Mixing solder alloys on the same board is never a good idea, but it is a cleaver thought. It won't be long before we are all required to "get the lead out" so let's consider the real problem: getting enough heat into the Modules to let them solder safely along with your BGAs.

First, make sure you are profiling the temperatures of the critical parts, at least the BGAs (those parts at risk) and your "Mdules" (the parts that are not getting hot enough). This data will be critical in finding the best reflow recipe.

With convection reflow ovens, you have three options: higher temperature, higher convection rate (air flow), or more time, and combination of the three. Since over temperature of the BGAs is the main concern, I don't recommend higher temperatures in the zones. This just opens you up to the BGAs going well above their kill temp.

You can increase the convection (air flow rate) IF you have this option on your reflow oven. Some do, some don't. This takes the form of fan RPMs, fan frequency (Hz), PSI control, a Low, Medium, High setting and other recipe settings depending on the oven.

This increased setting will allow you to heat thermally massive parts without increasing the set point temperature in the zones. This lowers the risk of getting the BGAs too hot, because the zone temp is still the same, while pushing more heat into your Modules. It will force the BGAs to get up to temp faster, thus increasing their time above temp, so you should make sure this time limit is not violated.

If you can't increase the convection rate, either because you do not have the control, or it is already at maximum, then the final option is to increase the time in each zone. This is of course done by lowering the conveyor speed.

More time in each zone, again without increasing the zone temperature set points, will allow more time for the thermally massive Modules to get to the needed temp without risking the BGAs maximum temperature.

Again, this will increase the time the BGAs spend above temp, so this should be watch to make sure you are not violating this time limit.

Another approach is to "heat shield" the sensitive BGA with the same composite material used to make wave solder pallets. We make wave solder pallets to protect parts on the board from the wave, and we can do the same thing for reflow.

Make a reflow pallet that covers (top side) the BGAs and other sensitive components and protect them from the heat convection because it acts as an insulator to slow the heat rise for these parts. Yes, the "pallets" would have to be applied to every board being reflow soldered, however, they are reused 1000s of times, just like in wave soldering.

If you have already attempted these ideas, then it may be time to try a different reflow process, like Vapor Phase. This process can force heat into the heaviest of thermal masses without risking temperature sensitive components, because the maximum the parts will see temperature is controlled by the boiling point of the liquid used in the system, typically 230C or 240C.

It worth a renewed look at this older, but returning reflow process, now that new "ozone safe" liquids have been developed.

The increase in the needed energy for good lead free solder joints to be formed is significant. If the target PCB has a wide variety of parts and the Reflow system lacking in thermal capability it become an issue of protecting smaller parts from reaching temperatures that exceed their operating specifications while providing adequate heat [energy] to reflow the heavier parts like BGA's and connectors.

The only true and safe method of guaranteeing a "not to exceed" peak temperature while ensuring the PCB and ALL the components reflow in an Inert environment is to reflow using Vapor Phase technology. A System that utilizes an inert vapor as the mechanism for thermal transfer.

When using a Pb-free profile for Pb-free BGAs & Sn/Pb paste, the good thing is that the Pb-free BGA balls will melt completely and a homogeneous joint is achieved.

Since a Sn/Pb paste is now exposed to higher temperatures, some of the things that need to be considered are:

• Is the flux in the Sn/Pb paste capable of removing oxide at the higher Pb-free temperatures of 235-245 deg C and allow for good solderability?
• Is the flux residue going to char / discolor due to the higher temperature exposure? This could be a cosmetic concern.
• If the flux becomes very hard due to high temperature exposure, it may not be easy to remove the residue after reflow (if a cleaning cycle follows reflow)
• When the Sn/Pb paste is exposed to the higher temperatures, depending on the flux chemistry, more voiding may occur - increased voiding could be a concern.

PCBs with a mix of tin-lead and lead free components or high mass and low mass components has a similar solution if redesigning the PCB is not in the cards. You can define each component with its own unique process specification visa-a-vie the thermocouple(s) attached, from which you let the prediction software find you a solution that balances your PCB.

What do I mean? Let's say you have a BGA with a peak spec of 235-250 C. 235 for the solder joint and 250 as the maximum temperature of the component. Meanwhile on the same PCB you have a few heat intolerant components that cannot go above 245C.

It doesn't help that one of these components is in close proximity to your higher mass BGA. So what do you do? Using KIC2000 software, you can choose to create individual specs for each thermo couple.

In this example, I would define the peak range for my BGA as 235-250C, while for my smaller heat intolerant component I would set a limit of 245C. Go ahead and run a profile and with KIC's Navigator software you will find out two pieces of information.

First, are both components in spec? Secondly, if not, what do you need to change with your oven recipe to bring these components into specification. In practice, I have found sometime there is no solution, especially if you are using an older reflow oven designed for tin-lead processes.

Nevertheless, if there is a possible solution the software will find it for you with the gear you have to work with. Sure beats having to redesign a board or send PCBs to rework.