Solder paste and surface mount components

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I have designed a circuit board and will shortly send it off for fab.
Some of the components e.g. AT90SCAN128 are surface mount.

I know that solder paste is used in soldering surface mount components, I have however never seen it, what is like? does it solidify as it dries? i have no idea!

My question is: should I include a solder paste mask in the PCB CAD design. Will it be OK in the post? Will it make it easier for me to solder the surface mount components?

Thank you

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sparkymark567 wrote:
I know that solder paste is used in soldering surface mount components

Depends; is this a project for yourself or is it something you designed for a company that is going to be mass produced?

If it is for your company, then yes - you will likely need to generate a solder mask. This is because the board, if there is any volume at all, will be sent to an assembly house and they will be using a pick and place machine to put the components on the board. Before they do that, they will put the solder mask over the board and squeegee some solder paste onto the board. As far as what solder paste is, imagine something the consistancy of toothpaste and the color of solder. It's sticky enough that the components will stay in place (unless the board is knocked around) after the component is put down by the pick and place robot. If you have mixed technology (ie: some parts are surface mount and some are thru-hole) then it may depend on the size and volume of the board. If there are a lot of surface mount components, they will probably be done first and the thru-hole manually assembled afterwards.

If the board is a personal project, you can build it any way you want. A lot of folks have had success with the toaster oven method of assembly. Myself, I just use a Metcal and microscope for my personal boards and it works quite fine. Check out the SMT assembly guide by Colin O'Flynn; there used to be a link on the main page before the reorganization.

Dave

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ok thanks. The board is a prototype this time round, although it may go in to production (perhaps after a few minor alterations). I have a weller soldering station and some very fine soldering tips. So I guess I'll be using a similar method to you.

Would brushing on some solder paste (by hand) hold the components in place sufficiently for me to solder the components by hand.

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Hi,

If you've got solder paste I would invest in some sort of hot air machine, or even just a toaster oven. The hand soldering doesn't work as well with the solder paste, although It is possible.

As well you don't need much solder paste at all - I use a toothpick to apply the solder paste, and just a very small amount.

Quote:
My question is: should I include a solder paste mask in the PCB CAD design. Will it be OK in the post? Will it make it easier for me to solder the surface mount components?

I'm not 100% sure what you mean. When you say solder paste mask do you mean a solder mask layer on the PCB where some areas of the board are masked off? Or do you mean a SMD stencil that applys solder paste to specific regions like at http://www.stencilsunlimited.com ?

Regards,

-Colin

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Thanks Colin. I have read your solding guide in the past...but can't get hold of it right now as the academy seems to be offline. Thanks for the advice anyway. A hot air soldering station would be great but I don't think it would be a worthwhile investment at the moment. So I will apply tiny ammounts of solder paste with a tooth pick as you suggested. Then I'll just have to get the magnifying glass out....and solder them by hand.

I'm not exactly sure what I mean by a solder mask. I think it's just a plot of the SMD pads where the solder paste should be applied. All I know is that my PCB software can provide an output file for a solder paste mask. The PCB fab people can more or less do anything.....e.g. solder resist, silkscreen etc, etc...and everytime I ask them about whether they can do something, the answer always seems to be yes and it doesn't cost any extra.......so I wondered whether they would also paste the board. But now I know that solder paste has the consitency of toothpaste it's probably not a good idea for them to send a pasted board in the post.

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I have populated many SMD boards with tweezers and solder. This works up until you get into 0402 packages, which if you sneeze, you will never find the component.
Get yourself a desktop hobby light / lens magnifier, a soldering iron and a pair of tweezers and you will be just fine. For IC's, place it roughly in place, solder one corner, then hold the iron to the pin while you get it perfectly into position. Then solder the other corner and finish up.

I soldered a MEGA32 MLF package by hand, under a scope. It was not all that bad. Fine tips are handy for this. Yes, this is no lie, I did it by hand, a MLF package.

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The solder paste layers are not used in the PCB manufacture, just the assembly, but you may as well plot them and include them - it costs nothing. If you want someone to build up the board for you, you'll need them.

