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surface mount soldering

Started Aug 2010.

Overview.

I used to think the change from through hole (TH) components to surface mounted technology (SMT) could be the end of the DIY electronics hobbyist. Some of the tight pitch devices are a challenge but most SMT device aren't all that difficult and don't require expensive equipment. Packages down to 0603 size and .127mm pitch ICs are not too hard.

Mostly I use a standard "hakko" clone iron with a normal tip. Usually I use regular rosin core solder (not paste) and a cheap ($20ish) magnifying headset. At a pinch 3X reading glasses ($4) will do.

My point is SMT is in reach of almost anyone who can afford basic soldering equipment.

Good goo.

The only "extra" you need is re-work flux (aka GEL FLUX). This is a honey like paste which is IMHO essential. It is the fairy dust of SMT - you need it to make the magic happen.
Farnell currently have this for about $80AU for a 10ml syringe - you can find it much cheaper elsewhere if you shop around. I only use a few ml per year. It goes hard over time but I get quite a few years life without refrigerating it.

Beginners learning regular soldering often make the mistake of using the iron to transport molten solder to the joint. There are exceptions but this is generally a bad thing. For some SMT work we do use the iron to move solder to the job and rework flux lets us get anyway with it. One reason for doing this is the lack of extra hands - with tweezers in one and iron in the other simultaneously applying solder wire would be difficult.

Surface tension.

When soldering big stuff we can ignore surface tension. When we work with small devices (a few mm across or less) or larger one with lots of pads. Surface tension is very significant.

On the down side devices will get sucked towards solder and stick to things in undesirable ways. Packages may stand up like tiny tombstones and do other naughty things.

On the plus side we can use surface tension to self align parts that would otherwise be difficult to position accurately. If all goes well slightly misaligned devices will self animate and moved themselves into position when all their pads are sitting on liquid solder.

Surface tension will even hold devices onto the underside of a PCB and stop them falling off during re-flow.

Provided surfaces are properly cleaned and fluxed surface tension will pull solder into the places we want it to go. You do not need to solder individual pins - you can, for example, solder one row of pads on an IC in a few seconds by passing a ball of molten solder along the row with a bucket tip.

Soldering iron tips.



I prefer to use a bucket tip for ICs but used a "normal" tip for years.
Bucket tips go by at least four names that I know of.
Bucket,spoon,flow type and "bevel with indent". I think they may also known as "wave" tips.

Here I'm soldering a TQFP32 AVR mega328.

In the past I used to tack solder two of the corners to keep the chip in place while I soldered the first side of the package. My new trick is to use two small magnets on either side of the PCB to hold the chip in place. The sponge I'm using as a working surface has a hole in it to accommodate the bottom magnet. I have to keep the soldering tip away from the magnet or bad things happen.

Image Here is a small clip showing a TQFP being flow soldered.


If you watch the clip you will see I had a solder bridge between the last two pins and this was easily removed. These chips are quite easy to do. With care you can solder the pins one at a time but flow soldering is much easier. The fancy tip is not essential but it make it easier.

After taking the video clip the magnets were removed. Having one side soldered is enough to keep the chip in place while the other sides are done.
Image Rework flux can be applied before placing the chip on the PCB but in this case I didn't. I didn't want quite so much flux but it was cold and it was hard to push it through the needle. It is better to have too much than too little.

Image Heating the flux makes it go transparent. This is optional but it is nice to be able to check everything is OK before soldering.

Image If all goes well all the pins on one edge are soldered in one smooth motion.

Image The result looks quite good when check under a 10X magnifier.



Bevel tip.

I've discovered my bevel tip works almost as good as a bucket tip.
These are much easier to get for hakko style irons and 3'rd party tips are quite cheap.
Even sparkfun have them.
http://www.youtube.com/watch?v=k8-TbMve_3E

Re-flow.

Re-flow is when solder which is already in place is remelted. Typically this is solder paste but I use the term loosely. I would say any time all the solder holding a device is melted you have re-flowed it - even if the rest of the board is still cold. Maybe you can re-flow a single join - I don't think the exact definition is important.

I have done solder re-flow using a hot air gun from a hardware store. Home-brew re-flow can also be done in a toaster oven or skillet - even using a soldering iron.

Devices such as 0603 resistors are small enough that a regular soldering iron can heat both ends simultaneously. Thus individual resistors can be re-flowed to self align them.

Special tips exist for specific SMT packages but I've never used them.

SF toaster page - http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=60
SF skillet page - http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=59

As stated above most work can be done with a regular iron. However there are a few devices which really need re-flowing - RFM radio modules and surface mounted crystals are the ones I need to do.

Equipment.

