Cause of Solder Cracks?

I agree that this is likely a thermal fatigue problem of lead-free solder. With this failure happening after the expected lifetime (warranty) of the product there are probably other parts close to failure, too.

issue is totally the lead-free solder period. good call eagle guy.

I have seen the same type of problems in a Trane Evaporator timer relay.

It was mounted in a location that protected it from extreme thermal cycling.

The bad solder joint appeared to have been in motion before it completely cooled;it had a cloudy appearance.

I repaired the relay by simply re soldering the joint,and all of the others on the board also.

I thoroughly removed all flux and coated the joints with urethane paint.Several layers of clear fingernail polish will work well if nothing else is available.

Factory coating is multi-layers .The first layer,next to the board,is a moisture barrier.The 2nd layer is a de-ionizing layer,to make any water non conductive,the final layer is another layer moisture barrier.

Cannot duplicate this process very easily,but I have had good results with urethane or lacquer.

I have seen control modules in vehicle fail from thermal cycling after about 10 years or so,but the appearance of the joint is is different.

Here is a link to the reasons for solder joint failure.

https://www.ansys.com/blog/top-5-reasons-solder-joint-failure

I'd suck out the existing solder; tighten the screws, then, re-solder the joints.

Ten years seems like much too short a service life for this sort of thing.

I agree with your belief that the joints were soldered before the screws were inserted. It would take several years for the mechanical damage to manifest as an electrical failure.

I had an identical failure on a windshield wiper control PCB.

The failure in the lead free solder joints was due to vibration created by a mechanical relay.

Just clean off any conformal coating, remove the existing solder, and replace with 60/40 and that circuit board will outlive the boiler.

In agreement with many others, the lead-free solders have caused a LOT of problems.

In my experience, the lead in solder problem has been greatly over-exaggerated. I have literally been soldering with leaded solder for 70 years. I only have two hands, so a lot of the time, I had (still have) the soldering pencil or gun in one hand, one or more parts in the other, and the (nearly always 60/40) solder in my mouth.

Whenever I repair any electrical device, which is a frequent occurrence, I nearly always use 60/40.

The joints shown were probably never done right. I have electronic equipment more than 50 years old. None of them has solder joints any where close to what is shown. A joint will stay looking new except if subjected to severe mechanical or thermal stress.

Sure! 50-year-old electronics were soldered with lead-bearing solder, most likely 60/40. It could flex much more easily, so didn't crack nearly as easily.

From my experience this is likely to have been a manufacturing problem caused by the reflow solder technique when the solder is not hot enough and has not adhered to the pins correctly. Too cold a solder joint will have a grey colour.

What to do, that is easy if you are handy with a temperature controlled soldering iron all that is required is to heat and manually solder all the joints on the board with a good 60/40 solder. It is even better to use a solder sucker to remove the old solder and redo the joint. I have fixed many a poor quality soldered board on plasma cutters, welders, inverters etc.

The lead free solder seems to be prone to this problem. I had to fix a lot of boards on a well known USA brand of wire feeder and 3 phase welding machines, (just out of warranty, surprise)!

If you check the photo you will see the solder is well connected to the pin.I would suspect that the problem initially has been caused by an over enthusiastic tech and over time it has degraded to a total failure.

Molex sockets are a bit stiff to assemble and disassemble, you would have noticed this in your work repairing welding equipment.

"It is even better to use a solder sucker to remove the old solder and redo the joint."

In this case where it is clear that the pins were not fully inserted before the flow soldering, it is important to: remove the old solder; remove the socket; clear any other solder from the area to allow the socket to be fully re-inserted; re-insert the socket; tighten the screws, then, re-solder the joints.

If the mechanical joint is not sound all the forces are on the solder joints.

Unfortunately it will probably be quite difficult to remove the socket, because of the solder between the socket body and the board, but, it's well worth the effort. Sometimes you have to keep: "sucking; adding fresh solder, and, re-sucking".

If you have compressed air in your shop,I have found that wearing a mask and heating the joints separately and hitting with a blast of compressed air will completely clean the joint and thru-the board ferrule.

This is very effective when de-soldering multiple-pin elements.Very good results for me.

I am not sure if the canned air products have enough pressure or volume to do this,but you could try it.

In my experience with a situation somewhat like yours, a solder crack issue was caused by vibration over time and I believe not enough solder was applied to the solder joints.

Here is a link that explains the characteristics of lead/tin alloys and how the ratios affect the finished solder joint.

https://ae5x.blogspot.com/2019/12/eutectic-or-non-eutectic-solder-does-it.html

The main point of failure with thru-the-hole soldering has always been the hole site.

That is one of the reasons manufacturers went to surface mount,(as well as faster automated production).

A solder joint is not a good mechanical joint.

Mechanical stability must be achieved before soldering.

A lot of thru-the-hole components are not properly anchored before soldering.

This is especially important in high vibration applications.

Even surface mount components will have issues with vibration.

Most automotive,and especially aviation, electronics are now potted in solid with epoxy or a silicon gel that absorbs vibrations,or both.

Relatively small runs of electronics still use thru-the-hole components.

I would be interested to know what the pins are made of.

I am with SolarEagle on this, thermal cycling and lead-free solder are most likely the problem. If the pins are from a high expansion alloy, this will exacerbate the problem.

I expect these sort of failures are going to become more prominent as all the lead-free products age.

I have seen similar joint failures on wave-soldered boards. Usually, the affected components were not under stress. In this case, it seems that the only component affected is the one under stress (the hold-down screws), so I concur with your initial impression that the wrong assembly sequence was employed.

Have had similar problems with Molex connectors on 2 way radio boards led free does not help but 60/40 did it as well, we were looked at thermal cycling,. tried silicon encapsulating socket will have to wait for MTBF to come up.

We did remove clean & re solder with 60/40 our sockets have no hold down screws though so think your problem could be not seated correctly, screws not fitted before solder or over tightened after.

It seems many of the observations have identified various possibilities of the failure in an effort to find the root cause. The product is over ten years old so the question becomes can this be reworked or repaired, to which there have been many suggestions leading to a positive outcome. Understanding and identifying the cause of the failure should lead to better manufacturing going forward. The one thing I could add regarding the process and sequencing is, was the mechanical connection properly specified. Applying a wrong torque can easily cause failure as it creates stress in the component and PWB. There are many ways to pursue if Honeywell was aware of the problem but as it is not a 20,000 foot issue hardly worth the effort.