Solder stencil helpl please

Flashlight Modding and DIY Parts
1

Hi all,

I need to reflow solder a large number of LEDs onto frames of mcpcbs. I have ordered stencils to do so, tried both the plastic and stainless ones but I still have some major issues. The LEDs I am working with are very sensitive to any imperfection in the thermal or electrical path. When I reflowed these individually moving the LED around to spread the solder and then pressed down on the dome to squish out the extra almost every single one I ever did worked perfectly. Failures were negligible. Sadly moving to the stencils I get about a 30% failure rate on the LEDs.

What I think is the problem is inconsistency with the paste. It seems like a large number of the failures have just slightly less solder under the center pad. I did also find a few where the LED did not sit down all the way or was not level but more of the first. I use kester EP256 solder paste.

Adding to the horrors of doing this project is that all of the LEDs must be viewed under a large aspherical lens so they need to come out 100% clean and neat hence the further need to get away from the individual method because of all the clean up.

My questions.

1. What do you think could be causing this issue? My technique? Equipment? etc…
2. What solder paste should I be using? Does it come in a bucket I can use a putty knife with?
3. What thickness of stencil?
4. Should I be getting framed stencils?
5. Would this help? https://www.ebay.com/itm/CYBRES-SP2421-Manual-Stencil-Printer-SMT-Schablonendrucker-/252416551098?\_trksid=p2385738.m4383.l4275.c10

Thanks in advance!!!

2

How are you forcing the paste through the stencil and clearing away the excess?

3

I don’t know much about stencils, but you could make that frame with about $30 worth of surplus aluminum extrusion t-slot and some hardware store threaded rod, no?

4

Maybe you could try a thicker stencil and a more rigid scraper?

5

1. Could you please share you pad`s shape?
Right pad shape (with right pcb coating) together with right solder paste are doing magic. You can put a small cylinder of paste across your pads, and it will find right place itself.

2. Do you keep solder paste in the fringe?

6

Always had good luck with the standard SS stencils from OSHStencils using the included credit card scraper.

I hold the scraper at a 45 degree angle and press hard just before the edge, forming a convex shape. That way the paste is first squished into the hole by the convex shape of the card, then the excess scraped away by the sharp trailing edge.

I keep the solder paste in the fridge, then let it warm up for an hour before using it. When reflowing, I follow the recommended reflow profile, letting the board soak at 150F for 60 seconds so that the flux can do it's work, before ramping the heat for a quick reflow.

7

Geez that looks super clean calipsoii

8

Solder paste has a shelf life. It keeps much longer in the fridge, first thing would be to try some fresh stuff.

It also dries out, so don’t scrape the excess off the stencil and put it back in the jar to be economical. Scrap it.

Then there is the thermal profile. The manual individual technique of applying far too much, whacking it onto a hotplate, then tapping the LED to squidge out the excess as molten beads once it starts to swim around, won’t work (and is not good for the LED, long term.)

A controlled warm-up, heat-up, cool-down profile will work much better, in particular the initial ramps dry out the volatiles from the paste before it melts, so it doesn’t bubble up, risking voids.

The LEDs also have a shelf life for solderability. Old ones, possibly not still sealed in their tape, or not kept in the recommended environmental conditions, may simply have lost their solderability qualities.

Using a component prepared for ROHS (lead free) solder may not work well with a traditional tin-lead solder.

Manufacturing companies spend a lot of effort developing their processes, with designed experiments and fine tuning of equipment. Expecting to get it perfect, first time, with more basic equipment may be unrealistic.

Or find an experienced contract assembler who could do this for you.

9

I have reflowed many LEDs and remove the old ones that were reflowed by big companies

often with lead free I heat my reflow plate up and push on the side of the LED with some force,
so the solder not really melts when the LED pops off

I have seen a lot of them with similar problems, bubbles in the solder ect.
like 0% of them had an even spread on the whole pad

I remeber 2 Klarus lights that had a died LED because almost none of the solder was sticking to the thermal pad

the only solution I see, without moving the LED gently and touching slightly on the dome to evenly spread the solder and force bubbles outside,
is either keeping the thermal pad fully covered with paste and the outer pads only with about 70% paste

there is some alcohol stuff in the paste that evaporates in the reflow process, likely if done too quick it will likely form bubbles,
and without moving the LED they will keep inside the big thermal pad

On top of that our LEds are different from resistors or MCUs that never cover 100% of the pad so the the solder is forced to move around on its own following the adhesion forces

