economic reasons lead to choices and/or compromises that end up “undermining” the result.
We won’t evaluate what’s already in the market, as enough has been written about that, but instead we’ll focus on a single finish that’s still relatively unknown, yet pioneering excellent quality results and where great expectations are placed:
Direct Immersion Gold, Known by the Acronym DIG
This is a highly reliable technology that involves the “direct application of a gold layer (averaging 0.2 µm) over copper, without the” interposition of other materials such as nickel or palladium.
This factor, besides quality and reliability, brings significant benefits in all applications working at high frequencies.
The solder joint created with this finish is one of the best in terms of integrity and long-term reliability, and it can also be used in “Wire Bonding” processes.
During the soldering phase, it helps create thin and uniform intermetallic layers. This has a beneficial effect and higher quality for joint strength, making it more resistant to “fatigue” and therefore more reliable over time since the intermetallic compound (IMC) created, Cu6Sn5, is the toughest.
The intermetallic layer formed with the DIG finish ensures better mechanical resistance, making it less sensitive to micro-cracks or breakage during thermal or mechanical stress.
The DIG finish has a thicker gold layer compared to the classic ENIG finish (IPC 4552) (chemical gold).
The DIG finish also has its pros and cons, which we analyze.
PROS
Reliability: offers excellent storage stability, maintaining surface properties unchanged over time.
High conductivity: “gold has excellent conductive properties, thus improving the” assembly performance.
Excellent solderability: makes the surface excellent for soldering in terms of both quality and planarity. Therefore, it’s beneficial where components such as BGA QFN etc. are present.
No “black pad” phenomenon: unlike ENIG finish, the DIG process doesn’t involve Nickel.
Corrosion resistance: resistant to corrosion. This ensures long assembly life, even in difficult and hostile environments.
Solder joint integrity: excellent joint composition, with minimal intermetallics compound IMC: Cu6Sn5.
CONS
Currently, this finish has a high cost: gold is an expensive material and therefore this finish affects the final PCB price.
Currently, there are no standards regulating the DIG finish; manufacturers follow the technical specifications of the chemistry used.
Initial tests with this specific finish are very encouraging; we’ll keep you informed about reliability tests in the medium-long term, stay tuned!
In conclusion, soldering a board with DIG finish produces high-quality joints with minimal intermetallic formation, resulting in one of the best solder joints obtainable today, both in terms of reliability and durability.
