Curious as to why the mixed technology on 3977

[amuron]

I took a look at a few of the designs, just found the site, its pretty cool. In a past career, I designed controllers, back when decent margins still existed. While it would be cheaper just to buy an assembled unit, there is nothing like the understanding one gets from diy, and its been a long time since I had my hand in the game, so it would be a cool educational experience.

One thing bugged me. Every 3977 design I looked at, mixed through hole and SMD, and the schmidit triggers were socketed. Since I hate drilling holes, and am not a big fan of hand soldering, ( I use a frying pan for reflow), I'm just wondering what the reason was for mixing SMD and all those through hole parts. Sure, if I was building 1 controller, I guess it would be fine, but I've got a number of ideas, so I might want to make up 12 boards. At that point, I think it might be faster to layout a board using SMD parts to make assembly easier. However, if I have missed something and there is one already done, there is no point in dupicating effort. More importantly, if there is a technical reason why not to, I'd sure like to know.

Thanks
Ron

[philba]

I'm a SMD bigot or perhaps better put, a "TH phobe". Especially when making my own boards. Some stuff doesn't come in SMD versions: large electrolytics, terminal blocks, .1" pin headers, .... Also, some things in SMD cost more - like sense resistors. I've made a lot of boards and have never done a truely 100% SMD board. The good news is you can get the number of holes down to a mere handfull.

Phil

[greybeard]

I've always been used to working on a small scale, and am pretty good with a small iron. But I've never tried smd components, and as Iwill be making my own boards, single sided will be the prefered option.
Any advice or tips on technique would be very helpful.
Thanks
John

[pminmo]

amuron,
I assume your referring to the ones on my website? The socket on the hc14 is always optional. On some of my designs I use an sioc part. The mix of smd and thru hole is just how it happens most of the time for me. If your trying to accomplish something in a single sided version, sometimes the thruhole approach makes fewer jumpers. I mean to run a trace between pin's on an sioc part is a tough sell to most DIY pcb people. Also it many times has to do with parts on hand or cost. To go any smaller than 1206 packages is just to small for my hands and eyes. As I build up a stock of smd parts I use them more and more.

[amuron]

That makes sense from the standpoint of making a board single sided. I have done the same, as I have yet to be able to find an easy way to connect the two sides of a board together (other than just ordering them). I use 1206's as I can run a trace between them. My process is not good enough to do so with an 0805. One time I designed a plasma controller board with 0402's, but I had my tech build it for me. Fortunately he didn't go postal, but he did go out in the backyard and do some shooting afterwords.

I like to buy partial reels of parts off ebay as I'm really cheap when it comes to my own stuff. So far I have been pretty lucky. I may go ahead and do an SMD board, I'll see if it looks reasonable enough, or else I'll just order some of the plate through boards off your website. I'm no guru when it comes to layout. I know just enough to shoot myself in the foot.

There are a few tricks to SMD. I use a magnifying headset, and that works ok. In my past job, we had Mantis viewers, and they made it simple. The other thing I've found is that flux is extremely helpful. RMA is good, do not use acid core, and be sure to clean things up when you are done. I have been less than pleased with no clean flux, although for a motor driver its probably ok, if its active enough.

For socketing a qfp44, get a good socket. The cheap ones are meant for reflow only, and they melt as soon as you get a soldering iron near them. The good ones will allow you to safely hand solder the pins, although it is tight. Another trick for soldering a QFP44 is to gob solder all over the pins, making sure you have good solder joints to the board, and then after it has cooled, flux it, and use solderwick to remove the excess. It puts less stress on the part, and is a lot faster, and shorts are a less likely. It does require some practice. The same works for a TSSOP if the board has soldermask. I have been too paranoid to try it on boards without solder mask.

Ron

[smarbaga]

i remove and replace tsops ( by hand ) whenever needed , i have 30 years experience in this stuff. i prefer the good old day , the tubes, but the consentration of functions makes for a lot less traces wires and soldering.

[garyc]

Power dissipation of the components and the dimension of the PCB may dictate the use of mixed technology. A power FET or voltage regulator in the SMD TO-252 package may requires many square inches of board space for heat sinking, while the To-220 part requires 1 inch including the heatsink. Some ICs in dip package dissapate more heat than the SMT version. If the SMT version is used, then it needs to be de-rated. Most SMD connectors also require a larger PCB footprints than the thru-hole version. Some electrolytic caps have a smaller diameter than SMT. I try to use all SMT components on my designs because they are mass produced, using SMT auto insertion machines. If I have to use thru-hole caps or resistors, they are usually axial leads as radial leads are not machine insertable.

I've designed a 2 axis controller using the 3977 (TSSOP version), all SMT except the electrolytic caps and connectors because of the reasons stated by Phil. To anyone who are building the 44 pin 3977, avoid using the socket and solder the chip directly onto the copper as the ground pins are heatsinks.

Gary

[philba]

when I first started using SMDs, I looked into SMD connectors - much more expensive than TH ones. Probably not as much demand.

On the heat sink pads: most of the SMD MOSFETs I've looked at (lately that is a LOT) want 2 oz copper with a 1" sq pad. With a good heatsink, a to220 way outperforms the various SMD packages, even the dpak ones.