In my opinion the best solder paste for hand work is Kester type R500. This costs a fortune ($42 a tube, KE1512-ND @ Digikey) and has limited shelf life, though if you keep it well sealed at the back of the fridge it may last a year. The good thing is the flux is water soluble and you can rinse it off under the tap. Don't squeeze out too much at a time because it doesn't keep overnight - it dries out and goes grainy like half-set cement, that won't spread and won't stick to anything.

Quote:
I have populated many SMD boards with tweezers and solder. This works up until you get into 0402 packages, which if you sneeze, you will never find the component.

cnewbie, you are so right. First rule of SMT is always have plenty of extra small parts!

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sparkymark567 wrote:
ok thanks. The board is a prototype this time round, although it may go in to production (perhaps after a few minor alterations). I have a weller soldering station and some very fine soldering tips. So I guess I'll be using a similar method to you.

Would brushing on some solder paste (by hand) hold the components in place sufficiently for me to solder the components by hand.

Actually, if you are doing the prototypes by hand, you just want to use regular solder - you don't need to worry about using solder paste (and it will probably be a lot easier too). However, just like having fine tips for your soldering iron, you need fine solder as well. Digikey has some Kester 0.015" diameter core that I find works very well when soldering the TQFP64 packages.

Dave

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Quote:
...so I wondered whether they would also paste the board.

If you wanted to do that you could get a prototype assembly place to mount the parts as well. You probably can't get a board just pasted, for both the consistency reason and the fact the paste will dry out.

Anyway a solder mask is handy for a number of reasons - one of them being it gives your board the nice green apperance for added appearence points (in my books anyway). It does help stop solder from going where you don't want it, but only up to a point. The thing is that for example in dense SMD IC's the solder mask can't fit between the pins so there is no solder mask seperating adjacent pads anyway. This means it isn't as useful as you might first think. I've almost always got it though anyway.

BTW - the article is a bit harder to find, but still there. Go to Academy, click on Articles at the top, then click on Read More. Finally hit the introduction button.. or go to https://www.avrfreaks.net/index.p...

Regards,

-Colin

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sparkymark567 wrote:
Some of the components e.g. AT90SCAN128 are surface mount./quote]
Some!
If you have a mix of thu hole and SMT, and the bottom has the SMT, then solder paste will not be used in the (mass) production process.
The SMT components will be glued to the bottom, then tru hole components inserted, then wave soldered.
Phil

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I've done quite a bit of SMT hand soldering - a great way to go cross-eyed!

I use flux gel that normally comes in a syringe. Tack the corner pins of the device with the soldering iron to hold it in place and put the flux gel around the part (we're talking about a Atmega128 or similar type part). Use the soldering iron and solder to wipe the solder ball across all the pins - you don't want too much solder nor too little and you don't need a super fine soldering iron tip. The flux gel stops the solder bridging the pins - if it does, use the iron to work the solder away - or use a little soderwick. With a little practice you'll be able to solder an atmega128 in less than a minute - once the flux is cleaned off (use pcb cleaner) - it looks like it was done by machine. I've used this technique for years - even on high density packages.

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Quote:
I've done quite a bit of SMT hand soldering - a great way to go cross-eyed!

A good note - if you can find a stereo inspection microscope, you will probably find it is worth its weight in gold. Well you might feel like you paid that much anyway...

But I got one of those cheap ones, and I love it. Makes soldering way easier, as you can actually check how your joints look. As well useful for other things, so if you can get one I'd recommend it.

Regards,

-Colin

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I've used solderpaste on MLF-28 packages in a toaster oven.

I got the paste from www.howardelectronics.com, it is AMTECH brand, comes in a syringe that is ideal for spreading a thin line, costs $16.00 and doesn't require refrigeration. (Much cheaper than the Kester stuff from DigiKey and I assume it is just as good since it worked.)

I controled the temperature using a cheap Sears multimeter that comes with a thermocouple (about $30) and the toaster oven was the cheapest I could find.

You don't need a solder mask (but it certainly would help) since if you put the right amount of the solder paste down when it melts the surface tension pulls it to the pads and off the non copper areas of the board.

Good Luck,
Smiley

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There is a nice tutorial on solder paste and SMD parts at http://www.pcbexpress.com. They have pictures of the stencils used to apply solder paste. It's under the Stencils tab.

People have been answering the question but I am not sure if they have been make a clear distinction between solder mask and the solder paste stencil.

