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July 29, 2014
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Using tin-lead solder on RoHS parts

We are a Military supplier of EMI filters and Circuit boards and we have been getting hit with long lead time on RoHS compliant only parts, but our customers want only lead bearing parts. What is the risk of using lead bearing 60/40 solder on RoHS compliant parts? Will there be any solder joint weakness or problems with these joints?

Brian Monnin

Experts Comments

The RoHS gull wing or J leaded components and passive components with end terminations should not have any issues. They will either have a Sn finish or some other finish that will not be detrimental when combined with 60/40 solder.

The only situation that will pose a problem is when you combine a RoHS lead-free BGA, where the bumps are made of lead-free solder material that melts at a higher temperature (217 deg.C) when compared to the 60/40 solder which melts at a lower temperature (~183 deg. C). You can use the following reflow profiling parameters to achieve uniformity of the joint. (Also check with the paste manufacturer.)

A Preheat temperature of around a 120 deg. C at the rate of 2-3 deg. C per second.

A soak time of around 60 -120 seconds for effective dryout of the solvents and good flux activation to minimize voiding.

The reflow temperature and time above liquidus are very critical, because of the BGA with SAC bump. A reflow temperature of around 225 degree C with an extended TAL (around 60-90 seconds) would be helpful in melting the bumps and also providing sufficient time for the lead (Pb) to mix within the bulk, and the solder to wet.

Maintaining a cool down rate of 2 to 3 deg C per seconds would also be appropriate.

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Bjorn Dahle
President
KIC
Bjorn Dahle is the President of KIC. He has 20 years experience in the electronic manufacturing industry with various manufacturing equipment companies covering pick & place, screen printers and thermal process management.

This is a backwards compatibility issue, using lead-free materials with a leaded solder. Many of the components manufactured in the last few years have been lead free, especially surface mount chip components as they are Tin plated.

Can you solder these with 60/40 tin/lead solders, the answer is yes and millions of solder joint have been soldered this way for years. The problem however is some of the Lead-Free component plated RoHS components have a SAC alloy for a lead coating and this is the issue.

The 60/40 alloy will not be hot enough to melt the SAC alloy to create a sound joint. Yes, many papers have been are being published stating that this will work, but keep in mind the thermal profile and the length of time the solder joint needs to be above the reflow temperature to all complete dissolution of the Tin/Lead into the SAC alloy coating or solder balls on the components. This is much different than what was used to the low temperature Tin/Lead alloys and dwell times above the reflow temperature is going to have to increase to create a sound solder joint.

If the customer only wants to use Leaded plated parts, then a separate process can be created to recoat these termination or leads with a Leaded solder. Retinning the parts is an acceptable process and will now make the components more compatible with the low melt 60/40 alloys being used to create the solder joints.

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Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

Traditional tin-lead materials are not compatible with lead-free device finishes. Because proper reflow for lead-free materials can only be achieved with higher temperatures, attempting to process lead-free terminated devices in 183C tin-lead conditions leads to incomplete wetting and the related issues of voiding and opens.

In order to compensate for this gap in process temperatures, many assemblers have attempted to adjust the reflow profile, only to discover that the higher temperatures exceed the capability of most SnPb materials.

In one such case, a manufacturer of power devices was experiencing wetting problems with an SOIC device and attempted to "fix" the problem by adjusting the reflow profile to a higher temperature than that conducive to SnPb pastes. But, the adjustment was not sufficient to achieve the necessary wetting required to pass inspection.

Different problems exist in the case of area array devices. When processing a SnPb bumped BGA or CSP device in a tin-lead process, the results can be disastrous. For Pb-free bumped devices, the SAC alloys generally used require a melting temperature of 217C, but SnPb solder pastes achieve liquidus at 183C. So, when lead-free BGAs and CSPs are processed at lower temperatures, the bumps usually do not completely melt or collapse, which can lead to undue solder joint stress and, ultimately, failure.

You will need to work with your suppliers to determine appropriate reflow parameters for your specific application. You may need to change materials that have more active flux systems capable of higher reflow temperature capabilities.

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Doug Dixon
Global Marketing Director
Henkel Electronics
Mr. Dixon has been in the electronics field for over twenty years and is the Global Marketing Director with the electronics group of Henkel. Prior to joining Henkel, he worked for Raytheon, Camalot Systems, and Universal Instruments.

The parts in your Bill of Materials you are using need to be reviewed individually. Many manufacturers of passive parts (connectors, capacitors and resistors) have sold parts for many years that did not contain Lead (Pb) on the surface plating.

So, if you have in the past used a part that now is called RoHS-Compliant, you may want to check with the manufacturer, since in many cases, the surface plating of the part has remained unchanged as they transition their production to meet the requirements of the RoHS-Directive.

In many instances, the part will solder just fine in both lead-, and non-lead-bearing solder processes. The use of a lead-bearing solder means a lower solder temperature and so the part will probably be just fine.

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Arnold Offner
Industry Standard Manager
Phoenix Contact
With over 15 years service at Phoenix Contact, Mr. Offner has worked in South Africa and Germany. He speaks both English and German, is based in Harrisburg, Pennsylvania and is responsible for the RoHS/WEEE Training activities in the Americas.
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