CONTRACT MANUFACTURING FEATURE
ACHIEVING HIGH QUALITY and high yields every time
With accurate temperature sensing and
precision control, and under a patented technique known as ‘soft vapour phase’, the PCB height is adjusted relative to the vapour layer. Time spent in the vapour, above the vapour or on the surface of the vapour allows for the creation of a reflow profile. However, the greatest benefit of using
Vapour Phase for soldering complex PCBs, is that when the board reaches the temperature at which solder reflow occurs, the temperature differences between the various sized components and areas of the PCB are virtually non-existent, regardless of thermal mass. This is due to the efficient heat transfer taking place as the vapour condenses on the board and its components. An added bonus is that since the
‘Vapour Phase’ reflow is in the running to be one of the best techniques for soldering mixed-technology PCB assemblies if you want to achieve consistently high quality results. Neil Owen, Speedboard’sManaging Director, explains why
T
he most common method of soldering surface mount devices onto PCB
assemblies is to use five-, seven- or 10- zone convection reflow ovens. The method is fully automated and suits fast production lines as the ovens are able keep pace with the pick and place line cycle time. However, there have always been two major drawbacks with convection reflow ovens. Firstly, the creation of the reflow profile (i.e. the temperatures of the zones and the amount of time the board spends in each) is a lengthy process, as each and every job requires a series of fine adjustments to assure high quality and yield. Secondly, it is difficult to produce uniform soldering temperatures across a complex assembly. This is because components and areas of the PCB with low thermal mass can get too hot, whereas those with a high thermal mass struggle to reach the optimum soldering temperature. Accordingly, if you take the view that
convection reflow is really only suitable for standard assemblies then it is clear that you need a different production process for complex assemblies; such as flexi-rigids, boards with high layer counts and/or areas/components with high thermal mass. The solution to this is Vapour Phase reflow.
A CONDENSED EXPLANATION Vapour Phase reflow is not a new concept. During the 1970s, a technique known as ‘Condensation Soldering’ was developed. It relied on liquids which used CFCs, and the process itself often caused tomb-stoning (a.k.a. ‘chip-lifting’). However, the process has improved significantly over the years, and is now more commonly known as Vapour Phase. The liquid used today is mainly Galden, a brand of perflourpolyether - an inert and more environmentally-friendly liquid – and the process equipment can now be controlled to achieve optimum soldering temperatures; to match solder, component and board specifications. Galden is available with different boiling
points. For example, one with a boiling point of 235o
C would typically be used for lead-
free soldering, as most lead-free solders have a minimum peak reflow temperature of 230o
C. In contrast, a convection reflow oven
might need to have its final stage heaters set significantly higher to ensure solder reflow for high thermal mass components. In a Vapour Phase oven, as the Galden
boils, a vapour layer forms. The vapour is several times heavier than air and basic physics dictates it cannot be hotter than the liquid’s boiling point; which removes the risk of over-cooking any of the components.
/ ELECTRONICS Figure 1:
PCB produced at Speedboard using a Vapour Phase oven
soldering is in an inert atmosphere, the aesthetics of the solder joints are improved. Other advantages are that a Vapour Phase oven requires far less electrical power than a traditional reflow oven and the use of airlocks minimises the loss (to atmosphere) of Galden. Vapour Phase is a ‘Must Have’ for high
complexity assemblies. Whilst traditional reflow remains the most cost-effective/ lean method of manufacturing the majority of PCBs, complex boards demand the use of a better soldering process to guarantee reliability and repeatability.
WINNING TECHNIQUE The PCB in figure 1 was produced at Speedboard using a Vapour Phase oven. The board has 75µm track/85µm gap over nearly all of its area, approximately 10,000 microvias, approximately 8,000 conventional vias, and 22 layers in a flexi- rigid construction (giving a total thickness of 2.5mm). This combination of features makes the board extremely challenging to assemble using traditional reflow techniques. For instance, the large thermal mass variations between the rigid areas and the flexi limbs, combined with the wide variety of component types, demand a very controllable soldering process. For boards of this complexity, and greater, Vapour Phase is one of the most successful ways of guaranteeing consistently high production yields.
Speedboard Assembly Services
www.speedboard.co.uk 01753 746 700
Enter 205 ELECTRONICS | FEBRUARY 2015 19
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