What decoupling capacitor for ICs at high frequencies?

SLK001

Joined Nov 29, 2011
1,543
1) I'm pretty sure that parts will not fly away with SMT methods.

2) This study from Vishay Electronics is quite intersting, and MLCC caps are the weakest component of the PCB.

3) About solder paste thickness: if you check recommended soldering specifications you will be surprised on how much thin the solder layer should be.
1) What do you mean by SMT?

2) What does the study give as a MTTF for chip caps?

3) Solder paste is thin, but not extremely thin. That's why I said use normal paste thickness.
 

Thread Starter

gimpo

Joined Jan 27, 2016
123
1) Surface Mount Technology? (ok, I'm going to check on google)

2) The study it's just an example, has not a statistical value. It's just an evidence from a manufacturer saying: "well... it happens". If you search on google you will see a lot of studies like that.
Understand me, this faults are acceptable on consumer electronics (throw-it-away-and-buy-a-new-one), but that is not my beer (like germans says).

3) Ah ok. The problem is that you need to be a machine to lay down an exact amount of solder paste. Even with stencil the process is quite empirical for humans.That's an important drawback in SMT for little hobbyists like me. Sometimes, when you deposit a too thin layer, the solder can contracts and nothing remains under the pins (but you cannot visually check that!) Dangerous.
 

Thread Starter

gimpo

Joined Jan 27, 2016
123
BTW, why a big amount of solder (like in this video :eek:) does not affect inductance while the lead of a PTH-mounted component does?
 

SLK001

Joined Nov 29, 2011
1,543
The problem is that you need to be a machine to lay down an exact amount of solder paste. Even with stencil the process is quite empirical for humans.That's an important drawback in SMT for little hobbyists like me. Sometimes, when you deposit a too thin layer, the solder can contracts and nothing remains under the pins (but you cannot visually check that!) Dangerous.
If you're a hobbiest, then don't even consider using solder paste, unless you have a reflow oven. Just bump the pads with solder, apply paste flux, put the part on top of the bumped pads and then use TWO soldering irons with appropriate tips to reflow the part in place. This technique works quite well and leaves beautiful solder fillets. Then clean the flux residue as required.

I've been using this technique for years.
 

Thread Starter

gimpo

Joined Jan 27, 2016
123
Good trick. Where you apply the two tips? On the part or on the solder?

I have a good hot air gun; what about soldering like in this video (obviously by using Sn-Pb paste)? Seems less prone to squeeze the solder away (leaving dangerous voids).
 

SLK001

Joined Nov 29, 2011
1,543
Good trick. Where you apply the two tips? On the part or on the solder?
I use the tips as tweezers and hold the part. Another technique is to apply solder to one pad, then apply paste flux, then holding the part with a pair of tweezers, flow the chip into the solder. Now solder the other side.

I have a good hot air gun; what about soldering like in this video (obviously by using Sn-Pb paste)? Seems less prone to squeeze the solder away (leaving dangerous voids).
Hot air guns have the tendency to blow parts all over the board. They are great at removing large parts like QFPs and BGAs. They are also useful at preheating a board that is being reworked. Heating a board to approx 150°F, especially for lead free boards, will make soldering and desoldering easier. Solder "squeeze away" is not a problem, provided a good flux is used. When you pull the tip away, the solder should still be molten. The surface tension will pull everything into proper alignment.
 

Thread Starter

gimpo

Joined Jan 27, 2016
123
I'm designing a new PCB board with pads in place of holes for caps. Then I will make some experiments by using your techniques.
 

Thread Starter

gimpo

Joined Jan 27, 2016
123
A curiosity.
Kemet produces some "J-Leaded" ceramic caps to keep them away from PCB surface:

The attached lead-frame mechanically isolates the capacitor/s from the
printed circuit board, therefore offering advanced mechanical and
thermal stress performance. (cit)
 

Papabravo

Joined Feb 24, 2006
12,862
A curiosity.
Kemet produces some "J-Leaded" ceramic caps to keep them away from PCB surface:

The attached lead-frame mechanically isolates the capacitor/s from the
printed circuit board, therefore offering advanced mechanical and
thermal stress performance. (cit)
That is an interesting innovation.
 

