Suggestion for Solder Paste and flux?

[gauravmp]

Hi,

Could someone suggest a good solder paste composition/brand and flux? 

I'm looking for these for smd soldering using the reflow method with an SMD rework station.

Thanks.

[nanofrog]

As you're in the US, I'd suggest Kester EP256* (63/37), and Kester RF741 flux.

* Kester only sells it in large amounts (i.e. 400 - 600g tubs & cartridges), but CML Supply breaks it down into 35g syringes for the hobbyist/repair specialist market (here). The RF741 comes in 30g syringes & 100g jars from Kester. CML offers a smaller size of this as well (here), though the 30g size is more cost effective (3x the cost, but 5x the amount).

[billfernandez]

Most solder paste has to be kept cold when not in use:  shipped overnight or in cold packs, kept in the refrigerator, then taken out and allowed to come up to room temperature over a period of hours before use.  Even then it only lasts (typically) a year.  (Those are the technically-correct rules.  DIYers probably bend them a lot.)

If you wish to avoid all this, Loctite has a relatively new solder paste, model GC10, that lasts a year with no refrigeration:  so shipping is easy and you can use it whenever you get the urge to solder. The flux is no-clean, and the only alloy currently available is the lead-free SAC305 alloy.  I've been hand-soldering successfully with it.

The downside, depending on your volume, is that it only comes in 500g tubs or 600g Semco-compatible cartridges.  A tub cost me about $85.

The full part number for the tub is GC 10 SAC305T3 885V 52U.  This has type-3 solder beads in it.

[uncle_bob]

Hi

Basic categories:

1) Solder that has lead in it

2) Solder that does not have lead in it

Which one do you need?

Next layer:

Solder for fine pitch stuff like 0.4 mm pitch 400 pad BGA's

Solder for "normal stuff" like 0805 resistors

Again, a choice based on what you are doing.

Next layer:

Solder with fluxes that are designed for a post solder cleaning process.

Solders that rely on the entire area being heated hot enough / long enough to blow off most of the nasty stuff

Which are you set up for / looking for?

Lots of twists and turns.

Bob

[HHaase]

For the hobbiest, I wish that Loctite GC10 was available in smaller containers.   We got a sample at work over a year ago, and waited until AFTER that year expired before trying it.   Not refrigerated at all, just put in a filing cabinet and left to sit.  I'll be darned, the stuff is still working!  It's also proven very stable in warm/humid conditions where most other pastes start to separate and run.   It is a lead-free, no-clean, and no other types are available.   

If you can stomach the high entry fee, since you have to buy the $80-$100 sizes,  it's worth a look.    Just make sure you're getting the right grind to it.  The T3 style will be fine for most anything you can hand solder,  but go with the T4 if you're doing anything fine pitch via stencil printing.    Though you can only get the SAC305 no-clean,  so if you need a different alloy or flux type, you're out of luck with the GC10

-Hans

[Corporate666]

As you're in the US, I'd suggest Kester EP256* (63/37), and Kester RF741 flux.

* Kester only sells it in large amounts (i.e. 400 - 600g tubs & cartridges), but CML Supply breaks it down into 35g syringes for the hobbyist/repair specialist market (here). The RF741 comes in 30g syringes & 100g jars from Kester. CML offers a smaller size of this as well (here), though the 30g size is more cost effective (3x the cost, but 5x the amount).

Strongly second the above.  Excruciating detail below (TL;DR at end) ->>>>

I have tried probably 15+ different pastes over the years.  Various leaded and lead-free options from AMTECH, Sparkle, AIM, MG Chemicals, Kester and others.  EP256 is far (FAR) beyond anything else I've tried. 

The stuff about keeping paste refrigerated and using within a short time only applies to medium to large companies with tightly controlled processes or those that need a chain of custody/QC.  For everyone else, it's unnecessary.  I get solder paste shipped unrefrigerated by ground service.  I buy expired paste.  I open it and don't refrigerate it.  I place boards that were pasted 2 or 3 days before.  I open a tub of paste and keep using it for weeks or months without bothering to refrigerate between uses.  I put unused paste back in the container once I've pasted the boards for the day.  Of course, I don't do this as a matter of course, I am just saying that sometimes shit happens... you come in after a long weekend and someone forgot to run the last few PCB's through the pick and place.  So you say "Hmmm, still feels tacky... let me see how they come out!" - and they come out fine. 

