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February 4, 2007

Reflow parameters for lead-free BGA

We have an ENIG plated PCB that is entirely leaded except for one BGA which has SAC305 for balls.

Using a water soluble 63Sn/37Pb paste, what are the optimum profile settings to assure no voids or "IPC acceptable" voids?

For settings I'm looking for times and temperatures for both the soak and above liquidous zones.

Mike Johnson

Experts Comments

Manufacturing with a mixed metal system is, unfortunately, becoming more common as high volume manufacturers move to Pb-free. This in turn causes the package manufactures to produce predominately Pb-free bumped packages. When these are used in a Sn/Pb process there can be a significant impact on reliability.

A worst case scenario is where the SAC bump is not reflowed or only partially reflowed. This typically occurs when a "standard" Sn/Pb reflow process is used, the incomplete mixing will have a significant impact on reliability Hillman, et al. (CMAP, Toronto, 2005) showed the disastrous consequences of not completely reflowing the solder joint and having a Pb-rich region in the solder joint.

More recently Pan, et al. (IPC/JEDEC, San Jose, 2005) summarized some of the previous work, including reliability studies done by Solectron where they showed an impact in reliability, but in some cases may be acceptable, as long as the correct temperature is achieved in order to get complete mixing. This paper also provides the methods to calculate the reflow temperature needed in order to get complete mixing (based on the bump size, alloy, and paste deposit). Of course it is going to be higher than a standard tin-lead reflow temperature. Therefore you need to insure that you Sn/Pb flux system is capable of withstanding the higher temperatures without suffering flux exhaustion.

There are a few vendors today who offer "transition" solder pastes, that are Sn/Pb alloy based, with a flux medium that can survive higher temperatures. Profile the PCB (with all the components populated) to be sure the BGA solder joints achieve the desired temperature, then cross section the solder joints to make sure the solder joint looks homogenous. Depending on your end customer you may need to provide reliability data.

Dr. Brian Toleno
Application Engineering
Henkel Electronics
Dr. Brian Toleno is the Application Engineering Team leader for Henkel Technologies. He is responsible for the technical service and application engineering for Henkel's electronics assembly materials, including solder paste, underfills, PCB protection materials, and underfills.

The profile should be according to the paste supplier's recommendations. You may want to run a hotter spike if you can with out damaging other components to insure a reliable bond to the SAC BGA I would recommend a 235C minimal spike, time above 217C for a minimal of 30seconds. Soak time and temperature is related to the chemistry you are using.

Karl Seelig

Deck Street Consultants
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

Since the paste is 63/37 eutectic solder paste I would recommend the following ...

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 dry out 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 to mix within the bulk and the solder to wet. Since it is ENIG finish, the gold (hopefully very low thickness, in the order of few microns) will precipitate as an intermetallic within the solder bulk, allowing the Nickel to interact with the Sn to form the intermetallic. The extended TAL will not form thicker intermetallic as with Cu and Sn, since Ni has self limiting characteristics when forming intermetallics with Sn.

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

Bjorn Dahle
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.
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