Build Details II – 74LVT16245B
Whew! Soldering all those channels must have been monotonous and boring. Maybe you used a toaster-oven reflow. We did not (too much risk to the ASIC we thought) so after installing the new channels we took a night or two off before doing the ’245. Take a break if you need, or keep going while you’re all warmed up – but the 48-TSSOP is not a job to be laughed at.
We’d like to take a moment to discuss the genealogy of the abbreviation “TSSOP”. First, there was the DIP package – Dual Inline Pin. It was a great package and could hold a big old die, but as geometry and die size shrank people wanted something smaller. The SOP (Small Outline Package, sometimes called SO or SOIC) was born. It too was a good package and could be readily soldered by hand. But high lead counts ended up in an unwieldy leadframe (the metal frame the packaged is molded onto). So a new bastard child was born, the Shrink SOIC, or SSOP. Similar proportions to the SOIC but smaller and finer pin pitch. But that device was determined to be too thick for some people (theta-JA and all) and so the THIN Shrink Small Outline Package, or TSSOP was born. It’s thin enough that you can bend it and crack the die if you want, and the lead pitch is pretty damn small. A standard DIP has 100 mil pitch, or 0.1 inch between each pin. The TSSOP has 25 mil pitch, or 4 pins for every DIP pin. GET A FINE POINT SOLDERING IRON TIP.
So here’s how we suggest soldering this beast.
Step 1 - Preparing the pads. Pre-Tin the PCB pads by painting them with flux and wandering over them with a little blob of solder on the iron tip. They should each grab a puddle of solder and look nice and uniformly covered. Now go back and remove almost all the solder using solder wick. Finally, use the iron to wipe any remaining solder out to the outside of the pads. This will create a flat surface that the IC leg can land on, which is essential – if one lumpy pad is preventing the chip from sitting flat, you can get leads that LOOK like they connect but really don’t and the debug is a bitch. The second aspect is that little reservoir of solder at the end of the PCB pad can be wiped back towards the IC to affix the leg once the chip is placed. That tiny pool is all you’ll need. If you try to bring in fresh solder, you’ll probably bridge over 3-4 of the little TSSOP leads. In that case, clean it up with solder wick and retry the wiping.
Fig 9 – Solder Management is key for the TSSOP
Step 2 – Attaching the chip. Place the TSSOP and align it. One side needs to be PERFECTLY aligned, the other side may wander and tend to drop into the spaces between the PCB pads. That’s OK as we will tack down each side seperately. Apply a little pressure to the top of the chip with your finger, and nudge/lever the body of the chip (don’t apply any pressure to the pins, they will just bend) until all legs on one side are aligned with the pads. Once that is done, use the soldering iron to tack down one corner pin. First, press the pin down with the tip of the iron – this melts the solder under the pin and the solder begins to “wet” the bottom of the pin. Now follow up by wiping from the end of the pad back up over the end of the pin. This should result in a nice sheet of solder covering the end of the pin and up over the bottom of the pin. Some more solder will also wick under the pin due to capillary action. Only do one corner pin at this time!
Step 3 – Tacking the opposing corner. Since only one corner is lightly held, the chip can be moved around a little bit to align the other side. As we said, the other pins usually fall into the gaps between the solder pads on the other side and will need to be adjusted. Stop pressing on the top of the chip for a moment and use your tweezers to adjust the chip so the other side’s pins line up with the PCB pads. We usually press the sharp tweezer points into the PCB right near the body of the chip and with a little lever action, we can precisely move the chip the fraction of a mm needed to get final alignment. With finger holding the chip in place, tack the opposing corner to freeze the chip placement.
Fig 11 – Tack the other corner once final alignment is completed
Step 4 – Solder the remaining pins, only 46 to go!. You will probably want to generously apply some liquid flux to each row of leads at this point to prevent solder bridges. Use the same technique of pressing each pin down into the pad and then wiping solder back over it. Sometimes, it is helpful to get a tiny speck of solder on the iron and rewipe over each pad-pin connection to insure it is well connected. One speck should last for 4-6 pins at least. You don’t want a drop, you just want the tip wet – no pun intended. But seriously, you can even rewet the iron by touching some other blob of solder like from one of the channels. The tiny layer of residual solder that the iron takes away is plenty for 3-4 pins.
The ’245 also need 4 bypass caps on it’s power supply. These are the pads located above and below the chip. Install 0.1uf or 0.12uF (100nF or 120nF) here, depending on what you have on hand.
Step 5 – Buzz out your board. That’s an old phrase describing the sound a multimeter makes on connectivity mode – beeping when a connection is made. It means to check the connections, of course. It’s a total pain in the ass, but we really suggest you do it to avoid blowing up a chip (or the ASIC) due to a shorted line. In order to touch the TSSOP leads, it’s best to clip onto a sewing needle or something with a very fine point. Stab the needle into the top of each of the TSSOP pins near where it enters the body of the IC. Now use the other lead of the meter to check that pin’s connectivity to the proper channel of the input circuitry. You want to check two things while buzzing out.
- Check that the pin is actually connected to the input circuit (or to the ASIC if you’re checking that side). The meter should beep (of course) when the input circuit and the TSSOP pin are touched. If not, it usually means that the bad pin is not actually soldered down to the PCB pad. That’s why you want to touch the top of the TSSOP pin – to make sure the path is good all the way from the pin, through the PCB trace, to the other circuit.
- Check that the pin is NOT shorted to any adjacent pins. This is easily done by checking the channel adjacent to the pin under test. The pin should NOT be connected to the channel before or after the present one, but it should beep nicely when the present channel is tapped.
An alternate test would be to stab the PCB pad (not the TSSOP pin anymore) and look for a GND->Pin ESD diode. By attaching the red lead of the meter to the PCB GND and stabbing each of the ’245′s PCB pads with the needle, you should see a diode with a value of about 0.5-0.6v pointing from GND to the pad. Again, you stab the pad to make sure the pin in question is well connected to the PCB pad.
You can check the ASIC side by looking at the value of the diode voltage seen. With the ’245′s diode in parallel with the ASIC’s diode the meter should read something like 0.45-0.5v. If you find one pin that is larger (such as 0.6v), then that pin is probably not connected to the ’245 and you’ll need to go back and resolder it.
It’s a good time to mention this if you have not done it already – you will need to cut the shorts between the 16 resistor pads on the back of the PCB now. Actually, you need to do this before you try doing a diode check on the ASIC side. Umm, because they would be shorting all those lines to ground and you would not see ANY diodes in that case. Here’s a pic from after we gouged out those shorts between the resistor pads on the back.
Fig 12 – Cutting the spare channel shorts on the back of the PCB
Fix any shorts by solder-wicking those pins and then resoldering. Fix any opens by repeating the press, wipe sequence and maybe another speck’s worth of solder. Buzz out again before continuing.
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