|Ask the Experts|
October 13, 2020
What Caused SMT Pads to Oxidize After Reflow
After our boards (ENIG) were run through reflow, the assembler is stating that the through hole/SMT connector pads oxidized.
We specified ENIG to our fabricator but are not 100% sure that is what we received.
Is there a possibility that any other finish would oxidize after reflow? Or after reflow any cleaning could cause this problem?
|Expert Panel Responses|
If the gold was too thin and, therefore, porous, it is possible that the nickel layer may have oxidized.
Depending on the age and storage conditions of the boards, oxidation can even be evident with the initial reflow. Obviously, a reflow cycle in an air environment would exacerbate the oxidation.
Technical Support Engineer
ENIG does not oxidize after a reflow process or normal cleaning process. You need to verify what finish you actually received by XRF and does this thickness meet the requirements of the IPC 4552.
S T and S Testing and Analysis
There are three possibilities here:
ENIG surface finish is typically resistant to oxidation from reflow, as compared to other surface finishes. Immersion silver, immersion tin and OSP are susceptible to oxidation and one reflow cycle could oxidize the through hole SMT pads. HASL surface finishes are very resistant to oxidation and typically do not show these type of issues.
The appearance of these surface finishes is quite different. You should be able to tell if the finish is ENIG based on the gold coloration. Hard and soft gold plating also have a gold color but the texture is usually more bright and shiny than ENIG. ENIG has a matte gold appearance.
Fluxes and cleaning chemicals might cause damage to the surface finish. The ENIG gold layer is very thin and can be porous which might allow chemicals and air to attack both the gold and the nickel underneath. Oxidation can occur to the nickel but the gold may look normal. The solder joint actually bonds to the nickel layer and oxide on the nickel usually causes poor wetting.
Flux and cleaning chemical attack largely depends upon how long fluxes were in contact with the finish, and when and how cleaning was done. If flux residues were in contact with the ENIG finish for an extended period of time, damage could occur to the finish. This is especially true for water soluble fluxes and solder pastes which are more aggressive than no cleans. Heating flux residues increases the possibility of attack.
For example, if flux residues from a prior soldering process were in contact with the through hole SMT pads, then were reflowed and finally cleaned, then the fluxes could attack the finish on the pads. Cleaning chemicals can also cause attack to the finish depending upon the type of chemical, concentration, temperature and contact time. The cleaning chemical supplier should be able to give details on this.
Most likely, this is the infamous "Black Pad" where the Nickel plating was not properly prepped prior to Immersion Gold application.
From IPC: The latest research shows excessive nickel corrosion during the immersion gold deposition causes the condition, says George Milad, national accounts manager for technology at Southington, Connecticut, USA-based Uyemura International Corp., a chemical supplier that offers ENIG chemistry to the electronics industry.
If this is the root cause, and it would not solder, then the entire board should be scrapped.
Capital Equipment Operations Manager
Specialty Coating Systems
Your question is quite common with many who specify ENIG PCB Surface Finish. Do you require IPC-4552 Specification for Electroless Nickel/Immersion Gold (ENIG) Plating for Printed Circuit Boards from your suppliers? Second, what IPC Class are you specifying ( e.g., Class 1,2,3)? Both of these will help to require the supplier to report on Immersion Gold Thickness, Electroless Nickel Thickness, Porosity, Phosporous/Boron Content, Solderability, etc.
Indirectly this specification will reference IPC-6010 and J-STD-003 as well critical to ENIG quality. We require our suppliers to perform XRF as well as GR&R via ENIG/ENEPIG standards traceable to NIST for correlation purposes with our incoming metrology requirements. You point out oxidation or contamination implied as a concern.
To determine if the Nickel is penetrating through the Gold due to either porosity or thin Gold you will need to have SEM/EDX performed on the suspect bareboards before and after exposure to reflow to verify this is not your problem? If this turns out negative, then I would recommend FTIR of the suspect bareboards Gold pads before and after reflow to see if you have an organic depositing on the gold ( e.g., contaminating it via reflow oven exhaust and depositing on Gold). If this turns out negative, then isolate before and after process boards via FTIR for wash issues. What do specify for your wash parameters( e.g., DI water , saponifier, etc.)?
VP of Advanced Technical Operations
There is a slight possibility that the Nickel in the ENIG oxidized after reflow. I have ran hundreds of ENIG boards through the reflow and I have never seen this problem. If your reflow oven is equipped with Nitrogen try running a bare board from the same lot and see if you still get oxidation.
Senior Manufacturing Engineer
I would suggest you have analysis done on the gold thickness. It sounds like you may have thin porous gold allowing the nickel to oxidize and will cause poor wetting and solderablity. Or they have a different finish.
President/Senior Technical Consultant
One would have to question the porosity and thickness of the gold immersion plating. Beneath the gold plating is a nickel undercoat and if the gold plating is too thin, then the nickel beneath will passivate and this will most likely affect the soldering of the gold coating.
Vice President, Technical Director
You should not see this effect, unless you're assembler:
Bakes your pcb before manufacturing. Washes your pcb. Process your pcb 48hrs of later from the first heat cycle. Check your assembler humility and temperature levels.
Generally ENIG finish PCB's have 5 heat cycles,(bottom, top, wave, bga remove/replace) so once the first heat cycle is applied, the finish will break down.
Process Engineering Manager - Electronics
Altech UEC, South Africa
The "corrosion" could also be caused by improper/insufficient nickel plating (plating skips), or too long of a time between nickel plating and immersion gold plating. If the gold is applied directly to the copper as a result of insufficient or missing nickel barrier, it will corrode.
As the co-chairman of the committee responsible for ENIG, rework of the boards post nickel plating is NOT allowed and is rejectable. The supplier needs to share with you the root cause for the skip plating, why the copper did not take catalyzation but would now take an OSP and be solderable. Additionally, given the source of free copper ions during plating , you should confirm that the ENIG plated pads are not exhibiting hyper corrosion. Unless these boards use super special material that is not available in a timely manner, I would reject them per the IPC 4552A.
S T and S Testing and Analysis
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