PCB Technical - 2 potential problems of ENIG surface treatment PCB pads?

With the popularization of smart phones, the miniaturization of electronic products and the EU's requirements for lead-free processes, the surface treatment process of nickel immersion gold (ENIG) is simpler and cheaper than other surface treatment processes. In addition, it also has its excellent repeatability, good flatness, suitable for thin-foot parts, long-term storage and not easy to oxidize. Therefore, more and more electronic products choose ENIG as their PCB surface treatment.

Therefore,when many people find parts dropped or poor solderability when using ENIG (nickel immersion gold) surface treatment on the pcb board, the first problem that comes to mind is usually "black nickel" also known as "black pad".However, it seems that few people really understand what is meant by "black nickel" or "black pad", so this article tries to discuss ENIG's "black nickel" or "black pad" from the perspective of working bears' understanding.

ENIG's "black nickel" basically has two main components: "phosphorus" and "nickel oxide".

"Phosphorus" comes from the electroless nickel plating layer. In the subsequent replacement process of "gold" and chemical nickel, because "phosphorus" does not react, it will stay between the gold layer and the nickel layer to form P-rich. Layer, and finally form the result of embrittlement on the welding strength.

"Nickel oxide" is basically composed of a complex chemical formula of NixOy (x and y are numbers).The fundamental reason is that the nickel surface undergoes excessive oxidation reaction during the immersion gold substitution reaction on the nickel surface (metallic nickel becomes nickel ions) It is "oxidation" in a broad sense),and the irregular deposition of very large "gold" atoms (gold atom radius 144pm) results in the formation of rough,loose and porous crystal grain arrangements,which means that the "gold" layer cannot be completely covered.Staying at the bottom "nickel" layer allows the nickel layer to be exposed to the air to continue its oxidation,so nickel rust gradually forms under the "gold" layer, which eventually hinders welding.

Because most of the solder, such as SAC305, SAC3005, SnBi, SnBiAg, etc., are basically based on tin (Sn), when the circuit board is heated by the reflow furnace, Sn and ENIG's nickel (Ni) will form Ni3Sn4 IMC (Common compounds). If the nickel layer is oxidized, it will be difficult to form ideal IMC. Even if it can be barely formed, the IMC is intermittently and unevenly. This will cause the welding strength to decrease, just like a brick wall or brick coated with cement.The cement between the wall and the brick wall is like IMC. If some places are not coated with cement, the strength of the wall will become fragile. This is the same reason.

In fact, the surface treatment of circuit boards also has "nickel-immersed palladium gold ((ENEPIG)"), and this kind of surface treatment can effectively suppress the problem of "black nickel/black pad" generation, but because its cost is relatively expensive, it is currently only Adopted by high-end board, CSP or BGA industry.

Two potential problems of ENIG pads and their prevention

ENIG's basic process

One of the biggest advantages of ENIG surface treatment of PCB circuit boards is the simple manufacturing process of the circuit board. In principle, only two chemical potions (electroless nickel plating and acid gold water) can be used to complete, of course, other potions are needed. ENIG surface treatment process is generally to first make chemical nickel deposition on the copper pad, and control the thickness of the nickel layer by controlling the time and temperature; then use the fresh nickel activity that has just been deposited to immerse the nickel pad in acidic gold water. The chemical displacement reaction replaces gold from the solution to the surface of the pad, and part of the nickel on the surface dissolves into the gold water. The replaced "gold" will gradually cover the nickel layer until the nickel layer is completely covered, the replacement reaction will automatically stop, and the process can be completed after cleaning the dirt on the surface of the pad. At this time, the gold-plated layer is usually only about 0.05um (2u") or thinner, so the ENIG process is very easy to control and relatively low-cost (compared with electroplated nickel and gold).

The formation and harm of black nickel

The quality of the nickel layer mainly depends on the formula of the nickel plating solution and the temperature control during chemical deposition, and of course it also has a certain relationship with the acid gold water treatment process. The process of electroless nickel plating is to obtain the plating layer through the autocatalytic reaction of hypophosphite and nickel salt on the surface of the pad.The plating layer will contain a certain amount of "phosphorus (P)". Many studies have shown that the phosphorus (P) in the plating layer is normal The proportion should be between 7% and 10%.If the formulation of the plating solution cannot be maintained immediately or the temperature is out of control, the phosphorus content will deviate from this normal range.When the phosphorus content is low, the coating will be very easy When the content of phosphorus is high,the hardness of the formed coating will increase significantly, which will reduce its solderability,and will seriously affect the formation of reliable solder joints. If the phosphorus content in the nickel plating layer is low, and the chemical replacement reaction gold plating is not properly processed,if a large number of cracked gold plating layers are obtained,the acid gold water will inevitably be difficult to remove in the subsequent cleaning process, which will lead to exposure to the air.The corrosion of the nickel plating is accelerated, and finally black nickel is formed, which is the so-called black solder pad.

The formation and harm of phosphorus-rich layer

ENIG surface-treated solder pads, in the soldering process,the real alloy with the solder paste is the "nickel" in ENIG,and its typical intermetallic compound (IMC) alloy is Ni3Sn4, and the phosphorus in the nickel plating does not participate Metallized,but in the nickel layer, phosphorus occupies a certain proportion and is evenly distributed. In this way,after nickel participates in the alloying, the local excess phosphorus will be enriched and concentrated on the edge of the alloy layer to form a phosphorus-rich layer. If the phosphorus-rich layer is too thick,its strength will be greatly reduced.When the solder joint is impacted by external stress,it must be destroyed from the weakest link first, and the phosphorus-rich layer may be the weakest link to be destroyed first. The reliability of the points must be obviously affected.

Prevention and control of black nickel and phosphorus-rich layer

Although the formation of black nickel and the appearance of the phosphorous-rich layer have a strong concealment,it may be difficult to detect and prevent by general means. But when we understand the causes, we can find effective prevention and control methods.

For the formation of black nickel,the main purpose of the manufacturing stage is to maintain the plating solution and control the process temperature,so that the proportion of nickel and phosphorus in the plating layer is in the best state. Acidic gold water also needs good maintenance,and it should be adjusted in time when it is too corrosive.

For users,

1.The best method is to use a scanning electron microscope (SEM) to observe the surface treatment of the soldering pad microscopically, mainly to check whether there are cracks in the gold plating layer,and use EDS to analyze whether the proportion of phosphorus in the nickel plating layer is within the normal range;

2.Secondly, you can choose a typical soldering pad to be welded by hand and measure the push-pull strength of the solder joint.When the push-pull strength is found to be abnormally small, there may be black nickel;

3.The last method is to conduct an acid gas corrosion test on ENIG samples. If powder or discoloration is found on the surface of the ENIG sample,it means that the gold coating on the pad is cracked, which means the possibility of black nickel.

Among these methods, the most convenient and quickest method should be the second method,which is simple and easy to implement.With these methods, problems can be found early before the ENIG circuit board is used, avoiding the production of a large number of circuit board components with reliability problems,and thus keeping the loss to a minimum.

For the production of the phosphorus-rich layer,when the proportion of phosphorus and nickel in the nickel plating layer is appropriate,it is mainly to control the welding process, control the welding time and temperature, and control the thickness of the intermetallic to the best 1-2 microns (um),when too thick intermetallic compound (IMC) is produced, too thick phosphorus-rich layer is bound to be enriched.