Reflow soldering of lead-free components with leaded solder paste

[Emanuel]

I am confused about the parameters of the reflow profile needed to solder lead free components.
We assemble boards for automotive modules required with leaded solder paste. The boards have ENIG finish.
The problem is that I see some wetting issues, very erratic, can be a single resistor with one side not soldered, not in the same place and not the same one, so for sure is not a component oxidation.
The profile is set according to the solder paste manufacturer data sheet, however the dealer recommended to increase the peak temperature and bring it to the lead free level.
While this makes sense as the components pads are coated with lead free tin, this exceeds the solder paste limits.
The solder paste is SnPbAg.
I will appreciate any advise.

[SMTech]

Curious, why are you using leaded paste? Even the automotive industry made that switch years ago for many areas, the exemption list gets smaller all the time. We assemble all sorts of automotive PCBs for dashes or adjacent functions and we have used lead free paste since ~2006.

Wetting could be an ENIG issue, or perhaps your paste needs to be slightly more aggressive. Either way you should perhaps run some tests, try the higher temp, try a higher melting point leaded paste or even try a HASL finish instead. Inspect the joints at a lab to check the metallic structures even.

[Psi]

Not all ENIG is made equal, sometimes the amount of gold they add is as thin as they can get away with in order to save money.
Maybe the nickel is coming through in places.


Usually this is only an issue on the ultra cheap and dirty china pcb makers, but i'm not sure were you are getting your boards from.
Maybe they are skimping on the ENIG gold.

[Emanuel]

There are manufacturers that require this, same as in other industries. In this case it is heavy duty vehicles.
The boards arrive with a test report and a sample of a soldered board, all the pads are well covered.
Also, as I mentioned the wetting problem of only one pad of the component is erratic, not on all boards, not on all the resistors of the same value/size and in different locations.
Yes, looking at some "statistics" there is a certain location where this is happening more often, near a SOT-263.
We started to use ENIG just 2 years ago because of 2 reasons:
There is no oxidation as with other options and compatibility with the leaded process, instead of fighting with both the PCB and the component now we only need to worry about the components.
It can be the ENIG, I was thinking about this.
The solder paste we use is KOKI SS48-M955, SnPbAg

[Emanuel]

I am talking about something like this. (not my picture, found on the net)
The pad is well covered with solder but on one side the pad is just on top of the solder.

[wraper]

I am talking about something like this. (not my picture, found on the net)
The pad is well covered with solder but on one side the pad is just on top of the solder.

It's tombstoning and has nothing to do with ENIG.

[wraper]

however the dealer recommended to increase the peak temperature and bring it to the lead free level.

Absolutely counterproductive suggestion. The issue happens when the paste starts melting, one side of component gets lifted above the paste during the reflow, so it no longer touches the solder. Running cycle to higher temperature won't fix the issue if it already happened. There are many factors contributing to this. Such as different thermal mass of the pads, uneven heating, too fast heating, components poorly placed and barely touching the paste on one side or shifted from the center, component terminal wettability, too large amount of solder paste applied, type of solder paste, its tackiness and alloy surface tension in particular. Solder alloys that do not pull component terminal to the center of the pad (low surface tension), are way less prone to this issue. Very small components can tombstone almost vertically, while larger and heavier components get lifted only slightly.

[Emanuel]

Thanks wraper,

I read a lot about possible causes to this problem, can say for sure that although it looks the same there is a difference between tombstoning and what I am describing.
In most places it is associated to poor wetting, while tombstone is commonly associated with uneven pads or one pad connected to a large surface without thermal relief.
But you mentioned very logical causes and there is some thinking to do, in addition to the possible bad ENIG plating.
The machine placements are very precise and each board is inspected before reflow.
The thermal mass of the pads, at least where this phenomena happens is equal. Funny, while this is happening with 0805 resistors, I have on the same board 2 tantalum caps size D placed quite close to each other in parallel and one of the pads is connected to a large shield surface without thermal relief, never had any problem with them.
I suspected the placement pressure, but I don't want to push too much as resistors break. Also, I have the same resistor in several locations on the board, if it was a placement pressure problem I would see the same on all.
I will check all of this again, very annoying situation.
 

[Emanuel]

however the dealer recommended to increase the peak temperature and bring it to the lead free level.

Absolutely counterproductive suggestion.

I am not sure at all, there is such thing that when soldering lead free components the temperature must be at the correct level and it is higher than for leaded components.
Here is something I found in data sheets of leaded solder pastes from Kester:
HM531 is capable of reflowing at 235°C peak temperatures required for fully collapsing lead-free SAC BGA'a for maximum reliability and remains easy to clean after these high temperature profiles.

