I am looking for the best way to design and assemble my first SMD PCB. It will feature a TQFP-48 package with 0.5mm spacing (0.3mm pads with 0.2mm spacing through BatchPCB).
The approach that I am currently favouring is to use a stencil, solder paste and a skillet, as shown on the Sparkfun site. Is this practical for TQFP-48 with 0.5mm spacing? Are the stencils usable with that kind of pitch?
The alternative is HVQFN-33 with 0.65mm pitch (no leads). It’s not ideal because there is less I/O on the part, but if it has a better chance of success then I might take it.
If you are only doing one or two of these boards, a stencil and solderpaste may be overkill for what you are trying to do. The TQFP package is well suited to drag soldering. Drag soldering works by tacking a couple leads in place with your iron, then you apply flux to all the pins, put a large enough ball of solder on your iron and drag it over the unsoldered pins. When you are done you drag a piece of solderwick over the pins to fix any bridges. You can do 100 pin parts in less than a minute once you get the hang of it. If you have a large volume of boards to assemble, a stencil, solderpaste, and a hot plate for reflowing will save you considerable amounts of time.
In the TQFP vs QFN debate, there are advantages and disadvantages to both:
For reflowing, both are easy to work with
For drag soldering, TQFP is far easier but QFN can also be done in this manner
Even though QFNs are physically smaller, they have a thermal pad which prevents signals from being routed underneith them. TQFP doesn’t have this issue which can be an advantage when working with a 2 layer board even though they are physically larger.
langwadt is right about making a custom footprint for the QFN if you want to hand solder them. Here is a board that I just made. U2 on the upper left side of the board is a QFN-36 motor controller. I needed thermal vias to bring heat to the plane on the back side of the board so I made the center one large enough so I could use an iron to solder the heat pad down. U3 on the bottom right of the board is a QFN-12 RS232 transceiver. Since heat isn’t a concern for this part, I simply used a square via underneith it that was large enough so I could use my iron to solder it. In both cases I extended the pads out farther to facilitate hand soldering (even though I ended up reflowing it with a hot air station).
