lead free joints on ICs, does this look normal?

[alexg]

Hi All, I received a batch of boards and solder paste that used on it is Lead Free type.
I noticed that on some ICs solder joint is nice and formed a fillet, however on some other ICs on the same board it is a bit clumpy. The thing is that everything seem to function fine.
I know that lead free solder is known for non-slick appearance but I wanted to ask if this look of a solder joint considered normal? Attached are two pictures, you can see that one one fillets are more pronounces than on the other one.
I can reflow it but if this is normal I rather keep board clean.

[ataradov]

It looks OK to me.  The pad extension is a bit longer than necessary, which resulted in a slight excess of solder paste. But I don't think you need to worry about it.

[SMTech]

Decent lead free solder should looks just as smooth as leaded, different fluxes will affect how shiny it is.

Your "clumpy" image is probably soldered OK but something isn't right, the solder fillet would be slightly concave, smooth and spread out onto the rest of the pad. instead as you say this paste appears to have tried to pull into a ball. I would suggest at the very least the profile wasn't great but possibly the paste was old and the flux no longer doing its job.

Here a doc yanked from the top of google with some decent solder joints and cross sections in it. https://www.nxp.com/docs/en/application-note/AN1902.pdf

[alexg]

Thank you for replies. I will definitely ask about changing solder paste or checking temp profiles.

[free_electron]

it depends on the alloy used. if this is SAC305 it looks normal. if this is SN99c it does not look normal ...

https://fctsolder.com/sn100c-bar-solder-fct/

sac305 and the other copper based alloys are garbage and should not be used anymore.

Sn100c
- is cheaper to make( no silver content)
- does not suffer from microvoiding
- does not suffer from shrinkage
- does not create excessive dross
- is not as agressive on the soldering machine (sac alloys literally eat the solder pot ... )
- is eutectic (sac is NOT eutectic ! it has a 3 degree band .)
- has better wetting properties
- less intermetallic growth
- less metallic strain ( reduces whisker growth chance)

drawback
- slightly higher melting point ( 227 vs 217..220 )
- only available from licensed manufacturers due to the many patents on the alloy.(patented worldwide. Patent holder Nihon superior of Japan ). Patent expired two months ago (march 15 2019) . So now the taps will open up completely and SAC and most other alloys will die off.

[alexg]

sac305 and the other copper based alloys are garbage and should not be used anymore.

It is sac305 ,
"GC 3W " paste
Flux Description 3W
Alloy SAC305

Any specific solder paste you may recommend I can look into and potentially buy and supply my assembly partner?

[SMTech]

@free_electron 's tirade against SAC305 is frankly horse***t as well as premature, SAC305 as a solder paste is the defacto standard lead free solder paste and has been for quite some time. Most of the advantages he/she lists apply to wave soldering, there are other potential pros and cons between the two when it comes to SMT work, you can freely read up on studies done between the two, although many of the differences could well be down to flux formulation. (voiding, IMC area etc).

SAC305s dominance means more work has been done on fancy flux chemistries that work well with it, in particular interest to low volume types such as many of those that inhabit this forum are the number of pastes with long room temp shelf life and extended tack times & pot open times. On the flow-wave front we've been using an SN100C alternative for over decade, there's not so many on the paste front.

GC 3W is a perfectly normal lead free paste from a major manufacturer, I'd leave it alone what you are seeing could easily be something other than the paste (e.g the boarg tinning) . Some pastes are very shiny (e.g Warton SAC305), some leave a tinted residue, some leave a clear one, some barely leave one at all. All of the SAC305 variants I trialled before choosing BLT gave a shiner less "clumpy" joint than in your image, but then I don't know how good your footprint is, I can't see what finish you have chosen and only your manufacturer knows if they've been scraping the bottom of an old tub, or crossing their fingers with a one size fits all profile.

[thm_w]

@free_electron 's tirade against SAC305 is frankly horse***t as well as premature, SAC305 as a solder paste is the defacto standard lead free solder paste and has been for quite some time. Most of the advantages he/she lists apply to wave soldering, there are other potential pros and cons between the two when it comes to SMT work, you can freely read up on studies done between the two, although many of the differences could well be down to flux formulation. (voiding, IMC area etc).

Well he does work on automotive electronics, so the requirements there are going to be higher than just about anywhere else. I think avionics still have the ability to use some lead.

https://www.aimsolder.com/sites/default/files/lead-free_for_mixed_technology_circuit_boards_alloy_compati.pdf
https://www.aimsolder.com/sites/default/files/lead_free_alloy_development_paper_rev_4.pdf
https://pdfs.semanticscholar.org/2d26/2d441459764013c17ac4a4204ee061d1095a.pdf
http://extra.ivf.se/eqs/dokument/Candor.pdf
http://www.circuitinsight.com/pdf/proliferation_lead_free_alloys_smta.pdf (says not good hole fill, probably not an issue with SMD)
https://blog.adafruit.com/2018/05/04/adafruits-solder-alloy-of-choice-sn100c-manufacturing/
"We finally decided to go with Sn100C bar and wire-spooled solder. We wanted an affordable eutectic solder alloy which would help us reduce dross (so critical in the lead-free selective soldering process) without damaging our machine."

[SMTech]

We do automotive too, SAC305 paste is not an issue here(at least for us), trace-ability and documentation are way more important. The only time I've had a customer specify a paste it was SAC305 with a water soluble flux so they could clean is afterwards, and that (very random) assembly went to an F1 team.

SAC305 is a very poor choice when it comes to wave soldering for all the reason listed above, however despite its higher melting point SN100ish pastes do in fact typically reflow within the same profile as you would use for SAC305 (as your peaks temps are often around 245 or more anyway).

Avionincs do or did still use leaded processes, that can mean sending their parts out to be re-tinned. I've visited a major manufacturer in this sector, its like nothing else you've ever seen. Absolutely no focus on cost, stock value or any of the things your accountant would be nagging you about, if they want it they buy it (or two or three) and failing that the toolshop makes it.

[free_electron]

@free_electron 's tirade against SAC305 is frankly horse***t as well as premature, SAC305 as a solder paste is the defacto standard lead free solder paste

All far east mass production shops ( Sony , Samsung , Foxconn  to name a few ) are sn100c and have been for the last 10 years. both 'big' smartphone makers are sn100c.
Also in europe it is the largest seller these days.