Solder mask, as Colin said, is a polymer coating that is applied to the circuit board and kind of acts as a resist for wave soldering or other soldering techniques. Solder doesn't like to stick to it very well and it helps to prevent solder bridges and such. The solder just beads up and pulls away and off of the solder mask. And as Colin pointed out, the mask really doesn't help much with fine pitch SMD components as the mask generally doesn't reproduce the fine lines between component legs and so is simply not there. There is an empty rectangle over the pads as a whole.

The solder paste stencil is really just another mask but is used to apply the solder paste for SMD components. At the PCBExpress website, they will sell you laser-cut metal ones and you squeegee the paste through all the slots and holes onto your circuit board before mounting the parts.

So solder mask is a large area polymer coating that has holes where you want to solder and the solder paste stencil is really just all the holes and slots where you want to apply solder paste. They are similar in where the holes are but not identical. The solder paste stencil will have all the individual pads cut out along the edge of a fine pitch SMD component (as noted above) while the solder mask will cover everything but the general outline of all the pads as a single cutout. If it doesn't make sense, look closely at the pictures of the stencil at the PCBExpress website and note all the cutouts are really fine and the size of the pads - and a cutout for every pad. Grab any circuit board that's green (common but not exclusively the color of solder mask) and look really closely at the coating around the pads of a tqfp44 package or similar. You'll see the soldermask generally doesn't run between the individual pads.

As to magnifying products for working with SMD parts, I found the perfect item for me. It's a Donegan Optical "OptiVISOR" DA-3 (http://www.doneganoptical.com/catalog/opti/. I got it at McMaster-Carr. Newark has magnifying visors but I sent the one I bought from them right back and told them they should be embarrassed to sell such junk. It really was junk. I forget the brand name but it's all plastic, wouldn't stay adjusted, and was uncomfortable as hell. The OptiVISOR, on the other hand, has a comfortable headband, has glass lenses, adjusts and stays adjusted, and really is just a joy. Stereoscopic binocular magnifier. And the beauty of it is that the glass lens wonderful OptiVISOR cost about $24 if I remember right and the really crappy visor from Newark was $19 or so.

I really recommend the visor - once you try one you'll love it. And save yourself some trouble and go straight for the Donegan Optical visor. It's unbelievable quality especially for that price. I would have paid $50 for it easy.

Please note - this post may not present all information available on a subject.

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If you are going to do any hamd soldering of cubic SMD components IT IS IMPERATIVE TO OBTAIN SOLDER WIRE WITH SOME SILVER CONTENT.

Ordinary tin lead eutectic alloy may leach the electrode connections on cubic components.

2% silver solder is ok.

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Quote:
It's a Donegan Optical "OptiVISOR" DA-3 [...] I got it at McMaster-Carr

McMaster-Carr has these with six different magnifications from 1.5X to 3.5X. What are You using and are You satisfied with it?

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ignoramus wrote:
If you are going to do any hamd soldering of cubic SMD components IT IS IMPERATIVE TO OBTAIN SOLDER WIRE WITH SOME SILVER CONTENT.

Ordinary tin lead eutectic alloy may leach the electrode connections on cubic components.

2% silver solder is ok.

Huh? I hate leeches but anything eutectic sounds downright scary!! what's a cubic component - anything with pins on four sides like QFP? Sounds like I need to get myself some silver solder .. but why is this only needed on cubic components - special coating on the pins on these packages?

Regards,
Scott

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What would I do without you guys!

Yep, solder paste is quite expensive I have found.
I definitely need to get some narrow guage solder (and I'll go for the silver content!). I had not previously considered the solder as an issue.... I probably would have made a real mess with 60/40 22swg solder. Now I have found some 32swg solder with silver content. Probably get some flux spray......I will also consider buying a magnifying lense/lamp or something similar.

SMD toaster or hot air station, solder paste, etc would be great but can't afford that at the moment; nor do I want to make/rig something up that will do the job. I have plenty of work to be doing and also I need to save the pennies for a scope.

Thanks everyone for your advice.You don't normally get to learn these tricks of the trade at university.

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Pick up some solder braid while you're at it. Sooner or later you are going to have a short between pins. The easiest way to get it out is to dab some flux on the pins and then go after them with the braid. Pulls the solder right out of there.

