a couple of additional pointers :
1) solder paste :
That stuff needs to be stored in a fridge for two reasons :
a) to avoid gravity from separating the solder out fromt he flux in the emulsion.
b) to prolong the life of the flux.
and if using water soluble there is a c : keep moisture out
Paste is to be stored in a closed jar , or the syringe is to be capped after use.
Not all solder pastes are made equal. Solder pastes in syringes are often the same formulation as the paste sold in jars for screen printing. that is NOT the kind of paste you want for this kind of hand job. Those are made avaialble for touch-up work and localized screen printing using mini-stencils. if yuo are going to do the 'squirt-n-dab route you need a paste that flows easily. Especially if you don't have a controlled application system that uses vacuum and air ( a 'calibrated thumb' clearly does not fall in this category )
Solderpastes are thixotropic in nature. shake or stir them and they become very fluid, so easy to apply. after a minute or so they return to a peanut-butter state. this is why , in industrial applications , the jar is opened , a scoop of paste is placed on the edge of the stencil , and the robot then goes through a program where it 'liquifies' by repeatedly scooping and scraping the paste. Personally i don't like the pre-filled syringes. i buy a jar of paste , scoop a dab out , stir it so it becomes pretty fluid , fill a bit in a syringe and when done squirt the rest back in the jar for reuse later.
the advantage of having the past in its liquid state is that you don't need to press so hard on the plunger and this gives you more control over the amount dispensed.
If you watch dave's video closely , a fraction of a second (time index 11:16) before he pulls the heat gun back you actually see the chip 'settle' on the board. You see that at that point all the solder ont he pads is in liquid state. That is the point you are looking for. This means hat ALL solder has now entered a liquid state and the chip is 'floating' on the solder. The surface tension and cohesion forces now 'align' the chip perfectly with the pads on the board.
One thing i noticed is that this board uses tin-flash as opposed to HASL. A couple of words of caution on that stuff:
Avoid that as the pest if you are going to hand assemble ! Shelf life is horrible. tin flashed or ilver flashed boards tarnish very quickly under open atmosphere. they need to remain in vacuum sealed packaging until ready to use. Get a greasy finger on them and you have soldering problems...
When you get these boards : run them all at once. keeping a few for assembly in a few months will give you only scrap boards unless you vacum pack them with dessicant.
Tin flash and silver flash do have superior flatness on par with ENIG or ENEPIG ( what is commonly called 'gold' or palladium-gold. The gold in these processes is only there to protect the nickel [nikcle tarnishes extremely bad ...] . during soldering the gold diffuses in the alloy and the solder joint is actually made to the nickel layer ) processes , are cheaper than the ENIG or ENEPIG but have very short shelf life.
You are by far better off having a good quality HASL board. A decent pcb fab has excellent control over the air-blade used in the HASL and the end result is almost as good as tin or silver flash. Hot-Air solder Leveling wors by dipping the board in flux and then dunking it in a vat f liquid solder and pulling it back out. a controlled blast of hot air (the 'air knife') where the board leaves the solder makes sure that solder is deposited eavenly. temperature control is very important in this process. the air blast needs to be slightly hotter than the tin temperature to ensure perfect release.
Now, here is how our 'soldering lady' ( we have a dedicated person in our lab to this kind of work. she's got all the IPC certs including Avionics and most of the MIL-SPEC's... ) places this kind of components.
- flip the chip upside down.
- squirt solder paste on the pads.
- heat with hot air gun so the solder 'beads' on the pads.
- inspect the solder bumps created and confirm all pads have an evenly spread of solder on them. there should be no area's that are devoid of solder.
- flux the board
- place the chip on the board
- heat with air gun and watch for the 'settling'
the advantage of this process is twofold
1) You have visual confirmation that ALL pads do indeed have solder on them. if a pad was tarnished the solder will have flowed to an adjacent pad. this avoids having a pad with no , or not enough, solder on it. Solder 'bumps' should be equal in size and flow over the entire pad surface. You can correct this at this step.
2) There is no risk of tiny solder balls or unbound paste remains under the chip that could create shorts. When squirting paste between the board and chip and then heating you have no way , apart from an x-ray, the confirm that all solder went where it should have gone. By placing the paste no the chip and then reflowing it you can inspect the chip for 'stray' solder.