hi i really want to know that what's
wrong with this pcb what's correct wat to solder smd component
That is not your own photo. I think that the photo has bad lighting and that the real board does not have such black soldering. In my opinion, there is nothing wrong with it.
I agree with @Koepel here. Without additional photos to show the issue more clearly, I would say it's simply shadows being cast by the components due to the position of the lighting when being photographed.
@itsashutamar does your board have black residue on the soldering (if so provide photos), or are you simply commenting on a photo you found on the web?
Mr. Solderfix...
yes i'm facing this problem in my protoypes
Can you upload more photos of the problem(s).
Look more closely. The dark color is only on the "hand placed" components. The reflow of solder paste looks fine.
Looks like epoxy rather than solder.
what is a problem?
I'm not sure - even 2 months on since the original question! 
If it works, why is it a problem?
If previously bright solder connections turn black, I suspect you're into an environment where acidic vapor is present. At least, that was my suspicion for a board that was exposed to vinegar vapor in a kitchen; most of the solder on the board had a black/dark grey appearance, though nothing was compromised(yet).
How/where are you storing your boards, and what's the air quality like? That's one reason why manufacturers bag their products, it lessens air quality requirements for storage.
The ONLY type of solder that will turn dark is silver bearing solder. The more silver the darker it will turn. Same reason your household silver will turn dark.
Excellent point, Paul.
its not looking good and after sometime stop working
sorry but i was bit busy in my project really sorry
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