Is there anyone know SMT parts?

[sarah]

I want to know someone engaged in SMT spare parts such as SMT cable .SMT squeeze,PC board,SMT machine,Oxygen analyzer and many other accessories.
Can anyone tell me?

[Garth]

I think you will have to be more specific. I have designed many SMT boards, but I don't know what kind of information you are looking for.

Some of the ways I can think of where SMT layout differs from thru-hole are:

• In thru-hole, every pin goes to every layer. This is both and advantage and a disadvantage. It's an advantage because you can make connections in any layer, but sometimes also a disadvantage because the pin and hole and usually associated pads take space on every layer. In SMT, you normally cannot have vias in pads, because when the assembler silkscreens the solderpaste on, if there's a hole in a pad, the solderpaste will squish through and come out the other side and make a mess. There is a way they can put vias in pads and then fill them and then plate over that so you can solder to the outside, but it adds expense. So in SMT, you'll need more vias, and they take space.
• In SMT, there are more types of packages, so it's not as simple. For example, a thru-hole IC may come only in a 14-pin DIP, whereas in SMT, it may come in SO-14 (possibly two different widths), the smaller TSSOP-14, the even smaller QFN16, maybe a BGA, and others. The smallest ones are not suitable for assembling by hand.
• Making it worse, various manufacturers of SMT parts will call for different land patterns for the PCB, even for the same package, for example a SOT-23. I will not give them each their own. They all get the same land pattern for a given package. So what I have had to do several times is to draw out the various manufacturers' desired land patterns' outlines on top of each other, in different colors, and then make the best compromise.
• SMT is of course generally smaller; but thru-hole allows you to put parts under other parts for example,
  
  
  
  or see here where I put resistors and diodes underneath DIP ICs in this product I designed in 1995. Look at the IC in the top-left corner,
  
  
  
  and then see the same board with that IC removed, showing all the resistors and diodes that were under the IC:
  
  
  
  In thru-hole, I have achieved across-the-board average densities as high as 40 parts per square inch, or more than six parts per square cm.
  
  Today our company does mostly SMT though, and the only parts that are thru-hole are the connectors which have to handle a lot of force. Here are two of my recent postage-stamp-sized analog designs. The second one has 66 components. The smallest parts are 0402 size. I am not allowed to go below that, for example to 0201, 01005, etc.:
  

  

  695Copper.jpg (57.64 KiB) Viewed 43629 times

  

  TT1-Aproto1b.jpg (41.37 KiB) Viewed 43629 times

• Thru-hole does allow you to put parts on both sides of the board, but it's more limited, and it requires more careful planning, not just for layout, but also for how the assembly process will work, so you don't design something you can't assemble. I just finished and ordered a small SMT PCB with parts on both sides, and I am working on another one with four layers and much more complexity. It is quite a challenge!
• SMT's reduced or eliminated lead length results in better high-frequency performance, making it more suitable for RF, high-speed digital, and switch-mode power supplies.

The larger SMT ICs, for example the SOJ-36 SRAMs I show at http://bradsprojects.com/forum/viewtopi ... 6895#p6895, and other ICs with .050" (1.27mm) lead spacing can easily be soldered by hand, even with a large soldering iron. I tell how at the link. I'm 58, and I did not use a magnifier. Chip resistors and capacitors at 1206 are easy, and 0805 aren't too bad. 0603's are getting difficult, and 0402, well, I don't want to try. Too tiny.

[brad]

That's impressive fitting all those components under the board Garth. Were auto routers a thing back in those days? or was it all laid out manually?

[Garth]

All manual. I still don't use autorouters. When you want it super compact, you have to route things while you place the parts, because every mil counts. If you try to sprinkle the parts in and then route the connections, you won't be able to. I'm always moving a part a mil or two this way or that to get the next trace in, or to close up space that wasn't needed, so I can get greater density.

At my last place of work (ending in 1992), we used OrCAD, which had an autorouter. Even with plenty of room between parts, it would often take off in exactly the wrong direction, wander around, and then say "You can't get there from here," even if it was supposed to just connect an IC's pin 3 to pin 4 for example. OrCAD had more bugs than an ant hill though. What can you expect from a company which still, to this day, doesn't know how to draw the schematic symbol for a resistor correctly. I'm using an old version of Easy PC Professional which was a fraction of the price and lets me do pretty much anything I want. The one thing I cannot do with it though is rotate parts at arbitrary angles.

[brad]

I've been using Diptrace for around 10 or 11 years and has been fantastic however similar to your software - I can only seem to rotate components 45° or 90° which can be a little bit annoying depending on what I'm designing.

[Garth]

In the pictures near the top, you'll see an inductor at 22° in the top-right; but I had to make it as a separate component rather than rotate the one I had made earlier. It's very quick and easy to make PCB components in this CAD, except that if it's for SMT, I can't rotate the rectangular pads at arbitrary angles. It doesn't matter for thru-hole parts where a round pad at any angle is the same thing.

[brad]

That's a clever idea - simply making a new part with offset pads - i'd never thought of that. I've always been sticking to my limitations of 90° or 45° angles!