The viscosity of the solder paste also varies greatly with temperature change 
that is easier to control. I've had to resolve the same issue at other 
companies and currently resolving it in this building and layout again.
Here are some things to consider before spending $500K.
1. Is the Receiving Dock and Incoming area totally sealed off from the SMT 
area. It is pretty hard to manage temperature and humidity control with 10' 
doors open many times a day. In one factory we put up a frame and sheet 
rock walls around the dock and another for the receiving area. Problem gone.
2. Are the air handlers equipped with programmable thermostates. If they 
aren't and they don't have the dehumidifier options in them then at night they 
will blow in cool damp air because it is cool enough the air-conditioner will be 
acting as a fan only. In the current factory we going to do this before summer.
Hopefully we'll also separate  our SMT area as stated in #1.
3. Try some of the newly released solder pastes. Indium just came out with 
6.3 WS tin/lead that is far better than the 6.2. Senju may not be well known 
in the USA but in an extensive DOE they were the best for all finishes. Senju is 
the #1 paste overseas. 
4. The paste storage and handling in the productin line to prevent the flux 
from separating is important. A low voume high mix shop may need to buy 
5000 gram or less and not the 7000 gram tubes since you can't put them back 
in the refrigerator. Large beads of paste over 1/2" may be easier for the  
operator but in higher humidity and temperature the paste on the stencil is 
going to change for the worse. The paste spec is written for beads up to 3/4" 
but that is in ideal RH and temp situations. Small factories that weren't 
designed for this industry are prone to the same situation that you have. One 
large factory had a huge sock like air exhange units that blew in whatever air 
is outside. In the winter  you can have small humidifiers added but in the rainy 
spring and humid summer you have to relay on using the best paste and very 
good handling controls. Keep the jar low on the rack or floor since since with 
air conditining the floor will be at a lower temperature approximately 72F. The 
temperature in the machine it may reach 78-80 if you didn't add the small fan 
for the summer months. In this way when the operator adds the paste you'll 
stay in the middle of the control range. 
 

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