Vapour phase or convection for lead-free?
Figure 2. PPM Soldering yields on test board product by convection and vapour phase reflow
Figure 3. Flux exhaustion during reflow leaving incomplete coalescence of all the solder balls. This can occur with some combinations of paste and convection profiles in air.
the process. There will also be existing system. It should be remembered that the the vapour to prevent it escaping, and this
engineers who will move back from convec- only reason for the secondary layer on was done with a secondary fluid which also
tion to VPS as a solution for lead-free. It older machines, was a ‘sacrificial blanket,’ produced a vapour layer boiling at a lower
was interesting two years ago to be listen to prevent the loss of the primary vapour. temperature. The condensing coils used
to development staff of a major machine In the batch and in-line vapour phase cold water running through them at a very
supplier discuss where noticeable improve- systems of today, systems do not have a low flow rate to recondense the vapour;
ment in convection technology could secondary vapour blank. the fluid then circulated back through
come from. There are areas of cooling and From a profiling point of view, the goal filters. Above the primary coil, another coil
energy savings still to be achieved, but was is to solder joints with the minimum ΔT was used to reduce the loss of the second-
a re-launch of VPS the solution? across the board and at the lowest peak ary blanket of vapour sitting on the top of
It can be seen that it is perfectly temperature. Minimise the time joints are the primary fluid.
feasible, by using more than one Vapour in a liquid state and control the speed of In operation, a basket loaded with
phase system, to carry out sequential sol- temperature rise to reflow. Gone are the assembled boards would be simply lowered
dering processes at different temperatures. concerns of damage of the boards due to through the secondary layer and into the
This may be a benefit to some companies excessive temperatures. Care does need primary. Prior to reflow, the boards were
and often for non-PCB applications. A to be taken on fixturing of small or light often preheated in a separate oven to
point on the cost of these liquids: they boards on carriers. If they are lost into the obtain a step heating process, reduce the
cost in the region £70-80 per kilo. Due to boiling sump, there is no way to retrieve collapse of the vapour and prevent the
this cost, it is clearly imperative that the them until the fluid cools. In the case of secondary vapour condensing on the board
design of the machine that is to be used designs which employ a heated plate for assembly. After the work had reached
with these fluids should be maximised in vaporising the fluid, as opposed to the re-flow temperature, the basket would be
terms of not allowing vapour or fluid loss traditional sump systems, retrieval may be raised. On the way up, it was allowed to
from the system. A typical batch process easier but still not recommended. dwell in the secondary blanket, in order to
is quoted with a running cost today of In the original batch vapour phase sys- contain primary vapour and fluid. Ideally
between £2-3 per hour. This also means tem, the primary fluid was boiled and gen- the board or basket would be very slightly
that the PCB design should not have, for erated a vapour layer. In order to contain angled to allow condensed fluid to run off.
instance, any fluid traps which could carry this vapour within the size constraints of The basket would then be raised out of the
this extremely expensive fluid out of the the holding tank, it was necessary to cover machine and allowed to cool.
www.globalsmt.net Global SMT & Packaging – December 2008 – 5
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