After much analysis – we decided that what worried people most about BGAs was not alignment – that turned out to be easier than everyone first thought. No, it was “has the flipping thing soldered?” You can’t have a quick squint under the microscope so how do I really know that the balls right underneath have gone?
SOLDERING METHODS So what methods are in use – and why are people uncertain? Most people in the SMT world had switched to convection (hot air) ovens long before BGAs arrived. Unfortunately if they had stuck with the first generation infrared systems they’d have been better off.
Problem is a BGA has all its connections underneath – the BGA body to PCB gap is a millimetre or so. Now you have to get even heating right under the BGA – just one cold spot means a defective joint. Hot air – even if turbulent just is not going to penetrate that well into such a narrow gap.
So what do solderers do? They either increase the temperature to ensure that every part of the device is hot enough or increase the heating time so as to allow the package time to heat through by conduction. Hence why IR is better – it heats the package rather than tries to blow heated air under a narrow opening. However, there is a third heating method – the one we use. It is guaranteed to heat every part of every device evenly, it is impossible to overheat from its specified temperature, it completely surrounds the job in an oxygen free, totally inert environment which helps the flux do its job better still.
What is this heating method: it is now called “condensation reflow” although many old hands at SMT know it as Vapour Phase soldering. A quick description is that the process uses a special chemical (basically a fluorocarbon) that boils at a known temperature – we use a 215- degree BP. The boards to be soldered are placed in a chamber in the bottom of which is a sump of this fluid, which is heated. AS it heats up it produces steam – which just like it does in your kitchen condenses on any surface cooler than itself. As it condenses it give sup its heat to the cooler item. Steadily the Allgood Technology Ltd – Electronic Assembly Services.