10

usually you take a really rigid scraper and hold it 45° but opposite to your stencil card
moving it over the stencil should push the paste in it and scrape off

if you notice your paste getting a bit to dry and its in the scringe type cut the end push all to a pot and add a tiny bit of isoprpanol or ethanol
its usually adding only a few mg alcohol per gram paste to soften it up again

and also very important is as calipsoii wrote a slow heat up phase in your reflow,
putting it in a pan and turn your gas stove at 50% output to get done relative quick is not good, not for the LED or the paste

if you can get a steady heat up 1°C per second you are perfec, also cool down relative slow

My tip, never use lead free paste its a piece of garbage, the majority of flashlight companies solder still with leaded paste

11

No, the Chinese ones do not. They all use lead free. You will not find any leaded solder in a Chinese-made torch. There were a number of serious scandals a few years ago then their government clamped down. Along with other toxic things. This is taken very seriously there, heads rolled (literally, well bullets in the back of the neck, charged to the families)

They are very clear about this, as are the competent border authorities.

RoHS applies, and it would be very foolish for anyone there to ignore this and try to sneak something illegal in. Consequences from Chinese authorities would be much worse than just the slap on the wrists from EU people.

RoHS also applies throughout the EU. So anyone, no matter how small, cottage industry etc, still using leaded, then selling commercially, or importing such things, is breaking several laws. Not going to tell though :wink:

Meanwhile, for now, we can still buy and use this good stuff (solder) ourselves, it’s an odd one, and in e.g. mil aerospace applications there is an exemption also allowing it to continue. Just not in commercial, domestic, consumer stuff.

Then I continue to shoot lead bullets and shotgun cartridges. One session might send many kilos downrange. Allowed (except over wetland).

12

So does this just apply to the solder paste in china. I see all kinds of solder wire on BG.

Just curious :slight_smile:

13

I think RoHS is meant for things like the millions of Tamagotchis and other products sitting at the bottom of landfills

14

It applies to assembled electronic items. And is the law.

You can buy any materials, made of anything not a controlled substance, that you want, and use it yourself, but if done so, don’t “put it on the market” here.

Not that anyone will look, once it inside our borders.

But, as I said, after some major scandals, not particularly to do with minor things like torches, China themselves self-regulate very closely.

Certain EU countries are taking this seriously, and rejecting imports at the borders, based on falsified CE and RoHS marking with laughable, if any, paper work behind it.

Many cannot even print the CE mark correctly, never-mind concoct a plausible bit of paper to include in the box.

Germany is quite hot on this at the moment, particularly for obviously high-value under-declared items, if you watch these forums. They get sent back. If it happens again, they are just destroyed. Hence certain big retailers/drop-shippers have started to introduce policies where they won’t accept orders of some things to “difficult” destinations.

For example the BLF GT was just a, frankly, pi*ss take, shipping it in two parts to try to skate under the radar. Whose cunning idea was that ? (Yes I do know).

15

Thanks for clarifying Tom Tom. :+1:
I was having trouble figuring that one out with seeing all the lead solder that was coming from there.

16

Snap it up whilst you still can.

It really is superior. A kilo of suitable material will last a lifetime in amateur use, and could be passed on to your heirs.

Solder paste is different, it has a life of very few years at most, even if purchased new and kept in a fridge. The solder particles oxidise, the flux degrades, all rapidly. Unsurprising because it all just a fine mixture of tiny particles that degrade in days, months, years, suspended in a medium of other stuff, chosen for other properties, lasting forever not being one.

Whereas I have solder “wire” with embedded rosin flux that is over 40 years old, but as good as new.

Nevertheless, components will continue to be soldered, but maybe with different lead-free RoHS metallurgy that might actually be an improvement.

However, when it comes to using an actual soldering iron (even on SMD stuff), I know what I like, and that still requires lead based solder.

Here is another broken wikipaediea link: Reflow soldering - Wikipedia

17

You are totally wrong!

The original RoHS, also known as Directive 2002/95/EC, originated in the European Union in 2002 and restricts the use of six hazardous materials found in electrical and electronic products. All applicable products in the EU market since July 1, 2006 must pass RoHS compliance.

Directive 2011/65/EU was published in 2011 by the EU, which is known as RoHS-Recast or RoHS 2.