Dave

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Already have the solder braid.....so atleast that's something! Shopping list is still getting longer though

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JohanEkdahl wrote:
McMaster-Carr has these with six different magnifications from 1.5X to 3.5X. What are You using and are You satisfied with it?

I've been using an OptiVISOR for about a year now and I love it.. The lens plates are interchangeable.

I originally purchased it with a 5 Dioptre (2.5X) lens plate and found it to be too strong. I've since purchased a 3 Dioptre (1.75X) lens plate and use it almost exclusively.

Don

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Cubic components are usually ceramic capacitors and film resistors.

Their endcaps ( electrodes) are generally made from material which readily disolves in tin lead solution.

Having a bit of silver in the solder will minimise the electrodes disolving away ( leaching) into the solder aloy.

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Some solder pastes have a short (sometime very short like 3 hrs) shelf life. They also should not be refridgerated unless instructed by the manufaturer. They are definetly not all the same and not all water soluble. I would avoid using it for hand soldering. The best advice is to do it by hand using a fine solder as mentioned in an earlier post.

Any mass production that uses pick and place machines will require a stencil.

For those of you out there using solder paste, have a close look at your solder joints if thing aren't working as you expected. Anything that doesn't shine should be fluxed and re-flowed. Solder paste (especially water soluble types) are sensitive to humidity and tend to deteriate when exposed to the element, they also require proper handling so as not to introduce air bubbles and other artifacts.

Best of luck

Glen

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You don't want flux spray! You want this:

http://www.surmountinc.com/Qstore/c000007.htm

As mentioned previously - use this and a normal soldering iron and solder - qfp's are not a problem. The flow solder machines rely on the same technique - in this instance you're just using the soldering iron to do the same thing - no magic.

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ignoramus wrote:
Cubic components are usually ceramic capacitors and film resistors.

Their endcaps ( electrodes) are generally made from material which readily disolves in tin lead solution.

Having a bit of silver in the solder will minimise the electrodes disolving away ( leaching) into the solder aloy.

Iggy--Can you point to some references on this topic? I pulled up datasheets on surface-mount components that we use from DigiKey, from manufacturers Panasonic, Yageo, & Susumu. I find no cautions against using tin-lead solder. In fact, a temperature table is often found which includes information for Sn/Pb solders.

Could you point me to a datasheet or other reference where the cautions or related information is presented?

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Quote:
Iggy--Can you point to some references on this topic? I pulled up datasheets on surface-mount components that we use from DigiKey, from manufacturers Panasonic, Yageo, & Susumu. I find no cautions against using tin-lead solder. In fact, a temperature table is often found which includes information for Sn/Pb solders.

Could you point me to a datasheet or other reference where the cautions or related information is presented?

Lee

I'm curious about this too, as I have been running with the same info as Lee. I do recall cautions
in the "old days" making repairs on Tektronix scopes with ceramic terminal strips. These had
plated-on terminals and Tek clearly warned about using only silver bearing solder on them.

These newer smds seem to make a point of having "3-layer" terminals which perhaps are
intended to solve this problem. (?)

Tom Pappano
Tulsa, Oklahoma

Tom Pappano
Tulsa, Oklahoma

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JohanEkdahl wrote:
Quote:
It's a Donegan Optical "OptiVISOR" DA-3 [...] I got it at McMaster-Carr

McMaster-Carr has these with six different magnifications from 1.5X to 3.5X. What are You using and are You satisfied with it?

I got the DA-3 which is a 1.75 magnification / 3 diopter / 14" focal length.

And I really do like it. I have a magnifying ring light too and now prefer the visor for most work.

Please note - this post may not present all information available on a subject.

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ignoramus wrote:
If you are going to do any hamd soldering of cubic SMD components IT IS IMPERATIVE TO OBTAIN SOLDER WIRE WITH SOME SILVER CONTENT.

Ordinary tin lead eutectic alloy may leach the electrode connections on cubic components.

2% silver solder is ok.

????

Can you say that in "plain" english , for all us non natives :-)

Well i understood the silver part ... but why ??

/Bingo

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Hi,

The silver content is a problem - I'd reference you on my book on SMD soldering but it is at home...

