Pb-free era

07 February 2006

BGA rework is a critical process in the Pb-free era. Roger Gibbs describes a number of tools that have simplified the task.

With temperatures close to that which would damage both components and board, rework of BGAs in the Pb-free age is a delicate operation. Not surprisingly, OEMs and EMS providers the world over spend a great deal of time developing thermal profiles - target temperatures, ramp rates and soak times - that will guarantee fastest, safest time to reflow. Creating a thermal profile is as lengthy as it is crucial; it requires tests, analysis of joint quality, thermal probes everywhere. It takes time away from the real business of rework, and it has to be repeated every time you change a board or add a new component.


Or does it? Roger Gibbs, Managing Director of PDR, doesn't think so. "Our clients use our systems because we've done all the thermal profiling for them - all they have to do is place the board, select the right profile, focus the heat source on the component to be reworked, and press a button".

How Focused IR works
IR heat sources directly heat objects, rather than the air around them, thus the heat effect is instantaneous, efficient and precise. It's also safe, as Gibbs explains: "We call it passive heat, as there's enough energy to get a component to the right temperature, yet you can safely put your hand in the beam, even at full power".

Control is absolutely essential to successful rework, and PDR's non-contact IR heat sensor, developed in 1996, is key, as Roger Gibbs explains: "Even for the big boards, our systems don't work at full power, allowing us to control how much energy we put into an assembly. With the sensor, we know the precise temperature of the board and component during the rework process, and as these temperatures are directly related to solder ball temperature, we know exactly when reflow occurs".

The heat sensor has really come into its own in the last year with the introduction of PDR's new profiling software. This automatically ensures that the board and component ramp at the right rate to the right temperature and soak for the right time for their size and application. The system controls how much energy is required 4 times a second and keeps temperature at all times to within 2(C of target profile.

Manufacturers can use their paste manufacturer's profile, or PDR's own default profiles. PDR's profile for mobile telephone boards, for example, will fit any mobile phone, independent of size and composition. One major European mobile telecommunications OEM uses one profile on PDR's equipment for its lead-bearing processes and another for its lead-free processes.

Focused IR vs Hot Gas
Gibbs believes that both IR and hot gas have their merits, and that any professional electronics manufacturer should have both: "Personally I think that gas is better for the very big QFPs, but with our temperature control capabilities, Focused IR is far better for BGAs. Gas systems would require hours of work to get to a profile as precise as ours - which is ready in minutes".

Focused IR or gas, nothing beats the top rework systems for professional BGA rework: "If you are handling the big active components, they must retain their integrity, especially where they are being placed after removal, so you really need an automatic or semi-automatic system. It's possible with a manual system, but it's not that easy".

Focused IR, he says, is more appropriate for applications such as mobile telephones. where there are a lot of smaller components, or for reworking large BGAs: "Large components need quite a lot of energy, which, for hot gas systems, means a higher gas flow. This can be a problem as BGAs are typically surrounded by small chips and resistors that can be blown away - a problem that increases when you get to higher temperature lead-free processes".

Accessibility on densely packed boards is an additional problem for hot gas systems, which furthermore call for a range of nozzle geometries for different component types. With Focussed IR, the operator simply positions the heat energy spot and adjusts its size to the component to be reworked - no access problems and no nozzle changes. The main advantage of Focused IR, though, is that the temperature of replaced components can be measured in real-time using the non-contact heat sensor - something that is impossible with hot gas systems.

Lead-free rework
This is all amplified by lead-free processes, and the higher temperatures and narrower processing windows they entail. From visits to countless electronics manufacturing sites, Gibbs concludes that consensus centres on a target temperature of around 245(C for the top of the component in order for the lead-free solder balls underneath to get to reflow temperatures. Yet many components can withstand maximum temperatures of only 5øC above that. It's a very fine line indeed, especially as it's not hard to overshoot the target temperatures if you don't have your profiles and temperatures under control.

For this reason, lead-free processes call for the top of the range equipment, whether it's Focused IR or hot gas: "You need enough energy on the topside to get to temperature, and you need absolute thermal and process control", says Gibbs, warning that this can be a problem with some hot gas systems: "As it is gas that heats up the board and the component, the gas temperature must be beyond the target temperatures, creating a risk of heat damage. The better systems avoid these problems with more effective back heating and better temperature control, but there are many more systems that could create damage".

On its own systems, PDR again recommends that the board be pre-heated from underneath to within 50°C or 60°C of target component temperature, to 185°C. The top heater then does the rest, following the profile exactly with no damage to board or component.

Once you've got your thermal profiling under control, Gibbs believes you are 40% of the way to good rework quality. Good alignment and placement systems will take you a further 20%. The remaining 40% is down to materials: "With no-clean and lead-free processes, solderability and wetting are harder to achieve. This means that solders and fluxes must be selected with care for their performance".

Conclusion

BGA rework is one of the riskiest processes within today's electronics manufacture - even more so with the introduction of lead-free manufacture. The quality of rework, so crucial to any OEM's or EMS provider's profitability, is down to mechanical accuracy, good materials, and thermal control.


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