Clean metal surfacesare chemically unstable. They reach a stable statethrough bonding of free oxygen, from the air, to unused bonding locations on the metal atoms.

Flux is designed to break the oxygen-metal bond so the solder alloy can form an intermetallic with yourpad. If there is too much oxide, the flux does not have enough capacityto clean the pad.

Keeping boards in bags is one way to slow oxidation. The better the bag and the less air entrapped in the bag, the better it will protect the board. Moisture barrier bags and other metalized bags provide the best protection. Vacuum packaging and the use of inert gas to displace air in the bag also reduce oxidation rates.

Oxidation on surface mount pads could be caused by poor pcb material manufactured (i.e. there should be a layer of surface protection on the pcb) or due to poor storage environment (humid and maybe over long period of time) of the raw pcb.

EH Lim has been in the PCB Assy industry since 1985, starting as a Process Engineer in Thomson/ Singapore for 5 years before moving to Electrovert Asia Pacifc. In the last 10 years, Lim was Sales Director for Vitronics Soltec (South Asia) prior to joining ECD in 2007 as Managing Director for Asia Pacific.

Oxidation on the surface mount pads is due to the exposure of the copper to the atmosphere. The degree to which the copper will oxidize depends on the type and quality of the pad surface finish. Some finishes provide greater protection when compared to others.

If the surface finish is an Organic Solderability Preservative layer then it is prone to handling damage and results in surface oxidation. Noble finishes (Gold and Silver) tend to last longer and prevent oxidation. If the surface finish is Tin (Sn) then the oxidation potential exists, especially if the finish is uneven and not coated completely.

Only pure gold does not oxidize. Therefore I assume you have tin alloy on you pads.

Tin will oxidize when in contact with air. It will build up a layer of oxide quickly if subject to a dam heat environment. Therefore you need to look at board storage or the assembly process, in order to have minimum oxide growth.

What pad surface finish are you using? Why do you suspect surface oxidation is the reason for BGA failure?

Metal surfaces can oxidize when exposed to elevated temperatures and humidity. Most of the surfaces used in electronics are based on noble metals (e.g. ENIG) that don't "oxidize" readily or have some organic component that retards oxidation (e.g. OSP).

Take a look at the board handling and storage procedures, keep PCBs stored in a temperature and humidity controlled environment and keep packages sealed as long as possible, minimizing exposure time.

Oxidation is the result of oxygen and moisture reacting with the alloys on the pads. In other terms, oxidation is Rust. Just like our cars rust after the protective coatings (paint) have been removed, PCB pads oxidize after sitting unprotected or exposed for too long.

Contamination could be an issue as well. If the operators touch the pads with bare fingers it is possible to transfer oils and dirt to the pads. Either way, once the pads are oxidized or contaminated a good cleaning is the best was to start.

To avoid the oxidation make sure to handle the boards properly as determined by the type of coating on the pads. Some board finishes are more forgiving that others. For example, ENIG may stand up to longer exposure than either OSP or Silver coated pads.

Follow the board manufacturer’s recommendations for handling and storage. No matter which board finish you are using, there are a few steps that will help extend the life of the board finish.

1. Keep the boards packaged in vacuum sealed bags with desiccant until immediately before use.
2. Once the boards are in process, finish the process. I have seen situations where double sided boards are processed through a bottom side operation only to be stacked in racks and left exposed for days and sometimes weeks. Those boards WILL oxidize no matter what board coating was applied.
3. If the boards must be opened and the stored, make use of a “dry box” or other controlled environment.
4. Handle the boards as little as possible. If the board must be handled by a person, handle the PCB only by the edges of the board. Do not touch the solderable surfaces with bare fingers. Finger cots or gloves, changed often can help keep contamination from the soldered surfaces.

In a nutshell, oxidation is the result of prolonged exposure of printed circuit boards to air. However, it is important to first determine whether you are dealing with oxidation as opposed to contamination.

Oxidation of pads can be difficult to determine and often requires the process of elimination. Oxidation is a natural process for metallic surfaces when exposed to the environment and cannot be detected visually until it is in an extremely advanced state. Boards finished in gold have a very limited chance of oxidation unless the boards are extremely old or in an environment that is high in humidity.

Boards should be received from your board supplier in a vacuum pack, clean and ready to take solder. Once the seal has been broken, the boards become subject to their environment. High humidity will accelerate the process and the longer the exposure, the greater the oxidation potential. Boards should be loaded within a six month (maximum) window.

The other possibility is contamination. If your boards are newly out of their safe packaging, are gold or are in a controlled environment, chances are the problem is not oxidation. Most contamination is created at the board manufacturing level and can usually be discovered when the packaging has been opened. Causes at the manufacturing level are a result of faulty conditions such as incorrect ph levels in the rinse bath, improper handling and poorly washed boards with surface film.

The safest way to avoid oxidation is the create a dry, clean environment when storing boards not immediately used and to make sure boards are used within a six month window. Contamination of boards is the problem of the board fabricator and should fall within warranty of their product.

In some cases where time does not allow for re-fabrication of oxidized or contaminated boards, it is possible to work with your stencil supplier to modify design of your BGA pads to circumvent poor solder joints.