Thread Starter

gimpo

Joined Jan 27, 2016
123
Hello,

Did you read the PDF's in this thread?
Decoupling or Bypass Capacitors, Why?

Bertus
Yesss, the PDF posted by thatoneguy is in my bookmarks since before yesterday.
Honestly I missed the 2 PDFs tha you posted after... :oops:

In the second pdf that you posted there are somethings confusing me... may I?
This sentence is quite ambiguous:

Keep lead lengths on the capacitors below 1/4′′
between the capacitor endcaps and the ground or power pins.
(page 2 / down-right)

Leads? What he means with that word? He is talking about caps with leads (no SMD) or he just means traces?

Another thing. In Figure 6 are drawn two planes, one for Vcc and one for Ground. I suppose that he is talking about a PCB with more than 2 layers, I'm right? Otherwise how can stay that Vcc in middle of the ground plane???
(I ask that 'cause I cannot design PCBs with more than 2 layers)
 

bertus

Joined Apr 5, 2008
20,314
Hello,

The "leads" are the wires used to connect the wired (through hole) capacitors.
The wires can make up inductors that will influence the effectivity of the decoupling capacitor.

A lot of complex boards, like a motherboard or a videocard, will have more layers.
A hobbyist will not be capaple of making such boards by themselves.
You can design it, but will need a PCB house to produce the PCB.

Bertus
 

RichardO

Joined May 4, 2013
2,271
I'm designing a new PCB board with pads in place of holes for caps. Then I will make some experiments by using your techniques.
Just remember to use the correct size pads. Pads that are too large will cause resistors and caps to tombstone on a production board during reflow.
 

Thread Starter

gimpo

Joined Jan 27, 2016
123
AH! Ok, so "leads" was real leads! :D
Thanks for the two answers.

We had a long discussion before this post about SMD vs Leads-mounted capacitors. There is no way, in your opinion, to take in account the added inductance when selecting a cap with leads? I read something about that during this day but I cannot find the link.

I always return back on this argument because to solder SMDs I have to learn a technique that is 100% new to me (as well as buy new instruments and stuff). On the other hand I feel my self quite able/smart in PTH soldering.
 

ian field

Joined Oct 27, 2012
6,539
Good trick. Where you apply the two tips? On the part or on the solder?

I have a good hot air gun; what about soldering like in this video (obviously by using Sn-Pb paste)? Seems less prone to squeeze the solder away (leaving dangerous voids).
Usually - I tack a couple of pins diagonally opposite so its easy to re-melt the solder and position the chip very accurately and all the pins line up.

Where the pins are way too small to solder one by one, I flow a bead of solder along the row, feeding cored solder in maintains the supply of flux.

Its a bit of a contortionist act - but if you hold the board up, topside down - you can flow the bead of solder off the pins and onto the iron tip.

If you got the flux supply right - this will usually take off all the excess solder. You may get one or two bridged tracks, usually the solder can be drawn off with the iron tip as long as plenty of flux is still there.

Desolder braid is another way of doing it, but unless you have a "Big Bertha" soldering iron - there's the risk of pulling up pins, tracks and who knows what else if you don't melt *ALL* its points of contact. But it does leave behind enough solder for secure joints.
 

SLK001

Joined Nov 29, 2011
1,543
I always return back on this argument because to solder SMDs I have to learn a technique that is 100% new to me (as well as buy new instruments and stuff). On the other hand I feel my self quite able/smart in PTH soldering.
Take 10 minutes and learn the new techniques. Other than a second soldering iron, what additional "instruments and stuff" do you need?

For your application, PTH parts will be fine, provided you observe good design and practices. If you are designing a PCB, be sure to flood ground everywhere you can and stitch top and bottom planes with many, many vias.
 
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