I recently bought a rework station. For about $400 I got the works - soldering, de-soldering, hot-air, hot-tweezers and vacuum pick up.
see http://www.ludatronics.com.au

To examine the boards I use a 10X magnifier. These cost about $30. USB microscopes are also cheap these days. I've ordered one for $70 - you will probably get to see the results.

I like proper stereo microscopes but have never gotten around to buying one.
Many like magi-lamps - I use them sometimes but they aren't my thing.

Different strokes.

Of the people I know doing SM soldering - some use skillet/hot-plate, some use flow tips and some like hot air guns. There is no right or wrong way. Several friend have use toaster ovens but aren't at the moment.

There are also differences in opinion as to the quantity of solder to use. There are many how believe less is better (for big stuff too). Some say it is easier to see dry joints if there is a minimum of solder and the solder form a concave surface as it bonds the parts to the copper.

My methods probably put down more solder that others.

I've seen experience techs make bad joints regard of their soldering philosophy - my boards are usually pretty good. This stuff is small - you really do need to check it over under good magnification. The $30 magnifiers work fine for inspection but you can't use them while soldering.

As time permits I plan to show some of tricks here along with some pics and video clips.

Hot plate.

21-aug-2010.
I went out looking for electric skillets/fry-pans. I came home with a $20 hotplate instead. I had already decided to try the lid from a die-cast aluminum project box as an extra heat spreading flat plate for the skillet - so I tried it on the hot-plate.

The first test showed the solder re-flowed just fine. The problem was that the hotplate had reached over 300C before the heat had got through to the top of the PCB. This meant turning the power off did not drop the PCB temperature for quite some time. The first test PCB was burned.

The second test was the same except I added some glycerol to aid heat transfer and I lifted the die-cast lid after the solder had melted and poked bamboo skewers between the lid and hotplate. This PCB looked fine but it still took a long time to cool down. It was at 230C for quite a while.

The third test was the same except I used olive-oil and after melting point was reach I slid the lid onto a cooling rack. Again this PCB looked good.

The first real PCB was a small mega328 pcb. This has a surface mounted crystal which needs to be re-flowed. I didn't use solder paste, instead I tinned both surfaces, applied gel flux and sweated it on using the hot-plate. It worked quite well.
Image This is my first job using a hot-plate. Looks good to me.

Hot air.

The crystals I soldered using the hot plate looked quite good but unfortunately I needed to change one of them because I wanted to run the micro at 3.3 volts. Using the hotplate was no longer an option because the micro and other components were already soldered to the other side of the board. I procrastinated replacing the crystal because I was expecting it to be a disaster. When I finally tried it it was trivial.
I used the hot air gun on the rework station. I set the temperature to 220C with the default air flow setting and small nozzle. I was surprised how quickly it heated to parts and melted the solder - maybe 15 seconds. I lifted the crystal using a small magnet.
I then pre-tinned the new crystal, added flux and placed it in position. I played with pushing it around a bit with the hot air jet and after a short time the crystal was pulled into place by the surface tension of the molten solder. The circuit works.
This probably wouldn't work for fine pitch components. Some way would need to be devised to hold them accurately in place while the temperature was raised.

This is sort of continued on t85soldering.



2012.

For new year 2012 I've put together another video clip.
This one shows a few different ways to stick resistors onto PCBs. This is not ultra high quality soldering but it is good enough for the hobbyist.
I've done quite a few boards and I don't have a problem with reliability.
Professional boards usually use much less solder - I let the parts have as much as they want.

Resistors a easy to solder and cheap (about one cent each in small lots from element-14).

In this clip I do both manual soldering and re-flow. I don't like solder paste and I'm not very experienced with it because I don't need it.
For production work it makes sense but for a simple board a soldering iron is quicker and easier.

I'm very much out of practice. This is the first time I've soldered for six months or so.
The manual work is done with a 3mm bevel tip. Some other tips are better for some of the operations I show but the bevel is the tip which can do it all.
For a basic hakko (clone) the cloned tips are $5 (ludatronics).
If you want to changes tips for different jobs then a knife,2mm bevel or chisel is worth a try.

The fry pan I used for reflow was the cheapest I could find - about $25. I think it is a little underpowered but it works.

The hot air come from my Aoyue rework station (ludatronics).

I will cover soldering other parts in a future clips.
http://www.youtube.com/watch?v=ClgwI36vlF4


March 2012.


A clip showing other parts being soldered using a 3CF 3mm bevel.

http://www.youtube.com/playlist?list=PLE669EF4F3FE29969

by eddie


Created by eddie. Last Modification: Monday 07 of May, 2012 23:57:19 AEST by eddie.

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