Anyway the silver on the leads would migrate away from the leads into the solder. This would leave ceramic which of course would have a terrible connector. However - I think most parts now use nickle plating to eliiminate the problem. I can't seem to find a cohesive online article on the topic, so I've added some links to a few little articles I found:

http://www.achesonindustries.com...
http://www.finishing.com/4600-47...
http://www.vishay.com/company/pr...

Regards,

-Colin

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Ah..I se i have been called to account for my words.

I am sorry I have no URL topint to a reputable reference.

I am not at home presently and shall not be till mid march, i do not have acces to my library.

Problem is evident with components which have silver /paladium ( AgP ) electrodes.

The silver dissolves into the tin/lead solution ( molten solder) and leaves paladium on the component.

Paladium does not provide a good electrode on its own and often the electrode disapears completely , that is disolves in the tin lead solution.

Addition odf small amount of silver to the tin lead alloy preserves electrodeand provides a reliable joint.

I did a quick search on this problem and came up with a 1997 report which highlights the component manufacturers recomendation to include silver in the solder alloy.

Here is the relevant excerpt from the report:

"....The first sample HV capacitors were hand-placed on the second panel.
This test demonstrated a problem with soldering the HV capacitors to the
PCB, which appears to be due to the Silver-Paladium electrodes not
soldering well with the tin-lead solder used by the board house. A 2%
silver solder recommended by the capacitor manufacturer will be tried on
a third sample panel in early May. The PCB house is quite confidant of
success...."

And the report can be found at
http://web.ask.com/redir?u=http%3a%2f%2ftm.wc.ask.com%2fr%3ft%3dan%26s%3dk2%26uid%3d06DA8FCAB63AB5F14%26sid%3d131C039326B5A2024%26qid%3dF9D470DCECA4C54DA97136A0A1D7D90F%26io%3d0%26sv%3dza5cb0d88%26o%3d0%26ask%3dsilver%2bpaladium%2bsoldering%26uip%3dcb679ecb%26en%3dte%26eo%3d-100%26pt%3dDIRC%2bApril%2bMonthly%2bStatus%2bReport%26ac%3d1%26qs%3d0%26pg%3d1%26ep%3d1%26te_par%3d156%26te_id%3d%26u%3dhttp%3a%2f%2flpnhe-babar.in2p3.fr%2fDIRC%2felec%2fmonthly%2fstaapr97.report&bpg=http%3a%2f%2fweb.ask.com%2fweb%3fq%3dsilver%2bpaladium%2bsoldering%26o%3d0%26page%3d1&q=silver+paladium+soldering&s=k2&bu=http%3a%2f%2flpnhe-babar.in2p3.fr%2fDIRC%2felec%2fmonthly%2fstaapr97.report&qte=0&o=0&abs=...a+problem+with+soldering+the+HV+capacitors+to+the+PCB%2c+which+appears+to+be+due+to+the+Silver-Paladium+electrodes+not+soldering+well+with+the...&tit=DIRC+April+Monthly+Status+Report&bin=&cat=wp&purl=http%3a%2f%2ftm.wc.ask.com%2fi%2fb.html%3ft%3dan%26s%3dk2%26uid%3d06DA8FCAB63AB5F14%26sid%3d131C039326B5A2024%26qid%3dF9D470DCECA4C54DA97136A0A1D7D90F%26io%3d%26sv%3dza5cb0d88%26o%3d0%26ask%3dsilver%2bpaladium%2bsoldering%26uip%3dcb679ecb%26en%3dbm%26eo%3d-100%26pt%3d%26ac%3d24%26qs%3d0%26pg%3d1%26u%3dhttp%3a%2f%2fmyjeeves.ask.com%2faction%2fsnip&Complete=1

I shall endeavour to locate a more apropriate refference.

PS.I have personally experienced this problem of silver leachingfrom some ceramic components.
While perhaps the modern components might use improved materials technology in the manufacturing proces I now exclusively use 2% silver solder in any SMD hand soldering I do irrespective of nature of component types used simply to eleiminate any potential for poor sooldering outcomes due to any silver related issues.

