|Ask the Experts|
September 9, 2020
Tarnished OSP Circuit Boards
We are purchasing circuit boards from China and they are coming in with a tarnished looking finish (they have OSP) and we are having soldering issues.
Could that be part of their processing issues? The boards we purchase are normally used as soon as they come in the door so I don't think it is a problem with aging of the OSP. Any thoughts?
|Expert Panel Responses|
Without more information it's hard to determine, but OSP is a good finish if the Copper is pre cleaned correctly prior to OSP application. Does the Copper look oxidized beneath the OSP in patches or just darker than normal Copper?
Also OSP if applied too thick will give poor wetting problems. Try to wipe off the OSP with a IPA solution or similar and them immediately process, if the boards process Ok then it's probably too thick and stops the solder wetting properly.
I assume you're using No Clean flux a slightly stronger No Clean flux can help greatly, so look for something around 35 acid value as OSP is removed also by the Acid, don't skimp on flux application either as you need to effectively wash off the OSP which will outlast any flux temperature wise.
Technical Sales Manager
BLT Circuit Services Ltd
Changing colors of the Cu on OSP coated boards and associated soldering issues is a good indication of an issue with the deposit itself. The IPC 4-14 committee has just tested OSP coated boards post 1 year of open on the shelf storage with no discoloration of the copper and no impact on wetting performance as measured by wetting balance - please check out the committee home page on KAVI.
You can do simple tests for the presence of OSP coating using silver nitrate solutions or you could have sample reworked and recoated with OSP by another supplier to confirm it is a coating issue.
S T and S Testing and Analysis
OSP point of view is easy to get tarnished. In this case need to check manufacturing date and when it's packed after the manufacturing by supplier. Is the supplier pack MBB? Is the supplier use backing at what environment or chamber?
It will affect solder wetting especially hidden joints at SMT thermal pads BTC And at PTH holes those can not sure thru X Ray Pl. And also look the humidity card in PCB pack is in right condition.
Engineer, Process Development
Sanmina India Pvt ltd
If you are seeing oxidation on the pads as they are received, that is certainly not normal. Either the OSP was improperly applied, or not applied. Any oxidation from fabrication is removed by the prep line prior to the actual application of the coating, so the oxidation you are seeing had to happen later.
If I were in your position, I would photograph the incoming condition, communicate it to the supplier, and ask for a root cause investigation. You also want to eliminate packaging/shipping as the source of the problem, so do review how the boards are packaged, and whether shipping entails extended exposure to uncontrolled environments.
For properly coated and packaged boards (moisture barrier bag, desiccant) even ocean shipment should not have a measurable effect on as-received condition.
The two conditions that will affect the OSP coating are being exposed to high RH and Temperature. This will tend to harden the OSP. The signs of this are darker shades of gold color on the pad versus a clean coating. Typical no clean flux products will not remove the OSP to allow soldering.
You can remove the OSP if you have water soluble flux and a wave solder machine. Turn the solder pumps off. Run the board though the wave with the fluxer and preheaters on. Wash the boards using a wash system. After they come to room temperature after the wash process the boards.
Sr Field Applications Support Engineer
OSP organic solderability preservative is designed to prevent oxidation and therefore loss of solderability. Having said that id does have a finite lifetime but this is months/years not days or weeks. If your boards are looking tarnished then this would suggest that there could be a problem with the OSP or it is not there at all, either way oxidation/tarnish of the copper could lead to solderability issues.
Each flux has only a finite amount of activity which is used up converting the metal oxide surface on the powder, pad and lead in to bare metal (required to facilitate the IM formation and good soldering). If there is excess oxide formation on any of those three surfaces it can lead to flux exhaustion and soldering issues.
Senior Applications Chemist
OSP is a permeable coating which slows tarnishing of the underlying copper. Tarnish is usually caused by air exposure over time, but other compounds besides air can permeate the coating. Its possible that the copper was exposed to sulfur containing compounds which can create a tarnish fairly rapidly. Some packaging materials like plastics and paper contain sulfur and these may create tarnish when in contact with the circuit boards.
The pre-cleaning steps of the OSP process may create a mild oxide on the copper surface. Mild copper oxides can look like clean copper but tend to darken in color over time. The circuit board supplier should be able to control the OSP coating process so that does not happen.
The OSP(Organic Solderability Preservatives)Process consists of the following high level steps:
OSP Process Steps
*Rinse / Clean
The tarnished finished along with soldering issues are part of Pros and Cons trade-off PCB surface finishes tradeoffs:
1. OSP Cons
Technology / Chemistry Challenges
Short Shelf Life
Difficult to inspect
Coating Skips/Oxidized Copper
OSP is also known as the anti-tarnish surface finish. It refers to a layer of organic finish generated on clean and bare copper by adsorption. The organic finish is capable of stopping copper from oxidation, thermal shock and humidity. It has to be easily eliminated by flux in the later process of PCBA soldering so that the exposed clean copper can be jointed with melting solder so that good solder joints can be generated in extremely short time.
With your observations some improvements and measurements you might try in order to ensure the correct OSP appearance and performance are listed below:
*The thickness of OSP must be controlled within a certain range of 0.2 to 0.5μm.
*The amount of micro-etching must be controlled within a certain range.
*During PCB fabrication, contaminants (gel residue, ink etc.) must be 100% eliminated in case partial abnormity and bad solderability take place.
VP of Advanced Technical Operations
Even though OSP over bare copper has a significant cost benefit compared to other SMOBC board finishes, OSP is not without its drawbacks. Since you appear to be doing an adequate job of monitoring shelf life with good FIFO practices and are still having solderability issues, I would suggest looking at the environment during transportation. Are the OSP boards being shipped from China via air freight or sea transport? Are the boards packaged in MBB (moisture barrier bags)?
Since the boards are being received with a tarnished looking appearance this is a pretty good indication of either long storage time at the board fabricator, long transit time, inadequate packaging, or a combination of all three factors. The typical thickness of an organic surface preservative should be 4-24 micro inches, but this is seldom specified in procurement specs.
Obviously the thicker the OSP finish is the better it will prevent the copper from oxidizing. It is rather difficult to measure this thickness so I would suggest having the OSP thickness measured by an outside FA laboratory.
After OSP application, the panels must be dried immediately and all holes must be blown out to ensure no residual OSP chemistry seeps out when boards are ready to be delivered.
The best practice is to use boards with OSP surface finish within 1 or 2 weeks of delivery. For boards with tarnished OSP that won't solder, OSP must be stripped then re-applied.
Director of Sales and Marketing
Tarnishing of the copper is one thing as this could be due to oxidation of the copper surface prior to the OSP coating application. This tarnish may be difficult to remove with low activity fluxes and this will result in poor soldering. If the flux is not strong enough to remove those oxide the results could be non wetting of the surfaces.
I would question the type of OSP coating being used, the thickness and viscosity of the material itself, and the time frame from manufacturing to the coating application. I would also question the method of OSP application, be it either spray or dip with one being more porous than the other as full coverage is mandatory to protect the copper surface. I would also have the tarnish analyzed to verify it is not a corrosion problem.
Vice President, Technical Director
A processing issue can be the root cause of this issue. However the SOP finish boards are hard to be processed and they get worse by the hour. How long is taking for the boards to your facility?
My recommendation is to have a supplier (domestic or international) that can ship you the boards immediately and you have them in the assembly process within 24 hours from the completion of the PCB build process.
Engineering and Operations Management
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