Problem with smd hot air soldering

[neazoi]

Hi, I have tried for first time to solder smd components using a proffessional hot air rework station and solder paste.
I have tried different air speeds and temperatures and different air approaching distances and the results are very dissapointing.
The solder paste is lead-free but it does not seem to joint together easily into a big solder bubble.
Look at the picture (2mb size).

What could the problem be?

 

[FvM]

I guess you used the pad-per-hole prototype board for test in lack of a real SMT PCB?

It looks like the nickel-gold surface finish isn't actually clean, as if the board has been laying around in a corner quite a while. Nickel-gold isn't best suited for reflow soldering anyway.

I also see that part of the solder past isn't molten completely.

 

[neazoi]

I guess you used the pad-per-hole prototype board for test in lack of a real SMT PCB?

It looks like the nickel-gold surface finish isn't actually clean, as if the board has been laying around in a corner quite a while. Nickel-gold isn't best suited for reflow soldering anyway.

I also see that part of the solder past isn't molten completely.

Ok I will try it with the right type of pcb.
any general rule of thumb for temperature/time?

- - - Updated - - -

I think that the problem (apart from the PCB you refer) is that the paste dries too quickly.
What should I do to make it dry more slowly?
Should I increase the temperature and decrease the air flow, or the reverse, or...?

 

[neazoi]

Ok I will try it with the right type of pcb.
any general rule of thumb for temperature/time?

- - - Updated - - -

I think that the problem (apart from the PCB you refer) is that the paste dries too quickly.
What should I do to make it dry more slowly?
Should I increase the temperature and decrease the air flow, or the reverse, or...?

Still nothing, I have tried tinned PCB (standard shiny finish pads and the results are exactly the same.
Maybe the lead-free paste is the problem?

 

[D.A.(Tony)Stewart]

Volatile solvents are slightly acidic to reduce surface tension while it is heating up so it becomes liquidus and spreads out then boils off all the flux before it cools.

Look at recommended solder temperature profiles and reduce the air flow to minimum to prevent premature drying the entire process might take a minute.

If the board is copper-plated with something you can check how it responds to surface tension with wet-dry sandpaper then clean. Oxidation degrades board surface tension with solder and usually boards are wrapped to avoid oxidation before use.

 

[neazoi]

Volatile solvents are slightly acidic to reduce surface tension while it is heating up so it becomes liquidus and spreads out then boils off all the flux before it cools.

Look at recommended solder temperature profiles and reduce the air flow to minimum to prevent premature drying the entire process might take a minute.

One minute?! Hm... I have only heated it for about 10sec or less.
When the "liquid" from the paste dries, there is no further activity afterwards.
Maybe the heating time is the problem, I will try to preheat the paste onto the pad, then slowly move the hot ait closer to it, to melt it and let you know how it goes.

 

[marce]

Look up reflow soldering profiling, also specific recommendations for the solder paste you are using.
As previously mention the quality and cleanliness of the PCB is critical.

 

[neazoi]

Look up reflow soldering profiling, also specific recommendations for the solder paste you are using.
As previously mention the quality and cleanliness of the PCB is critical.

There is not such a profile, the paste has been bought from ebay **broken link removed**

I have finally decided to go for the classic thin no-clean lead-free solder wire and fine soldering iron tip method for my project. For high density ICs I think I am going to try the flux and all-pins-solder-with-one-pass method.
I would really like to go for the hot air method because I have the hot air station already, but the results from this paste I have aqcuired are very bad, really useless as far as I have seen.

 

[EzPCB-Nila]

The pic looks like the paste not melt completely.The lead-free paste need about 290 degree temperatue.
And maybe the gold board exposed to the air long time and was oxided,this will lead to not easily solder.

 

[ads-ee]

The pic looks like the paste not melt completely.The lead-free paste need about 290 degree temperatue.
And maybe the gold board exposed to the air long time and was oxided,this will lead to not easily solder.

Since when does gold oxidize in air? It's inert to oxygen, water, salt, etc. https://www.webelements.com/gold/chemistry.html

So I doubt that is the problem.

Finger oils, dirt, etc might be at play, but it's more likely the wrong temperature profile and not enough time in the hot air.

I've watched supertechs use hot air guns and > 1 min of applying heat to get a part to reflow.

 

[FvM]

Although gold doesn't oxidyze, the ENIG (electroless nickel immersion gold) surface is rather sensitive to pollution. Other than a tinn (HAL, chemical tinn) surface, it doesn't melt during solder and deposited dirt can block solder wetting.

Even a clean ENIG surface isn't wetted as perfectly by leadless solder as a tinn surface.

 

[ravindragudi]

I think it will be difficult to explain in words on, how the reflow soldering has to be done manually. There are good number of videos on this topic available in youtube. Those videos will be helpful.

 

[kozacy]

As far as I know, HASL(lead-free) and ENIG are most used in the surface treatiment.

 

[neazoi]

I really believe is is something wrong with the paste, except if all lead-free pastes have this behaviour and need special technique for soldering. All soldering videos I have watched do not leave the hot air more than a few seconds (10?) for small parts.

 

[FvM]

All soldering videos I have watched do not leave the hot air more than a few seconds (10?) for small parts.

Doing so is likely to overheat the package (e.g.) with small ICs. May work with rugged parts like resistors or MLCC.

Generally you'll try to achieve a moderate heating rate, similar to that specified for production reflow soldering. It's required to achieve uniform heating. If the board respectively the vicinity of the component is heated up to e.g. 150 °C, the final reflow operation is relatively fast.

 

[rx8pilot]

I slowly pre-heat the PCB (or the general area) for about a minute, and then take another minute to bring the joint area up to flow temp. After flow, another minute of reducing temp to under 100C or so. This should allow the flux to do its work before the solder melts. The flux is activated at an elevated temperature but lower than the melting point. It needs to have some time at this temp to be active and clean the joint properly.

This works 100% of the time for me, even when the PCB is not perfectly prepared (it needs to be reasonably clean). As other mentioned, your eBay paste may be no good. Not all paste created equal. I have had various results with cheap paste, but perfect results every time with the premium brands.