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I'm creating a PCB and this will have an STM32 MCU on it. PCB

Just like this picture, my custom PCB has

  • STM32 MCU LQFP
  • USB connector
  • BGA QSPI memory
  • SMD crystal 32kHz
  • Buck converter QFN
  • Sensors VFQFN, SOIC
  • Pin Headers
  • 0402, 0603 metric chip resistors and capacitors
  • ELECTROLYTIC CAPACITORS, height 5mm

Since each component are packed to each other (it's a small PCB so each of them is quite close like 0.8mm),

I'm concerned about which component to start solder. If I start soldering pin headers, I think these will make a hassle to solder chip resistors nearby.

I'm curious; which part do you start solder in a smart way so that you don't mess up or damage nearby components? Thanks for your answers.

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    \$\begingroup\$ I usually go from small to large and solder all through hole components last. That means start with the 0402 and 0603 resistors and caps and do next whatever you think would be easier now than later. There's no general rule as it always depends on the board layout. \$\endgroup\$ – po.pe Jun 5 at 6:18
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    \$\begingroup\$ @Humpawumpa there's a BGA component on the board... you'd solder all these at once after placing them onto solder paste on the pads. \$\endgroup\$ – Marcus Müller Jun 5 at 6:19
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    \$\begingroup\$ Agreed, I didn't go through the list in detail I've to admit... \$\endgroup\$ – po.pe Jun 5 at 6:20
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Since there's BGA and QFN in there, you're not soldering these components one after the other, but you'll have to solder these all at once, using a reflow oven or at least a hot air gun, after applying solder paste to the pads.

When there's BGA involved, you'd very much want to apply that paste using a stencil.

The pin headers are special, because they are the only through-hole components: Reflow-solder the rest of the board without them, then solder them in manually (i.e. with a soldering iron). Unless you're really new to soldering, there's no chance you'd damage anything nearby when soldering these – pin headers must be the easiest thing to solder on this planet.


further comments:

  • if your memory is BGA already and you're space-constrained: you'd probably want to use a QFN or BGA package for your microcontroller, too. I promise that after layout, you'll realize how humongous the pin headers are compared to the actual components of your circuit.
  • If you're doing this board design for hand-assembly, do yourself favors like putting resistors aligned to each other: makes SMD component placing with tweezers easier, and also heating up with a gun uniformly.
  • Don't try to overdo it with the "smallness" of the board. At well-known manufacturers in China, a 5×10 cm board isn't more expensive than 5×6 cm, so saving space in a prototype doesn't pay at all.
  • Also, realize you'll build a prototype. Nobody in this world had a perfect board after they designed a complex board for the first time. That prototype can be larger than what you want to deploy in the end!
  • Add plenty of test points where you can probe with a multimeter – it's darn hard to figure out what goes wrong if you're not sure whether all parts are powered, for example. Not having test points was my main mistake in my first designs.
  • Honestly, the way you've been asking this doesn't sound like you should be designing a board where you expect components to be packed that tightly. Start easier!

Also, if you were really planning on doing 0402 components with a soldering iron: my respect, I wouldn't have the nerve for that. In the sense that I would really have to quit the caffeine a day before I tried to do that.

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  • \$\begingroup\$ +1 for test points. It would also be a good idea to think if they are only going to be used with traditional multimeter probes, or if an oscilloscope or a logic analyzer are going to be needed as well. \$\endgroup\$ – AndrejaKo Jun 5 at 6:47

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