Knowing how to assess the 7 key profile parameters leads to good joints.

The purpose of a reflow process is the formation of solder joints. The process ensures that solder paste reaches its minimum soldering temperature for a sufficient period of time to achieve this. A rule of thumb is the minimum soldering temperature should be approximately 25ËšC above melting point, and the joints that determine the soldering temperature are the ones that reach it last. The result is the formation of good metallurgical bonding between components and boards. A good bonding is defined as a joint that wets the surfaces and forms a layer of intermetallic with some elements from the base materials and solder paste.¹

To ensure that assemblies have good solder joints, a critical control of the reflow profile is required. A reflow profile includes these elements (or parameters): preheat slope, soak, time above liquidus (TAL), maximum temperature, cooling gradient, exit temperature, and total heating time. Figure 1 shows a typical Pb-free ramp/soak/spike reflow profile. Analysis of the individual parts leads to understanding the process and explains how to produce zero-defect assemblies.


Preheat slope. Preheat slope is the change of temperature per unit time. Assemblies heat from room temperature to a preheated temperature. When the ramp rate is out of specification, it can cause component damage and solder paste-related defects, such as bridging brought on by poor slump characteristics. J-STD-020D specifies a maximum average ramp rate of 3ËšC/s.²

Soak time and temperature. This is also known as preheat. Its purposes are to ensure temperature equilibrium across the board, beginning with the activation of the flux, and the cleaning or removal of oxides from all surfaces. Depending on the complexity of the assemblies and heat transfer capabilities of the reflow oven, this step can be omitted, and one may still be able to achieve all the purposes explained above. This type of profile is known as a linear profile.

Ramp rate. Assemblies are heated from maximum preheat temperature to peak temperature. J-STD-020D defines a maximum average ramp rate of 3ËšC/s. It is very important the flux retains its activity to ensure complete collapse during reflow.

TAL. This is the time the product should be held at a temperature above the melting point of the solder alloy to ensure good solder joint formation. Enough time should be assigned for proper foot and hill fillet solder formation. Insufficient TAL results in poor wetting and joint collapse. Excessive TAL results in a thicker intermetallic layer. This thick layer is brittle and can cause defects during field operation.

Maximum temperature. This is also known as peak temperature. It is the maximum temperature allowable for the different components. This temperature is defined by component moisture sensitive level. Absorbed moisture can damage the component during reflow. Internal delamination, cracks, bond damage, lifting and even external cracking of the package (popcorning) can occur. J-STD-020D recommends a maximum component temperature based on the alloy used, package thickness and volume. For example, for Pb-free alloys, package thicknesses and volumes less than 2.5 mm and 35Ëš mm³, respectively, the maximum component temperature is 260ËšC, while for component thickness and volume of more than 1.6 mm and 2000 mm³, respectively, the maximum component temperature is 245ËšC².

Cooling gradient. The cooling rate (gradient) of a solder joint after reflow is important because it changes the microstructure of Pb-free solder joints. Cooling rates of approximately 3ËšC/s result in the formation of stronger joints and less intermetallic formation within the solder bulk.³ J-STD-020D specifies a maximum average cooling rate of 6ËšC/s.

Exit temperature. This is defined as the temperature of the assembly when it leaves the oven. This is important with respect to handling the assembly just after reflow.

Total heating time. This defines oven throughput, together with other parameters such as conveyor speed and the distance between boards. A proper heating time is linked to the solder paste chemistry. Solder paste specifications should be followed to ensure joints with good collapse and fewer or no voids.

To ensure all solder joints in the assembly are within specification, thermocouples and data loggers are used. A minimum of three thermocouples should be used and placed near the lowest and highest mass areas of the board. The proper type of thermocouples and attachment process for an assembly are specified in IPC-7530. The data logger and the board pass through the oven and measure temperatures the assembly is subjected to. In this way, the profile can be reviewed and modified to meet solder paste and component specifications. The precision of the profile is affected by the internal temperature of the data logger, and type, placement and attachment method of the thermocouples.¹

Good reflow ovens should have proper control and produce repeatable temperatures to ensure the profile is the same for all boards. To monitor machine repeatability, SPC software is used. This helps monitor several machine parameters, detect shifts in the process and recommend preventive maintenance. In this manner, the user can be sure all components and joints reach correct soldering temperatures.¹

References

1. IPC-7530, “Guidelines for Temperature Profiling for Mass Soldering Processes (Reflow and Wave),” May 2001.
2. J-STD-020D, “Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices,” July 2004.
3. U. Marquez and D. Barbini, “The Importance of Cooling Rate in Developing the Totally Controlled Reflow Process for Lead Free and Eutectic Tin Lead Processing,” IPC Apex, March-April 2003.

Ursula Marquez de Tino is a process and research engineer at Vitronics Soltec, based in the Unovis SMT Lab (vitronics-soltec.com); umarquez@vsww.com.