Solder paste doesn't work very well in this situation as it is very hard to get the amount right. It also tends to get behind the pins whereupon it doesn't heat very well and wont wick easily. I practiced various techniques on an old SIMM card, and just couldn't get the paste to work well.

The approach that works best (for me anyway) is to use the iron to 'paint' the connection. The blob of solder on the end of a 1/16" tip is enough to solder several pins. Start by fluxing all the pins, position the new chip and tack two opposing corners. Verify the alignment, add more flux, and then drag the blob slowly along the line of pins, allowing a couple of seconds per connection. When each pin heats up it will wick solder off of the iron and into the connection whereapon it heats the other pin and results in a good solder joint.

    ||  ||  ||  ||  ||

         (OOO)

        [TTTTT]     Drag slowly --->

        [TTTTT]

 

The keys to this technique are flux and patience. If you don't allow a enough time per pin then the solder won't wick properly and you'll end up dragging a bridge of solder across _all_ the pins. If you don't have enough flux then you will also bridge adjoining connections. With a hundred connections to make it is likely that one or more bridges will be made anyway ('cos we're not pro's). Clean all solder off of the iron, add flux to the bridged pins and reheat. Often this is enough to wick out the bridge - the solder wicks back onto the iron. In stubborn cases where this doesn't work, fine desoldering braid will wick it out (with flux, obviously). When this happens you'll need to repaint the connection and also a couple each side of it as the braid isn't fine enough to only hit one connection.

This technique is used by many professionals to mount fine SMT chips to boards - they paint the pads and rely on the wicking action to draw solder under the pin. In this way they avoid physical contact with pins that are easy to bend. Allegedly they use the biggest tip size that they can get into the board as it holds the most solder.

I managed to stack 4 chips on my SIMM using this method. I haven't tested each and every connection, but they looked good.

DISCLAIMER - This post is purely informational in nature. I do not accept any responsibility for any actions taken based upon this post,