My first PCB is a simple LiPo battery monitor adaptable to the Adafruit feather line of products.
The thing I’m most worried about is soldering the IC to the board. It’s a 3 by 3 mm TFDN package with 14 0.4mm spaced pins, but most importantly an exposed pad. From a little searching on the web, I decided to add via in the IC landing that connects to a filled copper area on the back. Hopefully this will help with soldering the exposed pad by heating the copper area with an iron and a little solder. Do you guys think there’s a better way to do by hand?
Regarding fabrication, I’m thinking of going with BayAreaCircuits. They aren’t the cheapest but because I need the boards in a rush and I’m in Canada, I thought they might be a good option. If you have any other good recommendations for fast turn-around PCB fabs in the US or Canada I would love to know about it. I’ll be placing the order tonight.
Here are the rest of the components which are all through-hole so soldering shouldn’t be an issue.
The IC is a TFDN with 0.4mm pin pitch which is on the smaller side. I’m thinking of going with BayAreaCircuits for fabrication and I’ve selected the minimum track width spacing option of 5 mil.
Sorry I couldn’t post links to the components, since it’s my first post I’m not allowed to post more than one link. But these are the names of the components I am using.
Big THT components are also suspicious when we talk about electrical characteristics. Do you really know how to use TVS/ESD protection effectively? How about bypass capacitors? I don’t think the main IC has been designed with THT components in mind.
First:
imgur claimed they wanted to violate my privacy, so I had a short peek at some greyish fogged-out images.
From those, same comment as the other here.
Working with THT is a nuisance.
You have to bend leads.
You have to insert leads into the holes.
Then turn around for soldering, and the components fall of the board.
Then cut the leads to size.
With SMD components:
Apply a bit of solder to all left pads of each resistor / capacitor. Be quick with the iron, leave as much flux as possible on the pad.
Position resistors / capacitors with tweezers, then re-heat the pad.
Solder all the right sides of the resistors / capacitors.
This can be done without extra flux, but extra flux tends to make it easier, and it’s also mandatory to easily correct little mistakes you made.
I’ve never solderered IC packages like that, but seen some tutorials, which looked real easy when you get the hang of it.
Supposedly you first tin the pads (Again, extra flux is mandatory to make the amount on each pad the same) Just use a big tipped iron (but a clean tip!).
Then stick the iron through the hole from the bottom side, a bit of solder for good heat conduction and careful positioning of your part. After a small nod with the tip of the tweezers it should go back to the center of the pads, which again only works nice with extra flux.
Thanks for your reply eelik. It makes complete sense that the IC is designed for SMT components and not through hole. However I already purchased all the THT components so it would be ideal to use them. I didn’t think about TVS/ESD or bypass capacitors when designing this board, and simply followed typical circuit diagram from the datasheet. I’m very grateful for the document you linked describing PCB design guidelines for the chip as I didn’t come across it before. Most of the components I spec’d are within 5-10% tolerance. I’m just wondering what could go wrong using the THT components as opposed to SMT. As you mentioned the electrical characteristics get suspicious. I suppose I can re-spec the components for SMT but I really only want to do so if absolutely necessary.
Thanks for your reply paulvdh. I agree that SMD components are much better but I wasn’t aware of this when I initially spec’d and purchased the THT components. Do you think using THT will cause any electrical issues?
To give a valid comment on that I’d have to pay clocer attention to the circuit than I’d like to do right now.
You want to buy the exact number of resistors you need?
I’m more of a parts collector myself. I have all the THT resistors to fill up all my breadboards and recently added 0805 resistor, capacitor and transistor “books” from Ali. Complete E12 nicely organized in a compact multomap for EUR20 was more (less?) than I could resist.
I’m not a specialist (in anything), but the bypass capacitors are difficult to route effectively with THT component. I believe THT capacitors aren’t used for that. See Does the connected trace to VCC matter here?.
ESD protection may have similar problems. Placement and routing is critical for well functioning ESD protection. Something is better than nothing, but you don’t want your circuits being destructed by electrostatic discharges. See https://www.ti.com/lit/an/slva680/slva680.pdf?ts=1597091167374 (“ESD Protection Layout Guide”).
Thanks Piotr. Yes, I realize that having separate ground symbols is a better way to do things. The unconnected signal is actually an alert output from the IC which I’m not sure which pin exactly I want to wire it up to on the Adafruit Feather board. I’m thinking of soldering a physical wire to make that connection.
Thank you eelik. I have updated the PCB to make the bypass capacitors closer to the IC pins as well as shortening the path from CSN to the P- as recommended in the document you linked. Do you think something like this is acceptable? C1 and C2 are the b.p caps in the pic.
As for ESD stuff, rules for SMT and THT are the same.
ESD stuff has very narrow pulses steep flanks with high voltages.
So impedance is everything to control them.
Short leads, (no lead for SMT) are better, but not everything.
General rules:
Have a good GND plane, with the lowest impedance to everything.
Put your ESD components near the connector.
Connect ESD Directly to GND plane, multiple via’s are better. (4 should sufice)
Route from connector to ESD device, then from ESD device to your IC’s.
For high frequency content, each pcb track is an inductor, So route the board in such a way with the lowest feasible impedance to the ESD device, and possibly add some extra track length (= more inductance) to your IC’s.
The document mentioned above mentiones spark gaps, which can be completely SMD parts with no extra BOM cost.
(Unfortunately (weird?)) there seem to be none of these in KiCad’s default libraries.
Another Part are deliberate inductors made out of a coil of PCB track.
There are some generators to make these, KiCad is still not so good with round tracks.
Here is an example of such a design.
It is from a password generator, in a plastic housing, and millions of them are made. They use the mounting screws as connectors to (presumedly) do som in-factory programming before the things are shipped.
