How to use hot air equipment?

This general topic comes up quite often. People have used all sorts of approaches with varying degrees of success.

Firstly, you are using a Solder Mask, I hope - if not, all bets are off !

The 'simplest' approach seems to be to place a via or two in the large pad, and design the other tracks so that there is a heat path to the actual pads. This gives you somewhere to place your soldering iron tip to make the joint - the track/via will conduct heat to the pad.

Hot air (and infrared) work best when the back of the board is heated to a bit below solder melting point - the hot air then has only to provide enough heat for the final 20C. If you don't do this, then you must have lots more airflow, which does indeed tend to blow things off the board. Using infrared instead of hot air does not suffer from this problem.

My understanding is that you need to use some

Commercially this is done by screen printing the solder paste, placing the chips and then baking a bit to 'fix' things, followed by the final heat to melt the solder. Once the solder melts, surface tension takes over, and with a well-designed pcb layout the chip will self-align with the pads. [ Sometimes the parts are actually glued in place before heating. ]

My experience is that it is difficult to do a good job of applying solder paste by hand to such small footprints, even with a dispensing machine. In practise your board may already have enough solder on the pads to solder, so it would just need flux. If there isn't enough, consider building up solder on the pads first by applying solder paste, then a quick run over with a soldering iron, then clean the excess away and apply flux and solder in.

Good Question ! This a good reason for going infra-red. Unfortunately the kit is expensive for some reason that I do not understand.

Good Luck ! Dave

On Fri, 27 Jan 2006 04:45:12 GMT, Michael Noone wrote:

As you are no doubt aware the proper way to do this is using a solder mask, paste on the pads, and a reflow oven, which puts the board through a controlled thermal profile including a brief phase above solder melting point. However, depending how valuable your chip is, and your willingness to experiment, it is perfectly possible to do reflow in the kitchen. Obviously your chip may get overheated without that tight temperature control, and your board may also get stressed by sudden temperature changes. But the result may be OK for a 1 off. To attach just one component using reflow recently, I used our microwave oven that *also* has thermal elements. You can buy cheap domestic ovens that do this job, and even make fancy thermal controllers for same if you are keen, I'm making the Elektor mag design at the moment. I preheated the oven to 220C, and then put the board in with that one component on it, and could observe the solder melting pretty soon through the glass door. At that point I opened the door and turned the thing off. A crude but effective method in this instance. You maybe don't need solder paste for doing a one off. I just applied wire solder to the pads with the iron first. I reduced the amount to a reasonably small "bump" on each pad using braid, then applied non-corrosive flux using a pen, both to the component and board. The component ends to wobble a bit on top of the solid solder bumps, you can balance the whole lot on a metal tray for careful transfer into the oven if you want. Best of luck.

Steve

NO! Please tell me you are not now using this oven for food?

Did you read the MSDS on the solder paste?

I didn't use paste. I somehow think I'll survive one board being done in there ;-). Of *course* nobody else should do this at home.

Steve

Further to Dave's comments...

Hot air can work quite well on tiny parts (I regularly do 0402 SMD). The smaller parts will self-align due to surface tension on the solder, which turns out a nice result. And no, tiny parts don't get blown off the board with the right setup (you don't need much airflow; approach the 0603's from the top, not the side).

There is technique to it, so don't expect perfection the first time around. The hot-air pre-heating of the underside of the board is crucial; don't bother with a hot-air station if it doesn't have a pre-heater.

For example, tiny parts can "tombstone", where solder on one end liquifies before the other and surface tension causes the part to stand up vertically on that pad. Nothing a dental pick can't fix. If you're more aware of the thermal differences on the pads (e.g., power planes), you can avoid this easily through technique (from the top of the part instead of the side, heating both pads at once, with a little more heat on the larger mass).

You need a solder paste with a needle applicator tip - and a little goes a long way. (You don't need to refrigerate except for storage; one syringe lasts a long time. It's easier to work with if it's room temp before applying.) I use the stuff from

with the smallest black applicator tip (buy one tip per use). On a QFP, a tiny line across all the pads works great (i.e., you don't need to apply to every pad separately). On fine-pitch QFPs, I add a little more solder on a second pass, then use flux and wick to remove any excess. Solder mask (and generous flux during cleanup) is crucial for fine-pitch QFPs (and probably DFNs) so the solder automatically flows to the right spots.

You could probably heat the board and package enough from the top with the hot-air to reflow the large pad underneath using paste, but I haven't tried a DFN yet to say for sure.

FYI, Zeph's removal system is pricey but very effective at removing parts (it seems to work like Chip-Quik's). I've salvaged several PCBs this way, allowing me to just replace the toasted QFP instead of rebuilding a new board. With a lot of effort to clean the legs, you can even reuse a removed QFP if it's expensive.

HTH, Richard

You hold the chip down while blowing from the sides.

Not for QFN or PLL6 (in my prototype). The pads are completely hidden from the top.

SPE-0012 at zeph.com is lead-based. I don't know if they have lead-free (mandatory in July 2006), Lead-free are higher in temperature and harder to solder. On the other hand, Silver or Gold based solder pastes have higher surface tension and is better for fine parts.

Don't be cheap on masks and pastes.

"Dave" wrote in news:1138353866 snipped-for-privacy@sp6iad.superfeed.net:

Before I realized I had access to hot air equipment I placed a large via (1.5mm diameter) underneath the DFN chip. The pins on the DFN are just

*slightly* accessible from the sides, so I thought if I had to I could probabaly solder them since I brought the solder mask out a little past them. And yes I am using a solder mask - I can't imagine doing this without one.

How is the back of the board generally heated up? Could I use something as basic as a skillet? Would that damage the solder masks or traces on the bottom of the board? I should note that on the bottom of the board there are no very fine parts - just a number of 0603s, two SOICs, and a couple larger passives.

I have seen where people often just apply a line of paste across the pads. This technique would not be appropriate I take it?

I'll need it! Thanks,

-Mike