Reflow soldering

The term reflow soldering or reflow soldering (English: reflow soldering ) denotes a course in electrical soldering method for soldering SMD components. In the manufacture of thick film hybrid circuits, it is the most common soldering process.

Soldering

• In the first step, the solder is in the form of solder paste is applied to the printed circuit board / printed circuit board before the mounting in reflow soldering. Herein, the main difference is to other soldering processes, such as soldering iron soldering, dip soldering or wave soldering. There are various ways of solder application, for example by screen printing ( stencil, screen printing), dispensers, solder preforms by (preforms ) or electrically.
• In the next step, the components are assembled. The use of solder paste has the advantage that it is so sticky and hold the components during the assembly directly to the paste. So you need not be specially glued.
• During melting of leaded solder, the assembled components centered by the surface tension on the landing pads and put away. For lead -free solders (eg SnAgCu ), this effect is almost completely attributed (see also RoHS).

Possible solder defects that may occur during the reflow soldering, the wicking effect, the blurring and the grave stone effect are.

Common reflow soldering

Hot plate

The component mounting carrier substrate is placed and heated on a hot plate. After the solder is melted uniformly, the carrier substrate is removed from the plate. This method can be used for the inorganic support substrates. Here, the entire carrier substrate is brought to brazing temperature. Organic support (eg glass transition temperature for standard PCB 140 ° C) are usually not suitable for Kontaktlötverfahren on the hot plate due to its glass transition temperature. It is possible when Kontaktlötverfahren unilateral placement. There are systems in which the loaded carrier substrate without voids by a simultaneously -applied vacuum process and therefore has extremely reliable solder joints. Main applications of the vacuum contact soldering are the soldering of the power semiconductor chip (soldering without gaseous inclusions), hermetic sealing by soldering and flux-free contact. Through the use of a vacuum chamber inert process gas such as nitrogen, reducing process gases such as forming gas, to 100 % hydrogen or wet activating formic acid can be controlled used in the soldering process. In order to solder completely residue- free, vacuum plasma activation can be applied during the soldering process. Through these front-end capability can also MOEMS, MEMS and be reflow soldered on wafer level. A plasma enhanced brazing has advantages for soldering the power semiconductor chip, since the necessary cleaning process after soldering usually eliminated before the wire bonding.

Heated moldings, brackets and stamp

A housing adapted to the shape of the component to be soldered stamp or bar is heated with a resistance heater. This then presses the component leads on the solder joint and the solder melts on. The heater is then turned off and lifted the stamp only after solidifying again. The solder joints resilient component connectors can be soldered so sure. In general, as only individual components are soldered one by one.

Infrared Heaters

The boards to be soldered are soldered in Durchlauflötstrecken. The soldering material is thereby driven by a conveyor system through a furnace. The soldering may be controlled by the residence time in the various temperature zones. Usually there are four zones, one for heating the entire circuit, the second to enable the flux for soldering, the third and the fourth to cool. Reflow soldering with infrared radiators is a simple process to manufacture boards in series. A disadvantage of the infrared radiator is the strong absorption of radiant energy by black components ( IC package ) which has a non-uniform heat distribution result and can lead to local overheating.

In vacuum brazing ( lunker and without flux ) is the transfer of heat by radiation, in addition to the transfer of heat by contact physico only option. Transfer of heat by convection is not possible in a vacuum because of the lack of transmission medium. Therefore, in the vacuum brazing of both the transfer of heat by contact (contact soldering), and the transmission is used by radiation.

Full convection reflow soldering

The full convection reflow soldering systems resemble the infrared systems, but in this case air is heated and passed through a nozzle system to the part to be soldered. This achieves a more uniform heat distribution, than is possible with infrared radiators. A further advantage is the greater heat capacity of the furnace. In electronics manufacturing, this method is most commonly used. The disadvantage is that in the shadow of large components cold zones may arise because they are enclosed in the convection of air is not always 100 %. Each board must be soldered, passes through at least 4 compartments. Preheating the flux activation, the peak region and cooling.

Vapor phase ( condensation soldering)

The vapor-phase soldering (English: vapor phase ), utilized for heating of the assembly, the released during the phase change of the heat transfer medium from the gaseous to the liquid state heat of condensation. Here, a condensation on the surface of the part to be soldered will be held until the entire module has reached the temperature of the vapor. It boils the liquid forms over her a saturated, chemically inert vapor zone temperature is largely identical to that of the liquid, so that forms an optimal protective gas atmosphere and oxidation in the vapor phase soldering process are excluded.

As the heat transfer medium is perfluoropolyether ( PFPE, commercially available for example under the trade name Galden.RTM ) used. These liquid polymers are exclusively of carbon (C) -, fluoro (F) - and oxygen (O ) atoms up. The present in the molecule C = - and CF bonds are highly resistant. They are among the most stable bonds in the carbon chemistry. The bound to the central polymer chain fluorine atoms shield the carbon backbone from perfect and thus protect the delicate carbon-carbon bonds to chemical and thermal attack. They have excellent heat transfer coefficient and good dielectric properties. In contrast to CFC-containing gases which have been used in the past, PFPE having no ozone depletion potential ..

The heat transfer is fast and independent of geometry, there are no cold zones in the shade of large components. The precisely defined soldering temperature and uniform heating no overheating of the components is possible. This allows soldering with low activated flux. The requirement for preheating is less, by vapor phase soldering are usually more compact than infrared ovens. Main application is the serial production ..

A special form of vapor phase soldering is the vacuum vapor phase soldering. This technology is available in the industrial environment since about the year 2000. After the solder is melted completely, in the process chamber, a very high vacuum is produced. The negative pressure ensures that the gaseous inclusions to be displaced largely in the solder to the outside and be eliminated from the solder joint. The result is a largely void-free solder joint. This technology is particularly advantageous if the solder joints have to pay very high thermal power losses. Inclusions in the solder joints in the form of voids which would significantly increase the thermal resistance of the solder joint. Furthermore, the mechanical strength is significantly increased by the extensive Void-free.

Laser beam

The solder joints are heated with a laser beam, it can transmit exactly a lot of energy. The solder joint is time ( soldering time about 0.2-0.4 s ) and spatially in a very narrow heated. This occurs at the components virtually no thermal stress. A dealloying of the conductor tracks can be avoided. Due to the high cost of this procedure is actually profitable only in mass production or for highly sensitive components.