Vapor Phase Heat Transfer Print E-mail
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Written by Jochen Lipp   
Monday, 30 April 2012 11:39

Sensor-driven controls permit a range of profiles and eliminate tombstoning.

The past few years have brought a new vapor phase procedure and new machines. VPS now offers new features for soldering, especially helpful when changing to Pb-free soldering. We highlighted many of these in our February column.

Vapor phase can control heat transfer efficiently and precisely by heat level adjustment, which controls the height of the PCB in the vapor blanket. The more power the heaters have, the more vapor produced. Sophisticated design permits temperature profiles to be adjusted to nearly any required temperature gradient (Figure 1).

 

The assembly is lowered into the vapor blanket in multiple steps, where the machine will run any profile programmed into the machine. Independent of the loading condition, integrated sensor controls monitor the actual temperature at any time of the solder cycle and adjust the VPS to the current situation to achieve the same profile every time.

Newer vapor phase soldering systems use a series of temperature sensors to control each process. Cycles are monitored and compared against a programmed reference cycle. This helps ensure exact repeatability of the soldering process, in particular when processing complex applications such as BGAs and LEDs. If a certain profile must be run on a critical component, the temperature sensor can measure the temperature at this point, and the machine will follow the programmed profile
from that point forward, eliminating the need for multiple trial runs.

The process sequence of heat transfer in the vapor phase is performed by moving the assembly into the vapor phase chamber. There, it is either preheated by IR radiators or on the upper part of the vapor phase. IR preheat improves quality of the solder joint and permits control of the temperature profiles. The assembly then descends into the vapor phase, where it is heated and control of the heat transfer is possible.

A chemically inert fluid called Galden is used for the heat transfer. Most Galden liquids have boiling points of 200˚ to 235˚C, but it is possible to obtain Galden with higher temperatures. Because the Galden has a specific temperature set, overheating is physically impossible.

This even heat transfer pays off in reducing tombstoning (Figure 2). Tombstoning is the phenomenon where small components soldered on SMT pads suddenly flip up vertically on the pads. This phenomenon is determined by different tension forces that take over when the solder paste turns liquid. There is an unbalanced tension force from one end of the chip, which causes the lifting. The smaller the chips, the greater the risk for tombstoning. The solution to reduce or eliminate tombstoning consists of the following factors: the characteristics of the solder paste, the printed circuit board design and the reflow process.

Jochen Lipp is CEO of IBL Technologies (ibl-tech.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column runs bimonthly.

Last Updated on Monday, 30 April 2012 15:25
 

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