Vapour Phase Reflow Questions & Answers

Vapour Phase Reflow - Questions & Answers

Questions & Myths answered about Vapour Phase Reflow

Why Vapour Phase instead of other reflow methods?
Assemblers in progressive companies throughout the world are recognising the advantages of this reflow soldering method as currently the simplest and most reliable method of soldering.
It allows for the processing of ALL components without any complicated calculations or the need to maintain temperature controls. Vapour Phase provides long term reliable reproducibility of processing conditions.

How does it work?
As the liquid boils, a layer of saturated vapour containing no oxygen or other gasses forms above it. As the solder material is immersed into the vapour zone the vapour condenses onto it and transfers its corresponding heat.

Vapour Phase was very popular in the 80’s; the process gave good results, but fell from favour because the chemicals used contained CFC’s.
The fluid used in today’s Vapour Phase machines DOES NOT CONTAIN CFC’s. The chemical is environmentally compatible. It is non-corrosive and extremely stable.

The chemical used in Vapour Phase machines is very expensive, and machines suffer from excessive solvent loss.
Chemical running costs of a basic machine (IBL model SCL500 for example) are approximately 70p an hour or a little over 1p a minute.

Is Condensation Soldering different to Vapour Phase soldering?
They are the same. The same fluids are used, heated to their boiling point to create a vapour (or condensation), which allows the solder to reflow in an inert atmosphere. Any differences are to individual manufacturer’s machine design.

How do you control the risk of overheating?
The maximum board temperature is equal to the boiling temperature of the liquid used, i.e.200ºC. for SnPb or 230ºC for lead-free materials. Irrespective of how long the solder material remains in the vapour, irrespective of board thickness; temperature can never be higher than that of the vapour.
For example; it is possible to solder such disparate items as a 0.5mm thick PCB and an 18-layer board simultaneously. Both will get sufficient heat for soldering but neither will overheat.
Vapour Phase eliminates the risk of overheating.
The laws of physics dictate the process, i.e. it is impossible to overheat.

What about oxidation?
Vapour Phase Reflow produces a 100% inert/oxygen free soldering environment that eliminates oxidation problems. The vapour is heavy (in comparison to steam or air) and therefore displaces lighter gases, which are found above the vapour. In this way the vapour creates a protective gas atmosphere without the use of nitrogen (as used in other soldering procedures).

Boards are subjected to a large thermal shock when entering the vapour phase chamber. The PCBs are elevated from ambient to reflow temperature very quickly, and this can cause thermal stress and component damage.
The patented IBL design allows up to 20 steps prior to the soldering position. This provides a very controlled temperature rise prior to the board entering the vapour; temperature rise can be between 1° - 7°C per second.
Heat transfer to the board can be faster or slower, different temperature gradients can be produced to suit specific board/component types.
All steps are programmable.

Vapour Phase Reflow causes “tomb-stoning”
The ability to completely pre-heat the board, according to the material/component requirements (see previous answer), and create a very gradual temperature rise will help avoid thermal stress and tomb-stoning.

Today’s boards are frequently very heavily populated; and often include a great variety of small sensitive components, such as µBGAs, CSPs and Flipchips. Solder joints are mostly hidden; a uniform temperature profile is crucial. How can vapour phase provide this?
During conventional reflow in order to ensure that the hidden central joints of these delicate devices are heated sufficiently, the temperature may have to be raised as much as 50ºC above the melting point of the alloy. Overheating of the ‘external’ solder connections and permanent component damage is most often an inevitability. When using Vapour Phase (or condensation), heat is drawn by conduction from a saturated vapour onto the PCB; the heat is distributed evenly throughout, irrespective of whether components have a high or low thermal mass. Vapour Phase has a delta T of 5°C. e.g. Reflow temperature differentials across the assembly are always less than 5ºC.

How does Vapour Phase cope with Lead Free Solders?
Temperatures that are required for lead free materials are more extreme. Typically +10% for wave soldering and +20% for reflow, these additional temperatures create an even greater risk of material and component damage. Vapour Phase is the only reflow process which allows you to change to lead free (for example: SnAg 3,5 with a melting point of 221ºC) without danger of overheating the board or components by using a liquid with a boiling point of 230ºC.

Solder Wetting of lead free materials is a concern
Lead free alloys do not wet as well as SnPb, an inert atmosphere will be more important to employ the best possible soldering conditions to help overcome this problem. When using convection reflow the use of a nitrogen atmosphere will improve solder joints, but for low cost manufacturing the added cost of using nitrogen may not made it a viable solution. You can solder without nitrogen, however, long-term your product reliability could be jeopardised.

Vapour Phase reflow soldering automatically provides a 100% inert atmosphere, without adding nitrogen (and without adding additional cost), to guarantee the best possible soldering conditions, and the best possible long term reliability, for all your production every time.

What about the problem of pop-corning ?
As plastics are hygroscopic humidity will get into the body of these components (e.g. BGAs). When reflow soldering this can lead to high pressure of the moisture trapped inside the molded components, which is converted into steam and forced out under pressure during reflow. This can cause delamination of the substrate or 'pop-corning'. The higher the reflow temperature, needed for lead free alloys result in a greater the risk of pop-corning, therefore a maximum reflow temperature of 230ºC is desirable. As there is absolutely no danger of heating above a maximum of 230ºC Vapour Phase reflow, this provides a lower risk of pop-corning.

For use in the Semi-conductor industry an IBL Vapour Phase machine with vacuum is also available (model no. VAC665).

What modifications must be done to Vapour Phase machines which are currently being used for SnPb, in order that they can be used for Lead free solders?
NO modifications or additions at all are necessary. All you do is fill the machine with a fluid that has a boiling point of 230ºC (instead of 200ºC).

What about maintenance costs?
The design of IBL Vapour Phase solder reflow machines and the clean-running nature of the process mean that the maintenance requirement is minimal. (Operator maintenance virtually nil.) Machines are sold and supported in the UK & Ireland by A&D Automation (Sales) Ltd.