How to choose the correct SMD component?

Thread Starter

spikespiegelbebop

Joined Nov 30, 2021
63
Hello, what is the correct method to identify the correct version of a SMD component when you only have normal size component model as a reference?
What I mean is that I'm in need of finding the SMD versions of following components:

IC: Logic gate 74LS08
IC: PIC uC 12C508
IC: Voltage reg 78L05

For this project.

But when I've checked the JLCPCB website I found many variables of each IC and I still don't know how identify the correct one.
I'll also need to find the SMD versions of the rest of the components (All parts list is attached), but what I really need help with right now is the ICs.

For example, let us take the PIC 12C508, here https://jlcpcb.com/parts/componentSearch?isSearch=true&searchTxt=12C508, this is what we get from the search results:

PIC12C508A-04/SM
PIC12C508AT-04/SN
PIC12C508AT-04E/SN
PIC12C508A-04/P
PIC12C508AT-04I/SN

Taking that the component is 12C508, what does the rest of the letters and numbers mean? Can I use any of the listed components?

I'm in need of creating a small SMT board, with 4 to 6 layers. I'm still learning Kicad.

The other ICs:
https://jlcpcb.com/parts/componentSearch?isSearch=true&searchTxt=74LS08
https://jlcpcb.com/parts/componentSearch?isSearch=true&searchTxt=78L05
 

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MrSalts

Joined Apr 2, 2020
2,097
If the pcb already exists, you pick the one that is the right size based on the datasheet's dimensions for each package size.

Otherwise, if you are designing the board, you pick the one that fits the board and you pick one that you can solder (or have soldered).
 

Ian0

Joined Aug 7, 2020
5,885
The letters refer to two things: the size of the device, and how it is packaged - tube, tape and reel etc.
 

MrSalts

Joined Apr 2, 2020
2,097
@MrSalts So it does it doesn't really matter all those numbers and letters before and after the IC model, is it just the size that matters?
On this PIC device, comparing PIC12C508A vs PIC12C508 is the "technology" used and has to do with the resolution of the silicon wafer - 700nm vs 900nm. Nobody cares. Use which ever one you want.

Other letters are the package (SOIC vs narrow SOIC surface mount and DIP vs ceramic body with UV window.

This is a very old chip (23-years). Look into a modern upgrade. PIC12F508 will allow you to rewrite the code many, many times vs the one-time programmable PIC12C series.

Some other items the extra characters in the part number may mean the operating temp range (commercial vs industrial vs military temp range).
Them how all of the parts are delivered for mass assembly - as @Ian0 says, tube, tape reel size, etc. all meaningless to someone buying a few parts.

Best to read the datasheet. Sometimes, parts with similar numbers might have some extra characters to show it is 3.3v instead of 5volt power. Or some higher level of accuracy for voltage regulators, or who knows.
 

metermannd

Joined Oct 25, 2020
328
And let me point out that multi-layer boards are SIGNIFICANTLY more expensive than 2-layer boards.
Unless you need power / ground planes as a 'best practice', it should be possible to get by with 2 layers.

I had some boards produced recently - the 2-layer boards were about $1 each while the 4-layer boards were a bit over $4 each.
 

MisterBill2

Joined Jan 23, 2018
12,704
My most recent experience with an SMT single opamp ICwas that it was available with different connection locations for the inputs and power connections, depending on two of the the extension letters. This was for a prototype part build. Wile that made the PCB layout creation easier, I see it as setting up for a total disaster in production because purchasing would accept a substitute with the wrong connection arrangement.
( This was a pert sold by ST Semiconductor. I don't know if they still offer that many versions.)
As the application in this thread is for a new product, avoiding such a scheme seems like a much wiser choice.

And for multi-layer PCB designs, producing 4-layer or more reduces the reliability a whole lot, as well as the production yield, because of the many more production steps required and the much greater accuracy needed. And the inter-layer bonds can be a serious reliability issue. Blind vias are asking for trouble as I see it.
 

dl324

Joined Mar 30, 2015
14,905
So it does it doesn't really matter all those numbers and letters before and after the IC model, is it just the size that matters?
The suffix matters. DIP parts that end with N are generally through hole. Parts that end with D are generally surface mount.

