|Ask the Experts|
April 20, 2020
Reliability Concerns When Converting to Lead-free
We are converting to lead free soldering and limited to one SMT line, and we wish to convert it to 100% lead free, SAC305 in our case, because we are afraid of making mistakes with a mixed alloy line.
Engineering won't invest in changing the surface finish of about 100 old legacy PCB's from HASL finish, and these boards are not for the RoHS market. They are for industrial programmable power supplies. We are considering running these boards on the SMT line with the SAC solder paste. (Through hole solder would remain tin/lead to prevent contamination of the lead free solder pots.)
Since the HASL finish thickness can vary from extremely thin to about 1 mil, being 37% lead, if I put 5 mil of SAC solder paste (2.5 mil solidified) on top of the HASL, my lead content in the eventual solder joints could be from about 1 to 10%. (Up to 0.37 mil of 3-3.5 mil of solidified solder)
Assuming the reflow oven profile will keep the solder joint above liquidus long enough to create a homogeneous solder joint, the liquidus being a maximum of 227C, depending on lead percentage, is there any reason to worry about the solder joint reliability for the industrial market?
|Expert Panel Responses|
This is a problem. I'd rather order 100 new boards with ENIG finish. Solder joint reliability and function are highly diminished when you add SAC alloy paste to a 63/37 HASL-finished PCB. Consider the following:
First you have the formation of new alloys and intermetallic surfaces not suitable for soldering.
Second, the mixing of eutectic and non-eutectic alloys will be a challenge trying dialing in the correct profile for your oven. Finally, the two alloys will not mix evenly throughout the joint.
Senior Manufacturing Engineer
I wish it was as simple as converting from one alloy to the next. It is not!
Couple of quick things to check out as this is a backwards compatibility process, which is defined as soldering tin/lead with a lead free material, in this case SAC 305.
First the soldering process temperature of the product is going to have to be increased. The typical soldering range for tin/lead smt reflow is approximately 230C, whereas the SAC 305 is approximately 250C, with a longer dwell time.
The existing laminate material has to be evaluated to make sure it can withstand the rigors of the lead-free surface mount process while keeping in mind that most SMT boards have components on both side, so the total reflow time has to be considered when making this decision. I would suggest IPC-4101 Specification for Base Materials for Rigid and Multilayer Printed Boards to verify the electro-mechanical properties of the laminate material. I would also suggest J-STD-003 Solderability Tests for Printed Boards, to make sure the boards meet the thermal requirements of the Lead-free surface mount process. I recommend reviewing J-STD-004 Requirements for Soldering Fluxes to verify the fluxes can also meet the requirement of the new process temperatures.
Once the evaluation of the laminate material has been completed, other evaluations must be considered for all the components and materials which will be subjected to the Lead-Free surface mount process. Most through hole components cannot withstand the higher temperature demanded by the lead-free process, therefore this must be evaluated so not to degrade the functionality of the components and impact the reliability of the product. The same is required of the flux to make sure it meets the SMT requirements.
The soldering and inspection must meet the requirements of the J-STSD-001 and IPC-A-610, documents.
So, yes the conversion is not just switching the solder alloy, but to verify the existing product can withstands the rigors of the lead-free process be it either the through hole or surface mount process.
Vice President, Technical Director
I believe that reliability of a solder joint really depends upon the application, the environment, and the stresses that the solder joints are subjected to.
There are several papers discussing the reliability of mixed metal solder joints. I suggest searching the technical databases of IPC and SMTA for papers on the topic.
Don't do it. Previous work has demonstrated serious issues when mixing SnPb HASL with SAC305 solder.
CEO & Managing Partner
In my opinion, yes, there is reason for concern. There has been some research that suggests that Pb content in the 10% range or a little higher can have a strong negative effect on reliability. I personally would not mix SnPb HASL with Pb-free paste.
If I were in your position, I would look at ways to error-proof the line, e.g. bar-coding the paste containers and requiring them to be read in on start-up. Reflow oven recipes would also need to be keyed to board part number. Then you can run the paste that suits the particular assembly.
Please refer to my paper from 2001. Lead Contamination in Lead-Free Electronics Assembly
Deck Street Consultants
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