Panelists at APEX discuss misconceptions about the reflow process and how to Minimize Delta Ts, etc.
Mike Buetow of Circuits Assembly magazine moderates a discussion panel on soldering and thermal profiling at APEX 2009. Panelists include Keith Howell of Nihon Superior, Fred Dimock of BTU and Michael Limberg from KIC.
Much of the 30 min discussion hits upon how customers often confuse an oven’s recipe with a PCB’s profile/recipe. Factors such as density, delta Ts, belt speed, different components and extraction are used as examples as to why the oven’s set points don’t always match the temperatures on the PCB. All panelists agree that a fair amount of customers do not understand these important concepts.
Fred Dimock of BTU cites an interesting study he conducted to highlight the difference mass has on the peak temperatures a board experiences without changing the oven set points. The example he gives is a 100gram board that achieves a 231 C peak when compared with a 230gram board only reaching a 225C peak with everything else being equal. Panelists agree that customers often expect to see the same profile at a given oven set up, when obviously factors such as mass play such a critical role!
All panelists talked at length about how to minimize delta Ts as an important factor in producing quality PCBs. The PCB design and layout of components was discussed by Keith and Mike.
Fred cited a study that higher convection rates also yield a lower delta T, taking into account the need to maintain a stable environment early on in the reflow process before components have had a chance to take hold. Starting at low convection allowing the flux to become tacky (thus keeping components in position) and eventually raising convection in the peak zone can minimize large deltas.
Fred also shared a profiling trick with Ramp Soak Spike profiles he likes to use when trying to minimize the delta Ts at peak. In RSS profiles, one would run as close to the edge of the top of the spec of soak and get as high as you can in temp early before you hit the spike, but you need a quality profiler and good ThermoCouple attachment to pull this off, Fred added.
The session also covered briefly upon topics such as:
- Vapor Phase profiling: Keith & Mike
- Nihon’s SN100C paste: Keith
- How to Profile Expensive Components: Mike
- Importance of Cool Down and considerations, such as the roll of large BGAs: Fred and Keith
To watch a video of the session, click here: http://blip.tv/file/1969267/
