# Solder paste type



## videoman (May 22, 2013)

Looking to mount some XM-L's on stars but having a hard time finding which solder paste is best for this. I came across Mouser and they seem only to have the flux or the silver type like this one http://www.iso-tip.com/products-page/smart-paste-series/silver-bearing-solder-paste-7459/
any suggestions welcome.


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## RoGuE_StreaK (May 22, 2013)

I'll just tap onto this and add, is there one that is considered optimum for thermal transfer (eg. for direct-to-copper) that doesn't break the bank? I've got the cheap DX stuff, which does most jobs fine, but if I'm reflowing onto copper sinkpads I probably should investigate paste that's designed for better thermal transfer?


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## SemiMan (May 22, 2013)

RoGuE_StreaK said:


> I'll just tap onto this and add, is there one that is considered optimum for thermal transfer (eg. for direct-to-copper) that doesn't break the bank? I've got the cheap DX stuff, which does most jobs fine, but if I'm reflowing onto copper sinkpads I probably should investigate paste that's designed for better thermal transfer?



Most solders today are ROHS compliant and are predominantly tin, so thermal resistance differences are fairly minor between the various formulations. 

Semiman


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## RoGuE_StreaK (May 22, 2013)

I assume that means lead-free? Any sources of ROHS-compliant paste that doesn't cost an absolute fortune? I just checked my local large electronics supplier, RS Components, and the cheapest lead-free I could find was $81 for 25grams 

Getting sidetracked, but is there some threshold for level of manufacturing where below that threshold you don't need ROHS compliance? Pretty sure all of my components are ROHS compliant, it's just the solder that's the, um, sticking point.
eg. if you send one item that you've made to someone in Europe, will it be blocked for not showing ROHS documentation? Or is it only when you send a container of 1000 items? While I'm all for reducing lead, I'd probably have to make 100 boards to use the equivalent of one fishing sinker's worth of lead. And at my current rate, that'd take a few years...


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## SemiMan (May 22, 2013)

Check out Digikey, they have a much wider selection and at less money. This link should work for you:

http://www.digikey.com.au/scripts/d...k=1&rohs=1&quantity=0&ptm=0&fid=0&pageSize=25


Volume does not matter, but some products are exempt. 

Will you get blocked ... unlikely, but there are people better able to assess this than I am.

Semiman


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## RoGuE_StreaK (May 22, 2013)

SemiMan said:


> Check out Digikey


Well I found some reasonably-priced stuff ($24 for 35g), but then remembered why I never deal with Digikey (or Mouser for that matter); $35 shipping. Don't know why they bother having a .com.au version of their site when it's all in the US anyway. The search will continue...

Also found elsewhere some tin/bismuth solders cheap, but on reading up it sounds like they can have issues and their low temp (~135C) probably isn't suited to high-power LEDs.


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## LilKevin715 (May 23, 2013)

I got some solder paste from fasttech.com SKU # 1261003 for $3.28 shipped, works just fine.


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## jason 77 (May 23, 2013)

I bought mine from www.zeph.com, don't know about shipping to down under though as I live in the states....


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## SemiMan (May 23, 2013)

LilKevin715 said:


> I got some solder paste from fasttech.com SKU # 1261003 for $3.28 shipped, works just fine.



That is lead-tin, not ROHS compliant.


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## fyrstormer (May 23, 2013)

Just use silver solder for everything. The extra conductivity can only help, and the extra cost is inconsequential. If you can afford to spend $10-20 on a single LED, then you can afford to spend another $10-20 on silver solder that will last for years.


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## LilKevin715 (May 23, 2013)

SemiMan said:


> That is lead-tin, not ROHS compliant.



Info was for videoman who created the thread. If he is located in the USA then solder paste with lead content isn't a problem.


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## FRITZHID (May 23, 2013)

Pay shipping & I'll send you lead'd or RoHS solder paste. But for thermal efficiency, I suggest copper/tin/gold alloy solder, I'll send you some of that as well if you'd like.


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## SemiMan (May 23, 2013)

FRITZHID said:


> Pay shipping & I'll send you lead'd or RoHS solder paste. But for thermal efficiency, I suggest copper/tin/gold alloy solder, I'll send you some of that as well if you'd like.



I think you mean Tin/Silver/Copper solder which is better than Tin/Lead, but in application, the difference will be extremely minor. The most conductive solder tends to be Tin/Silver, about 50% better than tin/lead with most of the Tin/Silver, Tin/Silver/Copper about 20-30% better.

Tin/Gold solders are only about 15-20% better than tin/lead and really not worth the cost for this application compared to Tin/Silver.



Oh, thought Rogue had started thread ... but since Videoman, yup don't need ROHS, good practice though.

Semiman


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## WeLight (May 23, 2013)

fyrstormer said:


> Just use silver solder for everything. The extra conductivity can only help, and the extra cost is inconsequential. If you can afford to spend $10-20 on a single LED, then you can afford to spend another $10-20 on silver solder that will last for years.



You must have some magic paste, paste wont last for years as flux component will dry out. Please note Lead free on copper boards will require a fair bit higher reflow temp to get a good bond


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## FRITZHID (May 24, 2013)

SemiMan said:


> I think you mean Tin/Silver/Copper solder which is better than Tin/Lead, but in application, the difference will be extremely minor. The most conductive solder tends to be Tin/Silver, about 50% better than tin/lead with most of the Tin/Silver, Tin/Silver/Copper about 20-30% better.
> 
> Tin/Gold solders are only about 15-20% better than tin/lead and really not worth the cost for this application compared to Tin/Silver.
> 
> ...


 
Actually, I ment tin/copper/gold. I have a spool of it here & would happily send him some, along with water soluble OR NO-Clean paste of his choice.
& yes RoHS solders do require higher reflow temps, just make sure everythings clean and dry and you shouldn't have any issues as long as you don't over saturate the heat and cook the pill.
We have special 5 zone reflow ovens for just that purpose, but it can be done on a hot plate, toaster oven, or in gutsy attempts..... A clothes iron. 
(I prefer an oven or proper reflow equipment.)


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## SemiMan (May 24, 2013)

FRITZHID said:


> Actually, I ment tin/copper/gold. I have a spool of it here & would happily send him some, along with water soluble OR NO-Clean paste of his choice.
> & yes RoHS solders do require higher reflow temps, just make sure everythings clean and dry and you shouldn't have any issues as long as you don't over saturate the heat and cook the pill.
> We have special 5 zone reflow ovens for just that purpose, but it can be done on a hot plate, toaster oven, or in gutsy attempts..... A clothes iron.
> (I prefer an oven or proper reflow equipment.)




I have not heard of tin/copper/gold used for regular soldering, sounds more like die attach. Gold is used in solder mainly for mechanical properties and the elimination of flux due to superior wetting. If you want high thermal conductivity, Tin/Silver/Copper is the best and it is not overly expensive. It is harder to solder with as (in addition to temp issues) most copper alloyed solders have poorer wetting.

Semiman


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## Steve K (May 24, 2013)

hey guys.. I'm just enjoying the conversation! Never even heard of tin/copper/gold solder before. As a EE, I enjoy these sorts of discussions more than debates over tints.  

Is the thermal conductivity of solder significant when applying the average 3W LED? I would have thought that other parts of the thermal path would dominate the thermal resistance. Or is this more for special cases?


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## alpg88 (May 24, 2013)

i use digikey paste, paid iirc $16 for a surringe full. but the trick is, the paste has to be kept in a fridge, all the time, didgiky recomends 2 day shipping for paste, they mail it in termal bag, with ice bag in it. and you should keep the paste in the fridge, with surringe held in vertical position, before using let it warm up to room temp, for about 8-10 hours.


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## bshanahan14rulz (May 24, 2013)

Steve K said:


> Is the thermal conductivity of solder significant when applying the average 3W LED? I would have thought that other parts of the thermal path would dominate the thermal resistance. Or is this more for special cases?



I think the thinking is that the thermal path has the smallest cross-section at this point in the train.

I don't do any high performance modding or anything, but I just tin the pads with regular solder. When I solder it, I let it flow/self-center, and then push it straight down with an appropriate tool to squeeze out as much solder as I can from between LED and solder pads. This is probably along the same lines of "does it really matter?", but it makes me feel better about it, and it's MY light, so I do what I feel seems best to my ability.

With regards to the refrigeration of solder paste, I imagine it is to keep it from separating out, since you can't very well stir the contents of a syringe if it separates. BUT that is just a guess on my part.


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## FRITZHID (May 24, 2013)

bshanahan14rulz said:


> I think the thinking is that the thermal path has the smallest cross-section at this point in the train.
> 
> I don't do any high performance modding or anything, but I just tin the pads with regular solder. When I solder it, I let it flow/self-center, and then push it straight down with an appropriate tool to squeeze out as much solder as I can from between LED and solder pads. This is probably along the same lines of "does it really matter?", but it makes me feel better about it, and it's MY light, so I do what I feel seems best to my ability.
> 
> With regards to the refrigeration of solder paste, I imagine it is to keep it from separating out, since you can't very well stir the contents of a syringe if it separates. BUT that is just a guess on my part.



You are mostly correct, the paste is kept cold to preserve the flux however, I've used 3 yr old paste that was kept sealed with no issues, but moisture is the biggest issue with paste solders.
I use RoHS pure tin paste & tin/copper/gold water soluble wire for all my work. Its high conductivity, thermal, strength & corrosion resistances can't be beat. Not ti mention, with the gold alloy, tin whiskering is lesser of an issue,.... But to each their own.


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## SemiMan (May 24, 2013)

Steve K said:


> hey guys.. I'm just enjoying the conversation! Never even heard of tin/copper/gold solder before. As a EE, I enjoy these sorts of discussions more than debates over tints.
> 
> Is the thermal conductivity of solder significant when applying the average 3W LED? I would have thought that other parts of the thermal path would dominate the thermal resistance. Or is this more for special cases?




For an LED, thermal conductivity of the solder will be a very small part of the thermal path, no matter the solder.

Semiman


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## SemiMan (May 24, 2013)

FRITZHID said:


> You are mostly correct, the paste is kept cold to preserve the flux however, I've used 3 yr old paste that was kept sealed with no issues, but moisture is the biggest issue with paste solders.
> I use RoHS pure tin paste & tin/copper/gold water soluble wire for all my work. Its high conductivity, thermal, strength & corrosion resistances can't be beat. Not ti mention, with the gold alloy, tin whiskering is lesser of an issue,.... But to each their own.


\

Can you post a link to the tin/copper/gold you are using? I was looking at tin/gold a while back for something special where we did not want to use flux as we could not clean, but we also could not handle the residue of no clean. I like to keep track of things I may use in the future though you mention water soluble so I assume this is fluxed.

Semiman


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## VegasF6 (May 24, 2013)

RoGuE_StreaK said:


> Well I found some reasonably-priced stuff ($24 for 35g), but then remembered why I never deal with Digikey (or Mouser for that matter); $35 shipping. Don't know why they bother having a .com.au version of their site when it's all in the US anyway. The search will continue...
> 
> Also found elsewhere some tin/bismuth solders cheap, but on reading up it sounds like they can have issues and their low temp (~135C) probably isn't suited to high-power LEDs.


http://www.cutter.com.au/search.php?pg=1&stext=paste&sprice=&stype=&scat=&sman=


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## RoGuE_StreaK (May 25, 2013)

VegasF6 said:


> http://www.cutter.com.au/search.php?pg=1&stext=paste&sprice=&stype=&scat=&sman=


Hah, Cutter's intra-Australian shipping is almost as bad as ordering from Digikey! (not to mention that's standard 63/37 leaded solder anyway, which I already have)

RE: thermal conductivity, I reckon it might be a bit like thermal paste in that it won't matter much if it's thin enough, but might get to be more of an issue if you've got it slathered on.

