SMT LED alignment on PCB

[BobW]

A little project that's been sitting on the back burner for several years is an LED bar graph display for a certain special application. For maximum resolution, I'd like to use a large number of very small SMT LEDs, maybe 0402 package. If I could create a viable PCB, there might be a small market amongst fellow hobbyists to use these to replace a display in some obsolete equipment. (I'm being deliberately vague here because I don't want to give away my trade secrets.)

I would have to get the complete board built, with all of the components mounted on it, not just a bare board. So, I need to find a company that can build the whole board in small runs for a reasonable price. Secondly, I probably need advice in designing the PCB layout, because the alignment of the LEDs is important, so that the row of LEDs in the bar graph isn't uneven. I understand that as long as the solder pads are designed with the proper dimensions then the components should be self-aligning. Unfortunately, at this point I'm out of my comfort zone, because my past PCB design experience has been limited to hobby grade toner transfer boards designed with a generic CAD drafting (non-PCB) program.

My immediate concerns are that while I could design some sort of circuit board in Eagle or KiCad (I have both), I might end up designing something that couldn't be practically manufactured. Is there any source of design guidelines for practical PCB design readily available?

 

[KeepItSimpleStupid]

Take a look here: https://www.screamingcircuits.com/Home/HowItWorks for some general ideas of what's required.

LED's may have to be pre-selected because of brightness issues or you may have to order them in the right brightness level and "slightly taylor".

 

[Mickster]

I am adding this because it is something I recently read about.
If you decide to go the route of designing your own PCB, especially with packages as small as 0402,
pad size and orientation are pretty important, as well as pad matching.
Because these package sizes have very little mass, the surface tension on the solder, as it melts, can
more-easily result in 'tombstoning' or chip-lift, if the solder starts to melt at different rates.
Matching the pads to the correct size component can help to prevent the component sliding around
on the pads, orientation can help when the boards go through a reflow oven and matching the traces
going to pads so they are the same can also help to prevent tombstoning.
Component height is important when selecting pads too, as the likes of a thick capacitor will have
much more surface area on the ends than a resistor and be more subject to torque when heated
unevenly.

Just a few considerations for when doing the board layout.

Regards.

 

[BobW]

Thanks for the comments.
So then, it's obviously important that the mfgr. applies the solder paste as evenly as possible, and probably the minimum amount to do a reliable solder job. Now I'm thinking that maybe I'll stick with larger package sizes for this project.

LED's may have to be pre-selected because of brightness issues or you may have to order them in the right brightness level and "slightly taylor".

Good point. I hadn't considered that.

Also thanks for the link to screaming ccts. Very useful info.

 

[ronv]

Thanks for the comments.
So then, it's obviously important that the mfgr. applies the solder paste as evenly as possible, and probably the minimum amount to do a reliable solder job. Now I'm thinking that maybe I'll stick with larger package sizes for this project.



Good point. I hadn't considered that.

Also thanks for the link to screaming ccts. Very useful info.

They make bar graph displays. Have you considered the height of the other components when you think surface mount for the LEDs.

 

[BobW]

I've looked everywhere to find an existing bargraph display that would be suitable, but never found anything remotely close to the size and shape of what I need. So, this has to be a custom design. As for other components, there would be nothing on the display board except the LEDs. This display board would sit as a separate board on top of the main board.

 

[KeepItSimpleStupid]

I don't know if this **broken link removed** article can help you?

**broken link removed**

 

[BobW]

Interesting article. Thanks.
However, I'm using a microprocessor with an onboard ADC, so my chip count is lower than what they have in the article.

 

[KeepItSimpleStupid]

I added another reference.

 

[Wade_Hassler]

Here's one

 

[large_ghostman]

With smaller leds i have cheated and used super glue first then soldered them, I also did a template from thin ally (like a solder template) and used that for lining up, but to be honest i didnt have much problem soldering them as long as you press down while soldering

 

[BobW]

There's an remote possibility that I might try mounting and soldering the parts on a single board, but no way I'd do for a large batch.

 

[large_ghostman]

I found a good slpodge (technical term) of sticky solder paste and a rework gun with the heat high and airflow low does the trick, the trick with super glue in the center of the led works ok but is no good if you dont use solder paste. You can also tin one side only of the led and solder that while pressing down then solder the other and let the solder wick under.