RoHS 2 specifies maximum levels for the following 10 restricted substances

- Lead (Pb): < 1000 ppm

- Mercury (Hg): < 100 ppm

- Cadmium (Cd): < 100 ppm

- Hexavalent Chromium: (Cr VI) < 1000 ppm

- Polybrominated Biphenyls (PBB): < 1000 ppm

- Polybrominated Diphenyl Ethers (PBDE): < 1000 ppm

- Bis(2-Ethylhexyl) phthalate (DEHP): < 1000 ppm

- Benzyl butyl phthalate (BBP): < 1000 ppm

- Dibutyl phthalate (DBP): < 1000 ppm

  • Diisobutyl phthalate (DIBP): < 1000 ppm

and ROHS is an EU directive, not sure if chineese companies have to follow it for chineese only products

so RoHS means the final product has to contain less than 1000ppm or 0.1% toxic substances for lead,
impossible to achieve on a single driver board

but even on a seperate sold MCPCB with LED on it you can reach it and be ROHS conform using leaded solder
the usual 16 or 20mm copper MCPCB weights 2-4g and on a 3535 footprint so 2-4mg lead are allowed when sold seperately
in the solder 0,33x 0,25mmx0.05mm SN63/37 makes less than 1.5mg lead

so this is 750ppm so well withoin ROHS

Its is a matter of fact they solder with saimple 60/40 I can see it at the melting point, its not low temperature solder nor lead free definately a standard with lead
most of the drivers get soldered lead free but not all

What you mean are scandals in food insustry, but I doubt anyone will try to eat its MCPCBs with soldered gear
there are tons of ROHS exeptions of toxic metals in the semiconductor industry as they are substancil to make them,
but in the final part the concentration is low enough to be ROHS conform

18

Apart from stencil thickness and application methods (maybe some paste doesn’t stay on the PCB, but rather on the edges of the stencil cuts), have you tried cleaning the mcpcbs (with destilled water) to remove all residues from manufacturing(like salts, fingerprints,…)? Pressing the LED “into” the paste to make good contact with the LED pads and paste helped for me. But don’t press too hard.

19

No, I am not.

No, it is not the “final product percentage” that matters, it the is the percentage in any homogeneous material, such as an alloy, or plastic that matters. I.e. solder. More than 0.1% lead in your solder, and it is banned.

Study https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/31803/11-526-rohs-regulations-government-guidance-notes.pdf

Page 10.

“Maximum concentration values
25. For the purposes of the RoHS Regulations, a maximum concentration value of up
to 0.1% by weight in homogeneous materials for lead, mercury, hexavalent
chromium, PBB and PBDE and of up to 0.01% by weight in homogenous
materials for cadmium will be permitted in the manufacture of new EEE. These
values were established through the adoption of a Commission Decision on 18
August 200514.
26. “Homogeneous material” means a material that cannot be mechanically
disjointed into different material.
27. The term “homogeneous” is understood as “of uniform composition throughout”,
so examples of “homogeneous materials” would be individual types of plastics,
ceramics, glass, metals, alloys, paper, board, resins and coatings. ”

Solder is a homogeneous material. An alloy. Lead based solder is illegal in electronic equipment throughout the EU (except for the medical and military equipment etc. exemptions)

Playing numbers games to justify e.g. < 0.1% level on say an assembled MCPCB+emitter does not work.

If you are selling any sort of electronic or electrical assembly that has any, say 60:40 leaded solder, in it, in the EU, or exporting it to the EU, then you are breaking the law.

Period.

Also study RoHS Compliant - CE RoHS - RoHS Directive 2011/65/EU | F2 Tech Notes

There is no such thing as an RoHS marking. That is encompassed in the (legitimate, not fraudulent) CE mark. Seeing RoHS marked on anything just suggests that it is anything but.

Whoever signs the paperwork backing up the required CE mark, or imports the item, is personally responsible. Fines for non-compliance start at £5000 and are potentially unlimited.

It applies all the way down to component level. e.g. see Mixed-signal and digital signal processing ICs | Analog Devices

Leaded solder may still be sold, using the exemption (page 8)

“Exemptions
15.The RoHS Regulations do not apply:
…

• To spare parts for the repair of EEE that was placed on the market before 1 July
2006.”

Or you could just call it “very fine solder for use on tiny things, absolutely no electronics involved”

If you are selling stuff commercially in the EU, you need to be using only lead-free solder, and have a paper-trail to prove it.

If you are a hobbyist, or working on stuff pre-2006, carry on using the good stuff.

E.g. I know of a person who makes a good living from stripping down and re-soldering (with lead) high end audio and music equipment, built with early un-leaded solder prior to 2006, that soon became unreliable.

20

PS: these laws also apply to the metals used elsewhere in torches, since they are part of the electrical path.

Free-machining alloys, Chinese “brass” and “copper” debased with lead, etc.

Customs inspectors, particularly in Germany, have hand-held XRF analysers to look out for this sort of stuff. And do sometimes study the paperwork, make a few simple checks.

Like I said, things are being rejected, if discovered not to be correct.