PPS. On a very tenuously related note, I was shocked and disillusioned when a few years ago I found a NASA video on how to solder properly, and heard the pronunciation of "solder"; I started questioning whether been taught it wrong and been saying it wrong all these years. But nope, turns out there are two schools of pronunciation of solder, with Australia, New Zealand, most of the UK, and half of Canada being in the "l" school :huh: Language differences, who'da thunk it...
Weird Al's "White'n'Nerdy" lyrics made a bit more sense after that...


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## VegasF6 (May 25, 2013)

No kidding? Is it that high for all there items, or is it just this one? Perhaps they are sending it express, and keeping it cold? That would explain the expense. Otherwise, I guess that just doesn't leave you many options, does it? Too bad as I thought that would be more helpful to you.

As to the thermal conductivity, it seems to be splitting hairs, but they are your hairs to split. Soldering (No I don't say SOUL-dering!  ) is still many times better than thermal epoxies. I have seen comparisons of thermal efficiency of various solder types, usually wire solders, but not paid a lot of attention to them. I seem to recall indium solder being mentioned, but if it's unobtanium, then it wasn't worth my dedicating a lot of time to it. Then too, there are mechanical properties to consider. 
If you haven't seen it already then this place manufactures different pastes. I don't know if they are available in quantities that would interest you, but if nothing else, it gives a good idea of what products are out there. And gives access to a fancy chart 
http://www.indium.com/solder-paste-and-powders/

*edit* just noticed your first post stating you are currently using the DX lodestar stuff. Really this 63/37 and it's fresh, has to be better than that. You may be getting solder ball that don't even melt.


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## videoman (May 25, 2013)

Thanks everyone for all the suggestions. I ordered from Digikey the paste and will try it soon. I would have done without if the 3000K hi CRI XM-L2's were available on 20mm stars but I only saw them as bare emitters on Mouser and couldn't source them anywhere in US on the stars. I have put together a couple of them on the stars by using 60/40 wire solder by applying the hot solder on the star at the 3 contact points, placing the led on top and reheating with a heat gun. Works fine. Just wondering if the led sits as it should on the star instead of slightly floating on the solder as it looks flat to me.I used as little solder as I can get on it.


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## fyrstormer (May 28, 2013)

WeLight said:


> You must have some magic paste, paste wont last for years as flux component will dry out. Please note Lead free on copper boards will require a fair bit higher reflow temp to get a good bond


The flux doesn't have any water in it, so by definition it *can't* dry out. I assume you mean it will harden. That might be the case, depending on the packaging, but a syringe applicator will prevent the flux from contacting air regardless of how little is left in the package. However, even contact with air won't necessarily degrade flux; my dad has a tin of flux that's over two decades old and it is still usable.


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## FRITZHID (May 29, 2013)

SemiMan said:


> \
> 
> Can you post a link to the tin/copper/gold you are using? I was looking at tin/gold a while back for something special where we did not want to use flux as we could not clean, but we also could not handle the residue of no clean. I like to keep track of things I may use in the future though you mention water soluble so I assume this is fluxed.
> 
> Semiman



Correct, its a w/s flux cored solder. I'll dig up a link somewhere for ya, I order it thru work.


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## RoGuE_StreaK (May 29, 2013)

VegasF6 said:


> No kidding? Is it that high for all there items, or is it just this one?
> ...
> Too bad as I thought that would be more helpful to you.


Sorry didn't mean to sound rude/ungrateful but probably came across that way. No, Cutter seems to have a minimum shipping charge for anything of about $15 (last I looked), which negates them unless you are ordering maybe 10+ items. Sorry for getting off topic...


VegasF6 said:


> You may be getting solder ball that don't even melt.


I've done reflow of 0.5mm pitch QFN chips using it and seems fine, though probably need to start looking at ROHS stuff anyway. But sounds like for the moment the thermal transfer difference is negligible.

@FRITZHID, any idea if the tin/copper/gold is available as a paste format?

As I side note, during the past few days' research on the subject I came across a reference to silver content for solders (usually 2%), apparently some components may have silver in their legs/cases and some solder types can leach this out (or something to that effect), which can cause brittleness. The silver content stos this leaching from happening. Info was in regards to tin/bismuth solder, so not sure if applicable to others.


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