You should ascertain that sintering has occurred in the desired way, first. Even if sintering was correct, voids or pipes can severely alter the bonding result. I suggest a cross section microanalysis of the die after attach (optical microscopy or confocal one may be sufficient)

It is recommended to do the sintering in air circulated oven we could try in vacuum oven but I think that pipes void that you said is that much determinative. I was thinking if the silver metallization on the top of LTCC ceramic is bondable to naosilver or not? and is that silver metallization compatible with silver sintering process or no? because sometimes they use silver paladium instead of pure silver in metalization part, but I don't know between pure silver metalization and silver paladium which one work better for nanosilver sintering. The second doubt could be which metalization could be the best on the back of the die. We currently use Ti/Ni/Ag on the back die to be bondable with SAC solder but I don't know this die metalization works for nanosilver sintering or No. Please share your thoughts with me. Thanks

Ardalan

Not knowing your sintering temperature and time held at  the temperature, it is difficult  to comment, however, if you have used Glass Frits in the paste, it would improve the bonding and mechanical properties, specially shear strength nanosilver on substrate. Possible to tell something about your substrate.

Also try using the sintering temperature two-third of m.p. of silver and allow to hold for atleast 3hrs or more to allow the pores caused by burning of the bonding compound from nanosilver coating can be closedof by diffusion. Check it the porosity in coating and the bond between the coating materials and substrate using optical microscopy. Try it out and if possible  let me know.

Dr. K

Thanks for your response. The recommended sintering time and the temperature are 10 min ramp up to 260C and stay 1 hour in 260 C. two third of the silver melting point (960c) would be 640C for 3 hours. it is almost 3 times above recommended sintering temperature (260c) it is good for a try but I need to see the possibility. What do you guys think about metallization materials and compatibility of those with nano silver sintering process?

Ardalan - Comments I have made above are for the bulk components and  so  neglect  iit..

However if you are trying to sinter nanosilver joint to  power chips, then firstly remove the volatile materials from the nanosilver paste at ~100 to 125 C, and then try to sinter it using high pulse current of 1.0 to 1.3 kAmp for at least for 30second interval at up to 2 to 3 minuntes. Since the whole operation takes only few minutes, there is no need for any protection. This would give a good lead free die attachment with shear strength of about 25 to 50MPa depending on the sintering time at the pulse current. Hope it works out well and if possible let me know.

Thanks Navin

I need to check the possibility for pulse current. Yes, I use that module to attach the power die on the top. 

Also, I didn't see pulse current assisted option in paste data sheet. Do you think does that work on this type of nanosilve paster? Also, don't you have any other option in mind which could be just thermal sintering?

Ardalan,

My opinion is that your back-metallization is ok, it is standardized. I do not know if a pure Ni top metallization of your ceramic package would better fit nano-silver paste die attach compared to NiPd. That may depend on NiPd composition. However, consider that Pd addition reduces oxidation during die attach process, which is of course a good thing. By the way, if your paste is a commercial one then it is surely tested in several cases. Tou could try seeking for some application note of the supplier that matches your case as closer as possible. I agree that 260 degC as peak T should be enaugh, however the thermal profile of the whole process (that is in the range of few tens of minutes at most) has to be properly tailored, and an initial low-T step to degas VOCs is mandatory otherwise micro-bubbling can occur during sintering causing voids. Another concern may be the process atmosphere. I think that a flowing inert gas is surely better than air. If possible a mixture of nitrogen and a reducing gas (such as NH3) in small concentration (say less than 20% v/v) would also be better so to inhibiting Silver oxidation along the whole process.

I know all above is pretty general, but the best recipes would strongly depend on your specific cases. Hope these may be useful guidelines

Giuseppe

Ardalan  - Do you any facility to apply nanosilver coated chips to sinter under pressure? Although not knowing the particle size of nanosilver in the paste, but it would be around 2.0nm, and if so then it has a large surface area which could bring down the mp of nanosilver paste to 350C around one-third of the mp of silver. This would help if the coated chip is sintered sandwiching under pressure between 1 to 5 MP in two steps, first to 180C to remove any VOC substances and the then raise temperature to around 300C and allow time from 1 to 10 minutes. Thus by changing pressure, temperature and time, you should characterize the each condition for microscopically and joint strength. This  would give ample data to write up your thesis, if you are working for MS or PhD. My students do the same. 

Dr. K

1. Set the sintering temperature curve and atmosphere according to the properties of the substrate

2. The sintering temperature determines the type and content of glass powder added