 

[jpanhalt]

the trick with super glue in the center of the led works ok but is no good if you dont use solder paste.

I have used that method for more than 10 years and have never used solder paste. Why do you think one must use solder paste with it? For most hand assembly, I would think the problem, if any, would be when using solder paste.

John

 

[large_ghostman]

I have used that method for more than 10 years and have never used solder paste. Why do you think one must use solder paste with it? For most hand assembly, I would think the problem, if any, would be when using solder paste.

John

I found solder paste guaranteed wicking under the component and using tinning or other methods didnt always give me 100% wicking under components. Its also way easier for a beginner to use solder paste and avoid tomb stoning, I can only relay what experiences I have had.

 

[jpanhalt]

How do you get tombstoning when the SMD is held with CA to the board? Tombstoning is a problem during reflow when using solder paste.

John

 

[BobW]

I have enormously large fingers, and mediocre eyesight. A 0402 package is barely visible to me, and certainly not something that I could manoeuvre onto a PCB with any accuracy. That's why I have a dungeon full of slaves to do this. They're not allowed to have super glue. There's no telling what mischief they might get into.

 

[large_ghostman]

How do you get tombstoning when the SMD is held with CA to the board? Tombstoning is a problem during reflow when using solder paste.

John

You dont exactly but it can lift up at a angle which I hate, I also get more dry joints. If it works for you great, but based on my own experiences it isnt something I would recommend to a nooby to do, I try and offer advice that will give them maximum results. While glue will work with normal solder all smd soldering is easier with solder paste, the same way its easier with a reflow oven.
BTW dont get glue on the top of chips it takes the writing off .

 

[jpanhalt]

BobW I understand that problem with eyesight, fingers, and steadiness. I have several different eyeglasses of different diopters and a magnified headvisor to boot. I have even had two pair of glasses (1.50 and 2.00 power) and a headvisor stuck on my pinpoint head at the same time.

If you disassemble SMD PCB's you will sometimes see a dot of adhesive under components. In my very limited experience, it was red. A little searching later disclosed that LocTite (and probably other vendors) sell an adhesive for PCB assembly. A quick test (to a chemist) was consistent with that adhesive being a cyanoacrylic type (aka CA). That is, at moderate heat it decomposed to acrylic fumes. I tried to buy some, but found the only quantities available were industrial quantities. I then tried "slow" CA which works. Also old medium or thin CA that has thickened with time has been used. You need a CA that will allow a little pushing around to get final alignment before it sets. It does not hold that firmly at soldering temperature, but it holds well enough to get one side or one pin of your components soldered (for IC's, I rarely use CA). I use it mostly for two-terminal and and occasional SOT23-3 device. One nice thin about CA is that its polymerization is base catalyzed. The PCB itself (homemade) or solder resist is quite non-reactive. So, you do have quite a few seconds to get the part adjusted. Then press on it with a little music wire "toothpick," and it will set. To apply the spots, I put a small puddle of CA on scrap copper PCB, then transfer a microdot with the steel toothpick to where I want the device. That allows me to put several devices on the board, then solder them, rather than place device, solder, place device, solder, and so forth. A little too much CA is typically not a problem. It is vaporized easily by heat and, I suspect, might even act as a flux.

My concern with LG's description is that while he may have used CA, it sounded as if he was not doing it properly. Most fluxes are mildly acidic. Applying the flux,then applying CA might work to prevent its polymerization or adhesion to the board.

John

 

[large_ghostman]

Ah what you describe is described as loctite is sold here as loctite spline something or other, it can be had in small amounts but is incredibly expensive (like eye watering expensive), the stuff I used was clear brittle and also used to to stick skin in surgery! The working time is less than 40 seconds, so it does seem we are talking about slightly different adhesives.

There is also a red adhesive sold as a metal adhesive, this will work but I dont recommend getting it very hot as the fumes are awful. So for the record I would assume most in the UK would take super glue to mean the clear 'finger sticker' glue, its easy to forget similar products are slightly different around the world! You have 'Elmers' (love that name!) we have 'PVA' (rubbish name). You have 'sharpi' we have 'Marker', we could do with a kind of Anglo/American dictionary!!