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Some more, albeit aged ( vintage 1999 ) information:

"... Dave,
The 2% silver stops the gold from migrating into the solder. This is basically because the gold fit's the spaces between the tin/lead molecules so it disapear's into the solder and give's u sonme real trouble some joints. The 2 % silver fill's the gap's in the crystal lattice of the solder joint to block the migration of the gold into the solder.( shees must have been awake in that materials class after all)
anyway's them's the basic's I think, unless I'm confused again, it happen's but I'm pretty confident on this.

see ya

John

(http://www.smtnet.com/Forums/Ind...)

...."

The issue of silver solder is NOT going to go away especially since the industry as a whole is moving towards LEAD FREE soldering technology and the new components which hit the market as of 2006 in Europe are going to be compatible with the lead free process.

I am not shure what the North American market is doing but I would think that the lead free buzz word is pretty loud in both American and Asian marketplaces.

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I'll be getting my first SMD boards next week...how exciting :) I have some Multicore solder type 352 and the alloy is marked as LPM Tin/Lead Silver, I hope it's the right stuff. The UNOPENED Electrolube solder paste has a use by date of 23 July 04 :( :( I was going to work on this project last year but......So it's off to the bank for another loan to get more solder paste from Farnell :cry: :cry:

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Hello John,

Try exchanging the tube or atleast getting a credit with the locla Multicore rep.

I am shure that it would be less expensive that way. I have been out of circulation for a while so I cant give You any names but You might try to talk to Steven Chakovan at Machinery Forum in Sydney . He might set You in the right direction.

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Quote:

The issue of silver solder is NOT going to go away especially since the industry as a whole is moving towards LEAD FREE soldering technology and the new components which hit the market as of 2006 in Europe are going to be compatible with the lead free process.

I do not disagree with your statements about lead-free and the direction it is going.

However, I >>am<< questioning, as have others in this thread, your "the sky is falling" assertion about damage to components and/or their connection to solder pads when Sn/Pb solder is used. If this is true, then why wouldn't the mainstream component manufacturers that I mentioned have this information in the datasheets for the components? If this is an area that had problems and was solved with various coatings on the SMD component contacts with nickel, etc. 5 years ago, how can you make the blanket statement about erosion of contact areas if Sn/Pb solder is used?

I'm very interested to hear of which component lines on the market at the present time may be suseptible to damage if Sn/Pb solder is used.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Lee,

a fair enough question.

Personal experience with some ceramic capacitors purchased through an online distributor two years ago driven this point home.

Now may be the skieis have stopped falling now but I am yet to beconvinced.

As far as the leadfree technology is concerned European position to the best of my understnding is that LEADFREE technology is madated from 2006.

Leadfree solders are based on alloys of Tin, Silver, Bismuth and Copper . No lead.

The corolary of this is that if tin lead solder is used components not designed for tin lead soldering may suffer excessive leaching of silver from electrodes into the eutectic solution.

This may be questioned ahnd some might even claim that the component manufacturers might not take a retrograde step by removing some of the existing proceses to which make current crop of components somewhat imune to leaching but I am yet to be convinced that the manucaturers will not tattempt to cut back the factory costs if they ( additional plating processes ) are not absolutely necesary. Elimination of leadmay make them un necesary.

Many manufacturers already promote their lead free range of components. The usual label applied to these laedfree components id PbF .

There is a simple experiment that can be done...

Solder a ceramic component using standard SnPb alloy . Unsolder it and visually inspect it 's electrodes for erosion.

Carry out this procedure a couple of times . If no degradation occurs You are ok to use SnPb alloy.
Otherwise a certain percentage of silver will be needed.

I have tried to locate a URL which deals with these issues directly and explicitly.
No joy.

Here is a short article

http://smt.pennnet.com/Articles/Article_Display.cfm?Section=Archives&Subsection=Display&ARTICLE_ID=109047&KEYWORD=Silver%20Paladium%20Electrode%20%20leaching

and i quote a short excerpt from it"

"...Ag alloys typically are used with components (i.e., capacitors) that contain silver plating to prevent leaching of the silver plating during the reflow soldering operation...."

Note Ag is the chemical symbol for Silver

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Quote:
Try exchanging the tube or atleast getting a credit with the locla Multicore rep.
Unfortunately the solder paste is from Electrolube and purchased from Farnell...quite a few months ago. I'll try to find their invoice, perhaps they sold me a bad tube.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

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