So I've pushed the boundaries of paste, and EP256 is head and shoulders above in terms of results for regular use and in terms of how forgiving it is.  EP256 performs flawlessly.  I assume because of the alloy and the flux it uses.  It requires no post-reflow cleaning, I have no issues with tombstoning parts, components center perfectly.  The cost of solder paste is so low on a per-board basis that it's not worth trying to save money on paste.  We do several thousand PCB's per year and we go through maybe 4-5 500g jars/year.  At around $60/jar it works out to a few cents per PCB (each with 100+ pads).  The wrong paste can really cause problems.  I just this week finished fixing a few hundred boards that needed reworked because they didn't reflow correctly and got put aside.  I didn't get around to reworking for over a year, and it took me 2 days to fix them.  And I've had similar problems in the past when we tried a new paste only to get tons of passives tombstoning, or the flux wasn't very active and resulted in lots of bad connections or parts that didn't self-align during reflow, etc. 


For flux, are you looking at something for rework?  My go-to is Kester #186.  I have tried various no-clean fluxes (like Kester #951) and they just aren't "active" enough, or they are so thin they almost immediately evaporate when hot-air or soldering-iron reworking.  186 is a rosin flux, active enough that it reflows/remelts solder beautifully, but is thick enough that it doesn't instantly evaporate from the PCB. 

I have tried tack fluxes.  MG Chem #8341 is OK.  The upside to tack flux is you get a lot more working time on the PCB.  You can remelt a connection probably 2-3 times before it melts away from the connection and you need to add more.  #186 might get you 2 melts with an iron and is OK for hot air.  #951 you have to immediate hit it with the iron or it evaporates, and if you are using it on 2 sides of a SOIC, for example, the heat from reworking one side will evaporate the flux from the other.  The thin no-clean fluxes are useless for hot air rework.  My favorite tack flux is AMTECH NC-559-V2-TF.  The problem with all the tack fluxes I've tried is they come in syringes which really need an air dispenser to work.  Even with a 12 gauge nozzle, you have to apply a lot of force to the plunger to get any flow, and when it starts to flow you get a ton of it on the board.  With a pneumatic foot-pedal operated dispenser, the NC-559 is bliss.  But 90% of the time for a quick fix - it's the #186 I reach for.  I have tried many other fluxes - rosin fluxes from the dispenser bottles hardens and turns into a mess.  Paste fluxes in jars get everywhere and you throw away most of it. Many of the jelly-like fluxes turn to brown concrete on the board if you try to hot-air rework with them.  So I stick to the 186 and NC-559 in 99% of cases.


Probably more detail than anyone wanted, but it is what it is.

[gauravmp]

Thanks a lot guys

[nanofrog]

My go-to is Kester #186.

FWIW, I keep this ^ and 1544 on hand as well. Really good stuff IME. 

The downside, is Kester doesn't sell these in small bottles; they only package it in disposable pens (expensive), or 1 gallon+ sized containers. There are some eBay sellers that split it up into much more hobbyist friendly bottles for those in the US/CANk such as Dickie's Garage Sale (not sure if anyone does this in other markets). For those that aren't familiar, 186 is an RMA that borders on RA (186-18 is the lower solids version, which is easier to clean), and 1544 is the liquid version of the RA flux used in Kester 44 solder wire.

MG Chemicals 835 (RA) is decent as well, and comes in hobbyist friendly sizes (100ml & 1L). But I'd rate it second to the Kester products listed above as it's more prone to leaving a tacky residue, and given the 50% solids content, makes it a bit tougher to clean as well (best cleaned right after the joint is made, while it's still a liquid or tacky).

We're talking small increments/improvements between the Kester & MG Chemicals products mentioned above, but you'll notice the differences if you ever get the chance to compare them IME.

[technotronix]

Kester 186 Flux Pens.