Contract Services
By Nicholas Abel
Inovar, Inc.

What about mixing a leaded BGA on a board using Lead-Free paste? Or vice-versa?

There are two things that we need to consider here.  We need to look at both the temperature that the BGA ball and solder on the board during reflow, along with the long term effects on the joint, due to the grain structure of the reflowed board and BGA.  One point to keep in mind is that BGA balls (and other items similar) have a larger share of the volume in a ball and solder paste combination.  This means the alloy that the ball is made out of will have the majority effect on that joint.  This makes the conditions that occur for a BGA in a mixed alloy assembly unique.  There may also be various grain structures, depending on the alloy combination used.

With that in mind, let’s look at the case of a leaded BGA, in combination with a board using a lead-free paste.  In this arrangement, the tin-lead solder of the ball will undergo temperatures that are normally for a lead free reflow profile.  The component may or may not be able to handle the temperatures typical of a lead-free profile, or the temperature ramp-up needed to achieve that – generally above 217° C, with the profile peaking above 240-260° C.  What does this do to the solder joint?    The ball will definitely reflow, although the heightened temperature may cause a larger intermetallic layer growth, which tends to be more brittle than the remainder of the solder joint.  The combination of the tin-lead ball and the lead-free paste can cause long-term joint temperature cycling problems.  While the leaded solder ball is liquidus, the solder paste may vent up into the ball during reflow, encouraging voids to be in the ball.  Due to the amount of difficulties that may arise, this is not the best combination for mixing alloys.

How about the alternate case of the combination of a lead-free ball and a tin-lead solder paste?  We know that heightened temperatures are needed to achieve a temperature that the ball can reflow in.  Depending on the type of flux that is contained within the solder paste, it may burn off the majority of the flux before it reaches peak, allowing some oxidation.  The increased amount of tin within the alloying ball and solder paste will cause some of the tin to migrate together, and some of the lead will tend to solidify earlier than the rest of the solder joint; dendritic growth may occur within the joint.  Insufficient temperature in reflow may result in stratification or layers of decreasing reflow status; this condition may encourage premature failure in a shock or a cycling condition.  A tin-lead solder paste with a higher temperature threshold must be used, to avoid excessive flux burn off and joint embrittlement.

Typical industry recommendations are to use a lead-free paste, with tin-lead components and a tin-lead compatible board finish for achieving forward compatibility, or a tin-lead paste with lead-free components and finish for handling backwards compatibility.  Caution and considerable attention must be used when using a BGA that combines alloys, in trying to achieve these compatibilities.  More and more manufacturers are specifying how to use their lead-free ball BGA’s in a tin-lead paste with a higher temperature threshold.