The prefixes are the wild wild west. Most manufactures start their 74 series LS TTL with 74LS. Texas Instruments likes to be different and they use SN74LS, but I don't think they always did that.

For 4xxx series CMOS, most manufacturers follow RCA's CD4xxx, but Motorola wanted to be different and used MC14xxx. That was in the good old days. Now other manufactures are trying to be difficult and companies like Philips use HEF4013 instead of CD4013 (or MC14013). ST uses HCF. Samsung uses some bizarre mish mash of TTL and CMOS nomenclature, something like 74HCTLSxxx that aren't even TTL parts.

There are parts CMOS parts that are pin and function compatible with TTL, e.g. 74367 and CD4503.
 
Last edited:

Irving

Joined Jan 30, 2016
2,991
You also need to be aware that different SMD packages can have significant performance differences, especially thermally, compared to the through-hole version.

For example, the TO-220 case version of the 7805 can dissipate 2w with no heatsink up to 50C ambient - that's 12v in, 5v out at 250mA, but the equivalent SMD part in a DPAK-3 case can only manage that when the tab is connected to a pad that's at least equivalent to 20 x 20mm of 2oz copper (400mm2) if the board is mounted vertically in free air, 35 x 35mm if horizontal. The SOIC or SOP-8 parts need double that, and the smaller SOT-223-4 cased parts would struggle with 1.25W on a vertical pad 40mm x 40mm (1600mm2) of 2oz copper.

Those heatsink requirements could potentially scupper a proposed miniaturised SMD version of a product. You may not hit this problem, but whereas a through-hole design in many situations didn't need more than a cursory thermal calculation, the SMD solution may need significant attention to detail to be thermally sound and reliable. That could include providing multiple regulators in the circuit.
 

Irving

Joined Jan 30, 2016
2,991
Guys, can I use a PIC16F15313 instead of a PIC12F508?

EDITED: Nevermind, its size is different.
???

Physical size is same, for 8 or 14 pin variants in PDIP,SOIC, MSOP, TSSOP. F508/9 also in 8-pin DFN.

Later devices have more ROM & RAM, operate to 32MHz instead of 4MHz, and have more peripheral functions. The older devices are c2009 and are not recommended for new products.
 

Thread Starter

spikespiegelbebop

Joined Nov 30, 2021
63
When I've searched for it on Google Shopping tab, I've got a different size. Never mind, it was just there.
Now come to think about it, there's another problem, if I order the pcb assembly, how am I supposed to program the PIC? Because PIC16F15313-E/RF is the component that I'm thinking about using:

1658791605380.png
 

MrSalts

Joined Apr 2, 2020
2,097
Tell the company assembling your boards that your microcontroller needs to be programmed. They can load the code before they mount the chip (preferred).
Or
You can leave some copper pads that you can press a programmer against (preferably with some pogo pins ) or add a row of male headers to connect your programmer to and program it yourself if you don't overload the programming pins too much (look up in circuit serial programming).
 

Irving

Joined Jan 30, 2016
2,991
When I've searched for it on Google Shopping tab, I've got a different size. Never mind, it was just there.
You must read and digest the data sheet for the device(s) you propose to use. You cannot do this with Google and hearsay alone.

if I order the pcb assembly, how am I supposed to program the PIC?
Congrats for realising that before designing (or worse, assembling) the PCB!

Option 1: Program the PICs in a jig before assembly (and repackage to ship to assembler) - only really viable for self-assembly.

Option 2: get assembler to program them - can add significant cost.

Option 3: (most common) Provide the In Circuit Serial Programming (ICSP) connections & logic to the PIC and the ICSP header pins/pads to program the PIC after assembly.

Read, completely understand and implement, paragraph 2.4 AND the whole of section 35 of the data sheet.
 

MisterBill2

Joined Jan 23, 2018
12,704
The burden with almost all processors is that they must have code loaded to be able to do anything useful. That means that code must be written, and often compiled, and then a program loaded. Calling a program a sketch only serves to hide what it really is.
 

Thread Starter

spikespiegelbebop

Joined Nov 30, 2021
63
@Irving
I have the code, but I asked about programming before assembling because jlcpcb said they won't program the IC, well, unless they got me wrong and thought I was asking them to create the code...

@MrSalts

"Or You can leave some copper pads that you can press a programmer against"
That is a great idea. I'll probably try this.
 
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