Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Micro crack for lead free wave solder

Views: 3420

Joseph

#35822

Micro crack for lead free wave solder | 1 August, 2005

Dear all,

We observed visible micro crack at top side of solder fillet during lead free wave solder process. We did not see any visible micro crack at bottom side, and it always located at joints of transformer.

Any input be much appreciated.

Best regards,

Joseph

reply »

#35823

Micro crack for lead free wave solder | 1 August, 2005

Joseph, What is the topside temp reading of your board before reaching the pot? Do you have top-heater in your wave machine? It sounds like; you got an insufficient per-heating issue.

reply »

#35824

Micro crack for lead free wave solder | 1 August, 2005

Based on your description, it sounds like the solder connection is defective. Further description of board and solder materials and assembly processes would be helpful in moving forward.

reply »

Joseph

#35834

Micro crack for lead free wave solder | 1 August, 2005

We use SAC305 and top side particular joint temp. is 110 deg C, and the terminal finish for that particular lead is pure tin. It look like hot tearing at the primary side of PTH.

reply »

#35850

Micro crack for lead free wave solder | 2 August, 2005

IPC-A-610D Section 5, shows a variety of photographs of: * Good lead-free solder connections * Lead-free soldering anomalies

Hot Tear / Shrink Hole is one such anomaly. This shrinkage effect appears primarily on wave-soldered joints, but can also show on hand-soldered joints. Such surface shrinkage effects are not considered a defect.

We seem to remember something about lead contamination inceasing hot tearing of SAC wave soldered connections.

reply »

Ken

#35904

Micro crack for lead free wave solder | 3 August, 2005

Yes, lead can promote fillet lift. In order to understand this defect you must begin looking at your lead frame mateials.

some lead frames are more susceptable to this disorder. This is most likely a cte mismatch disorder.

Lead contamination in the part is also a possibility. However this tends to pool at the hottest part of the joint, causing fillet lift.

reply »

#36036

Micro crack for lead free wave solder | 10 August, 2005

Hello Joseph, Sorry for the bad news but microcracking is a well known potential condition occuring with the SAC305 alloy.

Microcracking is a condition being seen with many joints soldered with the SAC305 alloy. It is technically a shrinkage cavity caused by the growth of the tin dendrites during solidification and the shrinkage of the substrate. The further off eutectic the alloy is the greater likelihood of the shrinkage cavity to occur. SAC305 is off eutectic by at least four degrees Celsius and will display this condition, particularly if the joint is slow to solidify or is large. Rapid cooling of the joint can possibly eliminate this condition by this is sometimes difficult to achieve in many waves and ovens used in production. It has been recommended by the SAC305 patent holder to use the hypereutectic composition of tin/4.0%silver/0.9%copper to reduce the chance of shrinkage cavity formation. The tin/copper/nickel alloy (SN100C)is a true eutectic and the presence of the nickel reduces the ability of the tin dendrites to nucleate, which eliminates the shrinkage cavity potential. In addition, the eutectic characteristic of the tin/copper/nickel alloy gives the joint the smooth, shiny appearance seen with the 63/37 tin/lead joints. The paper given by Senju (SAC305 patent holder) can be found at: Ueshima, Minoru, �Mechanism of Shrinkage Cavities and Method for Restricting Them in SnAgCu Alloy,� 11th Symposium on �Microjoining and Assembly Technology in Electronics�, Yokohama, Japan, February 2005. I could fax you a copy if interested. bgilbert@fctassembly.com

reply »

#36038

Micro crack for lead free wave solder | 10 August, 2005

Bob,

Great theory if you're selling SN100C, but I have to disagree that an alloy being 4�C off eutectic will cause cracks. When a solder joint exits the solder wave, there is an immediate drop in temperature of 100�C. so being off 4�C is irrelevant. Lets not forget that lead-free is still in its very early unexplored stage, and nothing can be stated as fact. One of the biggest differences between leaded and lead-free alloys is that leaded alloys are ductile and lead-free alloys are brittle (more susceptible to cracks). When a board passes over a solder wave there is a tremendous amount of thermal stress applied to the board. The board warps when in contact with the wave and goes straight again when leaving the wave. When the board exits the wave and goes from warp condition to straight the solder joint is already solid and the stress applied to the board will create cracks with the brittle lead-free alloys (independent of composition). Reflow will not have this phenomenon because the board is not thermally stressed as much as in wave soldering. The reason why the cracks are formed on the topside of the board is: When the board warps when heat-shocked in the wave the PTH is stressed wider at the solder side and narrower on the topside. When the board leaves the wave and stresses back to its original shape the opposite happens creating a significant potential for cracks on the topside of the fillet.

It will also be interesting to learn more about the galvanic properties of all the new metals and alloys used in our industry, because lead was also used to eliminate the galvanic corrosion potential between different (noble) metals.

It's about time we reinvent the need for lead in the soft soldering process and simply implement a recycling program before we get ourselves in some serious trouble.

Pat

reply »

#36045

Micro crack for lead free wave solder | 10 August, 2005

Joseph,

In addition to my previous post The reason why the cracks are most visible at the transformer location is due to the weight of the transformer. The stress on the boards is the highest because the weight of the transformer will warp the board more (it�s a weight/temperature combination). Try supporting the board avoiding the warp and see if you can reduce the problem.

reply »

reflow oven profiler

Reflow Oven