I have watched a number of videos showing the mounting of SMD parts using a reflow oven and/or blower and also hand mounting - or rather reworking.
I am aware that when having PCB's made one has the option of specifying certain coatings e.g. HASL / ENIG / solder mask.
There are a few things I am trying to get a better understanding of.
When using the reflow oven, no additional solder is applied. Is this because of the HASL coating provides the solder for the joint?
The solder does not spread. I this because the solder mask limits the spread of the (HASL) solder?
I have seen the parts "line-up" - what causes this?
I have seen one video where SMT parts were hand mounted using flux and solder. So when is additional solder and flux needed - is it because the PCB had no HASL layer?
I have seen a video where paste was added - when is paste needed.
I guess in my mind am trying to formulate a table which tells me given a particular surface coating, what are the required steps to mount the parts.
Finally when ordering a PCB, what is the recommended coatings for reflow mounting.
This is what I understand of it.
HASL (hot air solder leveling) is a way of "pre-tinning" the board. Adding a very thin layer of tin over the otherwise bare copper pads. Done by the board house.
A solder mask is the (green) paint, printed on the board, to prevent the solder flowing everywhere.
That solder mask is e.g. between two IC pins. It prevents the pins shorting out during reflowing. This mask is also done by the board house.
Before you put the parts on the board, you apply a special solder paste. Small balls of solder in a flux.
Do not confuse solder paste (lead/tin/silver + flux) with solder paste (the flux only).
Crude way to apply is with a toothpick, or syringe.
Best way is with a (lasercut stainless steel) stencil from the board house.
Then you drop/press the parts lightly on that paste.
NO extra flux is added. The right amount is already in the paste.
You can use lead/tin paste (easy) or leadfree paste (harder for a beginner because of the higher temps).
I reflow/melt the solder on a custom made hotplate, with the help of a hot air rework station.
The hotplate is set to ~180C and pre-heats the board. After a few minutes I reflow with the hot air.
The parts re-align themself (if you have used the right amount of paste) because of surface tension.
This is a 16 channel home lighting LED constant current driver/12-bit dimmer (16x~20watt) board I made with Eagle-lite, ordered through a board house, lead-free solder paste done with a stencil and soldered with a hotplate.
Leo..
Solder doesn't like being on soldermask or FR4 - like how water beads up on some surfaces. Solder beads up on solder mask, and most other non-metallic surfaces.
You have to apply solder paste before reflowing, unless your board already has solder on the pads (extra cost option from board house, I guess? I've never heard of this - wouldn't they need to somehow embed flux in it too?).
Hand mounting SMD IC's requires flux ("no clean gel flux") - you tack the part down, put flux along the pins, solder on iron, and drag it over the pins, and like magic (assuming you didn't use too much solder - a solder sucker is invaluable here), it's soldered. It's amazing how well it works - especially after trying to do it with just an iron and a roll of solder.
I make simple boards at home (single layer - these reflow just fine even w/out solder mask, though I'm sure it wouldn't work with fine-pitch parts, it's hard to etch those at home anyway), and get anything more sophisticated made by OSHPark. I reflow with a ControLeo2 (made from their conversion kit from that cheap toaster oven they recommend) and it works great for leaded solder, and okaay for lead-free.
Extra flux for new ICs/boards in NOT needed. You only use flux if you repair/remove/resolder parts, because the original flux has burned off. Added flux is bad, and should be washed off.
Look at the pic of my board. That IC has a 0.65mm pitch, hand mounted, done with a stencil and silver solder paste. And yes, that solder bridge has to be there.
A hot air station is NEEDED. Soldering irons are for through hole components.
Leo..
