Along with what Peer stated, there is an IPC Standard that provides instructions on solder paste handling. It states that the solder paste jar or Nelco tube or container should not be placed back into refrigeration once it has been removed. The initial removal does not usually condense moisture because the container is full, with the inner lid and the aluminum foil seal above the inner lid providing a seal from air. You cannot get condensation if no air is present under the inner lid.
Upon removing the inner lid you expose the paste to air, and if you mix it the air is mixed into the paste. You cannot reseal the jar at this point and refrigerate it without having some condensation occur.
Secondly, there are some solder pastes that are much more susceptible to spattering because while sitting on a shelf in the Nelco tube the flux separates from the thixotropic material in a very short time. You can see this when you get a tube of paste and it is dark brown or green near the nozzle where it comes out of the tube. Upon removing the red cap, sometimes it is so bad that the flux will actually pour out, sans fines. When you perform the spattering tests and the wetting tests on copperclad board per the IPC-TM-650 test methods, these pastes will always fail. They usually also fail the slumping and print definition testing as well, even after they have been re-mixed.
It has been my experience that once you have qualified a good solder paste that passed the above tests and the paste is being handled properly (use up all of the single container you remove from the fridge within 2-3 days, do not return it to the fridge, FIFO, etc.), then the cause of spattering is usually moisture trapped in the vias because the pwb was washed between side one processing and side two, and not properly dried prior to printing/placing/reflowing the second side. The other frequently-seen cause is contamination on the pwb/components. Both of these happen quite often.
Most solder pastes today (but far from all) are made with a flux formulation that allows the flux residues to remain on the board between side one and side two reflows, and yet will wash off properly after both sides have been completed. If you are not using a paste of this type, your paste is probably outdated. I think it is a good idea to evaluate new solder paste candidates once every two years to make sure you have the best one for your process.
Another item that can cause spattering, but less frequently, is the type of solder paste in terms of particle size. If you are using Type 3 paste, you should not see spattering unless it is from one of the causes I described above.
However, if you are attempting to use Type 4 or higher, which has a smaller grain size, then you really need to have a handle on your print process, as any overprinting due to misregistration, aperture size too large, etc., will really have an effect on the number of extraneous fines being seen, and this is not really spattering at all, but is often mistaken as such.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of P. Langeveld
Sent: Wednesday, February 07, 2007 12:17 PM
To: [log in to unmask]
Subject: Re: [TN] Max Humidity allowed
The main course of satellites found around solder joints after reflow soldering comes from moisture that is trapped in and/or stirred in the solder paste before using it.
And again the main course is a jar with solder paste or cartridge that has not been conditioned to room temperature when used. When the paste is too cold, moisture in the air condensates on the paste when opening the jar or using the cartidge..
A 500 grams jar needs at least 4 hrs conditioning, a 75 grams cartridge needs 2 hrs.
A standard does not prevent satellites, only describes tests and evaluates the reflow results.
Soft Soldering Processes
Peer Langeveld, Consultant
The Netherlands
2007/2/7, Dehoyos, Ramon <[log in to unmask]>:
>
>
> One way to mitigate the problem of solder balling is to bake the boards for 30 minutes at 70 deg C or store them in a dry room or box prior to processing. Also slow down the reflow process to, once again, dry the board and components to prevent the steaming of the humidity in vias or in the paste which is believed to create solder balls. The ideal environment that I have heard the most is 70 deg F, 70% humidity. That much humidity lessens ESD. Based on the records that we keep, air conditioning does not allow more than 68% humidity in the building we work during the summer and a lot less in the winter. That is due to the fact that when air is cooled, humidity condenses on the air conditioning system. On J-Std-001D 4.2.2 it says: Temperature and Humidity/ When humidity
> decreases to a level of 30% or lower, the manufacturer shall verify
> that electrostatic discharge control is adequate, and that the range
> of humidity in the assembly area is sufficient to allow soldering and
> assembly materials to function correctly in the process, based on
> vendor recommendations or documented evidence of process performance.
> ****For operator comfort and solderability maintenance***, the
> temperature should be maintained between 18°C [64.4°F] and 30°C [86°F]
> and the relative humidity should not exceed 70%.
> For process control, more restrictive
> temperature and humidity limits may
> be required.
> So if you are in an air conditioned building, keep the material dry and adjust the reflow process, the problem should be very small.
> Ramon
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
> Sent: Wednesday, February 07, 2007 2:58 AM
> To: [log in to unmask]
> Subject: Re: [TN] Max Humidity allowed
>
> Surely that depends on the chemistry of the paste, the hold time before and after the component assembly and before reflow, as much as the RH?
>
> Brian
>
> Sklenar Vit (RBAU-AE/MFE3) wrote:
> > Hi Techneters,
> >
> > Please is anybody aware or can you refer me to any standard which
> > addresses the maximum allowed humidity in electronic production
> > in order to prevent microballs creation during reflow process?
> >
> > Thank you fro your reply
> >
> > Vit Sklenar
> > Robert Bosch (Australia) Pty. Ltd.
> > RBAU/MF33 - VS, Process Eng.)
> > www.bosch.com.au
> >
> > Tel. +613 9541 7734
> > Fax +613 9541 3909
> > [log in to unmask] 2
> >
> >
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