lead free solder on regular HASL pads

[kellogs]

Re: mixing lead solder with lead free solder

Quote from:    free_electron on November 21, 2012, 08:02:10 pm

But on surface mounted boards it is problematic. There are plenty of studies and articles in magazines like PCB manufacturing that show you all the crap that can happen when you take a SN/PB HASL and solder lead free or take a lead free HASL and solder SN/PB...  it is completely unreliable. Especially when you throw packages in the mix that have ENEPIG plated electrodes. Even mixing 'lead-free' alloys is a no-go . if the board was HASL'd with sac305 you can squirt sn99c paste on there and reflow it. it will be a mess.

This just begs the question: Why would then so many engineers prefer ENIG pads, which, supposedly, goes with both leaded and lead free solders ?

[tooki]

ENIG doesn’t oxidize (unlike bare copper, and to a lesser extent, pure tin), and it’s perfectly flat (unlike HASL).

The amount of gold is so tiny that for nearly all applications it’s irrelevant. For high-reliability applications like spacecraft, it’s standard procedure to tin the gold pad, then suck off the gold-contaminated solder before use.

[kellogs]

Does that mean it is fine to use lead free solder on regular HASL pads for non-spacecraft applications ?

[wraper]

For high-reliability applications like spacecraft, it’s standard procedure to tin the gold pad, then suck off the gold-contaminated solder before use.

It's not a standard practice and cannot be done without severely affecting pad flatness and causing ionic contamination. This is done only with component leads where gold layer is an order of magnitude thicker than ENIG.

[wraper]

This just begs the question: Why would then so many engineers prefer ENIG pads, which, supposedly, goes with both leaded and lead free solders ?

Remains solderable for longer, is always perfectly flat and less ionic contamination. HASL sucks for fine pitch and BGA.

[tooki]

Does that mean it is fine to use lead free solder on regular HASL pads for non-spacecraft applications ?

You should order lead-free HASL if you’re going to use lead-free solder.

[tooki]

For high-reliability applications like spacecraft, it’s standard procedure to tin the gold pad, then suck off the gold-contaminated solder before use.

It's not a standard practice and cannot be done without severely affecting pad flatness and causing ionic contamination. This is done only with component leads where gold layer is an order of magnitude thicker than ENIG.

Nope, not in current standards: they say that gold shall be removed “From 95% of all surfaces to be soldered of surface mount components regardless of gold thickness.”

https://www.eptac.com/j-std-001-revision-changes-regarding-the-requirements-for-gold-plating-removal/
https://globalsmt.net/articles_&_papers/getting-gold-critical-requirement-high-reliability-applications/

[wraper]

For high-reliability applications like spacecraft, it’s standard procedure to tin the gold pad, then suck off the gold-contaminated solder before use.

It's not a standard practice and cannot be done without severely affecting pad flatness and causing ionic contamination. This is done only with component leads where gold layer is an order of magnitude thicker than ENIG.

Nope, not in current standards: they say that gold shall be removed “From 95% of all surfaces to be soldered of surface mount components regardless of gold thickness.”

https://www.eptac.com/j-std-001-revision-changes-regarding-the-requirements-for-gold-plating-removal/
https://globalsmt.net/articles_&_papers/getting-gold-critical-requirement-high-reliability-applications/

And where it says PCB? If you mean gold thickness on the component leads, I wrote not about standard but that gold plating on the leads is much thicker than ENIG (except cheap connectors which won't be used in aerospace anyway).

[tooki]

Absent the full text of the standard, we can’t be certain what the intent is. For sure the thick gold on connectors, etc is the major concern.

[kellogs]

Do specific fabhouse lead-free HASLs call for specific lead-free solder pastes then ? Or, perhaps, maybe some of the solder paste formulations are easier to work with on certain lead-free HASLs ?

[wraper]

Do specific fabhouse lead-free HASLs call for specific lead-free solder pastes then ? Or, perhaps, maybe some of the solder paste formulations are easier to work with on certain lead-free HASLs ?

I assume they generally use something like Sn99Cu (since pads look shiny) which should be safe to mix with SnCu and SnAgCu solders.

[jeremy]

Definitely do not mix Pb solder with the low temp Bismuth-based solders (also there is the different melting point issues, but the alloy that forms is awful). Joint becomes very brittle due to strong intermetallic growth and easy to break with even what I could consider moderate force. For example, with the right (or wrong) mix of solder and a decent grip, I can pull SMD aluminium polymer caps off the board with my bare hands.

I'm pretty sure remember reading a super old IBM article where they investigated this Bi/Pb mix for some temperature sensitive parts in a mainframe, and it was a disaster. Can't find the article now though 

Also from http://www.surfacemountprocess.com/low-temperature-lead-free-solder-paste.html

Caution must be used when using tin / bismuth alloys. It is dangerous to mix tin / bismuth with lead containing alloys. Tin, bismuth, and lead can form a very low melting combination that melts around 95 °C. This could potentially lead to solder joint failure due to natural heating of the assembly during use.
 
Tin / bismuth alloys are safe to use in combination with other lead free tin-based alloys. In some cases surface mount assemblies are made with SAC305 on the first side and tin / bismuth on the second side. The low reflow temperature for tin / bismuth minimizes intermetallic growth in the SAC305 solder joints and allows for soldering of thermally sensitive components.

[kellogs]

Since it has ads all over the site... my fab house is JLCPCB. They have already answered that there is no recommended solder paste formulation for their LF-HASL.

Also from http://www.surfacemountprocess.com/low-temperature-lead-free-solder-paste.html

Caution must be used when using tin / bismuth alloys. It is dangerous to mix tin / bismuth with lead containing alloys. Tin, bismuth, and lead can form a very low melting combination that melts around 95 °C. This could potentially lead to solder joint failure due to natural heating of the assembly during use.
 
Tin / bismuth alloys are safe to use in combination with other lead free tin-based alloys. In some cases surface mount assemblies are made with SAC305 on the first side and tin / bismuth on the second side. The low reflow temperature for tin / bismuth minimizes intermetallic growth in the SAC305 solder joints and allows for soldering of thermally sensitive components.

Of course not to mix anything! Actually, this is the first reference that I see advertising safety to mix alloys of any nature. Would be pretty neat for my case, very helpful. Oh, and for SMD electrolythics - I can pull them out of pure low temp SnBi alloy without a problem. 

[xavier60]

I got some PCBs made by PCBWay with OSP treatment. The solderability is excellent. Yet to see if there is any deterioration during storage.
OSP and even bare copper cost more than HASL.

[kellogs]

I still got a thought lingering in the back of my mind...

I have not come across all that many datasheets specifying whether the part is meant for lead-only soldering or LF-only soldering. It seems this lack of spec allows both types of solder to be used with the part. The part being the same, it has the same sort of pads / pins....   

[wraper]

I still got a thought lingering in the back of my mind...

I have not come across all that many datasheets specifying whether the part is meant for lead-only soldering or LF-only soldering. It seems this lack of spec allows both types of solder to be used with the part. The part being the same, it has the same sort of pads / pins....   

Lead-free parts can be used with both leaded and lead-free solder. Although some precautions may be needed when parts with thick layer of solder are used with leaded solder in high reliability equipment. Parts with terminal finish which contains lead shouldn't be used with lead-free solder.