[wraper]

however the dealer recommended to increase the peak temperature and bring it to the lead free level.

Absolutely counterproductive suggestion.

I am not sure at all, there is such thing that when soldering lead free components the temperature must be at the correct level and it is higher than for leaded components.
Here is something I found in data sheets of leaded solder pastes from Kester:
HM531 is capable of reflowing at 235°C peak temperatures required for fully collapsing lead-free SAC BGA'a for maximum reliability and remains easy to clean after these high temperature profiles.

The suggestion was for particular problem. And this particular problem will not be solved by increasing peak temperature. When adjusting reflow temperature profile to reduce tombstoning, you want to slow down temperature ramp speed. Not excessively fry the components.

[Emanuel]

The suggestion was for particular problem. And this particular problem will not be solved by increasing peak temperature. When adjusting reflow temperature profile to reduce tombstoning, you want to slow down temperature ramp speed. Not excessively fry the components.

Let me put it this way..... there are 2 separate issues:
One is the process itself, soldering lead-free components with leaded paste. The problem is well known, practically the joint is not always reliable when using the leaded profile with the peak at 225°C, it looks good but if analyzed in special labs it clearly shows that is not. In fact aerospace industry requires lead-free components to be sent to re-plating, a costly process where they take out each component and dip it in leaded solder.
The other problem is what I see on some components, if you want to call it tombstoning is OK. The question is if this is not related somehow to the first issue.
As I said, the KOKI dealer recommended to increase the peak to 235°C exactly as Kester are suggesting, the thing is that while the Kester paste is made for this temperature the KOKI is not.
So now I am checking how to order a jar from Kester and test on a batch of boards.

[glenenglish]

IMO tombstoning has little  to do with paste type (lead free/leaded)  or plating...

thermal  profile, solder quantity, stencilling accuracy / consistancy, component placement, component  terminal oxidization/corrosion....

and make sure you rub the boards with isopropanol before stencilling - I see residue on my ENIG boards that I remove with a bit of a rub with isoprop.

All my boards are lead free SAC305 and I have zero tombstones on all ranges of component sizes. .......

[wraper]

tombstoning has nothign to do with paste type or plating

Paste type has a lot to do with it. It may not matter much if everything is tuned perfectly, such as reflow profile, paste amount, pad thermal mass, pad size, component placement. However if some of those are suboptimal, paste type makes a huge difference.
https://www.vse.com/blog/2023/10/05/tombstoning-prevention-design-analysis/
https://resources.pcb.cadence.com/blog/how-does-solder-paste-composition-affect-tombstoning
From personal experience I can say that with my imperfect prototype reflow setup I got quite a bit of tombstoning with SAC305 Loctite GC10 T3 and almost none with SAC305 Chipquick SMD291SNL T3 with everything else exactly the same (PCB, stencil, reflow profile).

[glenenglish]

Paste type : I meant lead or leadfree shouldnt affect tombstoning all other things being correct and equal

beware - dont  mix lead free and lead containing solders.  the mix isnt pretty.

[Emanuel]

https://www.vse.com/blog/2023/10/05/tombstoning-prevention-design-analysis/
https://resources.pcb.cadence.com/blog/how-does-solder-paste-composition-affect-tombstoning

What puzzles me while looking at the possible tombstoning causes is that if any of them exists this will result in some repeatability.
But how is it possible that on a board with about 25-30 resistors, some of them the same value and size this phenomena is erratic, not on all the boards, not the same resistor and sometimes it shows on capacitors or diodes.

[wraper]

But how is it possible that on a board with about 25-30 resistors, some of them the same value and size this phenomena is erratic, not on all the boards, not the same resistor and sometimes it shows on capacitors or diodes.

It always happens randomly, although some component positions may be nearly immune to it and other very susceptible. If you place components by hand, then it becomes much less repeatable as positioning and how deep components were pushed in the paste matter a lot.

[Emanuel]

If you place components by hand, then it becomes much less repeatable as positioning and how deep components were pushed in the paste matter a lot.

The components are placed by a machine and the board is 1.6mm aluminum substrate, 90x90mm therefore quite rigid so there is no bending.
I was thinking to add some pressure even by giving the machine a component height less say 0.1mm although this is a lot for a resistor which is 0.5-0.6mm thick.
But I am worried of possible cracking the component, it happened many years ago but with a large 2512 resistor.

[tooki]

See the tips shown here:

https://www.surfacemountprocess.com/surface-mount-troubleshooting-guide.html

…and here:
https://smtnet.com/library/files/upload/Reflow_Soldering_Processes_Troubleshooting.pdf

The second one is a whole book with lots of detail.