Small parts coupling together in MSA Sn (Tin Plating) bath

A. Rich,

I know that some surface-mounted capacitor manufacturers have invested pretty heavily in process development against this exact problem and the problem of the tin bridging across the body of the capacitor. It may be that the coating or the profile of the unplated surface is encouraging two of them to stick together through surface tension. You may need to modify the barrels or the parts to discourage this capillary attraction.

A. Hi Rich,

I understand the problem you are facing. I worked on similar project few years back. We were plating Chip resistors and Chip network resistors or different sizes. 1206, 0805, 0603 etc. Since they are flat they tend to get struck due to surface tension and as the plating goes on the get coupled together. This is called "coupling" in SMT components plating which particularly appears during Tin/Lead plating.

We were also using a MSA bath to attain a composition of 80/20 Sn/Pb using 8.5 ± 0.5 gpl of Tin and 1.2±0.2 gpl Lead in the solution. The important thing to maintain is the composition of the bath constituents. Prior to Tin/Lead plating we do Nickel plating and provide a thickness or 3 to 5 micron. This provides a very good barrier layer during soldering. After nickel plating we have observed that the components don't get struck with each other as nickel layer is smooth and hard. But when the same components go for solder plating, they get struck together and Couple. (i.e., softer the metal the better it couples)

First we ensured that sufficient thickness of Nickel is built up so that the barrier layer is intact. The composition of the Sn/Pb in MSA bath is very important. If the concentration is not maintained properly then you will have such problem. The problem occurs when the concentration goes on the upper end. i.e., It is not that you maintain the metal salts ratio in the bath. But the total concentration should be lower.

Try the following.

1. Maintain Sn and Pb concentration at lower end. Don't control the ratio alone. Keep check on the total metal content on the bath. (if possible operate with the lowest ratio recommended for the bath)
2. Limit the rate of plating. Try to plate at a LOW CD. We were plating at 0.3 to 0.5 ASD for 25 - 30 min to attain sufficient thickness.
3. Do not build excess thickness. Plating thickness of 5-7 micron is sufficient for good solderability.
4. Keep the bath clean (pls refer letter 15140 - Tin sludge formation in MSA solution with a tin reactor).
5. If the metal contents are high the coupling formed will be hard to break. But under lower metal content there will not be any coupling and even if it is formed it will be very soft (tends to break easily).
6. After plating in MSA bath Neutralize in a Tri Sodium Phosphate bath (10% soln) followed by rinse in Hot DI water with Ultrasonic. This will break the coupling formed. (in case if it had been formed)

Lower metal concentration in bath and Slow plating rate is the key to solve this problem. Hope this should help you.

Good luck.....

A. Hi Rich,

I am actually working in the same multilayer capacitors field right now, and what I can tell you is that the coupling problem you are having is very familiar to me. I think current is one of the most important factors to consider in this case, along with the size of the media, and also the pH of the MSA tin bath. I have noticed that at pH below 2.0 this problem occurs more often, but I don't think this is your case. If you plate in 6" dia. barrels and the volume of the chips and shoots is below 1/3 of the inside volume of the barrel, I have had very good results plating at 4 Volts (No amps). Then a hot 1% TSP bath and finally a hot rinse.

Good Luck!

I hope we can talk about this problem again.

Bye,

A. Hi Rich:

We have found that there are both chemical and equipment technologies that can help prevent coupling of small passive components. There are alternatives to the standard MSA process commonly used in this industry, and these alternatives are now in production and plating 0201 sized parts in barrels without any coupling.

As you know, alternative equipment to the barrel can also prevent coupling. The Gets RFT plater is such a machine, but has other issues that make it not a universal solution to this problem. A new machine called the SBE Plater (Spouted Bed Plater) has been introduced to the small parts plating industry, and this technology can be retrofitted into existing barrel lines. This equipment pumps a stream of plating solution up into the bottom of a plating chamber. The stream lifts the chips into an internal tube in the plating chamber. As the parts exit this tube, they are deflected back down to the bottom of the plating chamber. The deflection helps to eliminate the coupling problem, yet does not damage the parts because the deflection takes place under solution. Plating occurs when the parts reach the bottom of the plating chamber. Circulation of the parts throughout the chamber is quite rapid, which also reduces coupling. An added benefit is that the thickness distribution is far superior to any barrel operation.

Best regards,

A. Instead of a traditional plating barrel, try a "vibratory plating barrel". If you google that, you will find examples. There is a well known precious metals supplier with an equipment division near you that makes them. They are designed for applications such as yours.

  Thank You Peter, I will look into that.

A. Hi Arlene. With essentially five nine's satisfactory production, don't even think about tinkering with a good line by tweaking your process chemistry :-)

Obviously there is a problem here, seemingly related to larger inductors, which must be solved either by better inspection or by figuring out a way to separate the inductors better so they all plate. Perhaps either the barrels or the parts need some kind of dimples or reverse dimples to prevent capillary action from holding parts so tightly together or to the barrel wall that plating does not occur in the interspace?
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi again Arlene.
Your reliability results strike me as already excellent for barrel plating. Although I am not personally familiar with the specialized equipment involved in spouted bed electroplating, as mentioned by George Federman on this page, I do see that the concept of spouted bed processing of small items is widely applied these days in many diverse fields. If I were you I think I would study up on the technology so you are able to expertly discuss it with your team moving forward.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey