Reflow profiling of power PCBs: create a profile that takes thermal density variance into account.

Ed.: For the complete article please visit circuits assembly.com/cms/content/view/3348/

Developing a reflow profile for high-power PCBs requires the same procedure as for other boards. The challenge in creating a proper thermal profile for power boards is assessing the thermal density of boards and components and recognizing the thermal density variation on a given assembly. Several parameters must be taken into consideration.

To illustrate this point, compare the EMPF-009 board and the high-power ETO board (Figure 1, online). The largest component on the EMPF-009 board is a 169 I/O PBGA, which measures 23 x 23 mm and is 0.54 mm thick. The board also contains two 169 I/O PBGAs. On the ETO, one of the largest surface-mount components is a power MOSFET, measuring 6.73 x 6.22 mm and 2.38 mm thick. The ETO contains 68 power MOSFETs.

If the entire PBGA were made of plastic, with a thermal mass of 873 [e.sup.-6] J/[mm.sup.3] C (Table 1), 49.88 J is required to raise its temperature from 20[degrees]C to 220[degrees]C (T = 200[degrees]C). Conversely, if the ETO's power MOSFET is made of ceramic, 59.06 J is required to raise its temperature 200[degrees]C.

Both boards are made of FR-4. However, the ETO occupies more than 3.5 times the area of EMPF-009. The ETO is a six-layer board, versus two layers for EMPF-009. The ETO has two copper ground planes with 4 oz. copper while half the EMPF-009 board contains a single ground plane with 1 oz. copper. For the ETO to raise its temperature from 20[degrees]C to 220[degrees]C, it will require approx. 9,469.97 J. The EMPF-009 board will require 556.43 J. If the EMPF-009 board was the same size as the ETO, it would require 1,969.44 J, still significantly less than the ETO's heat requirements. This does not include the effect caused by the quantity and types of components on each respective board.

When comparing the soldering process, several differences in the equipment setup are apparent. Tables 2 and 3 (online) show the temperature settings for the oven's heating panels, the cooling zone setting, belt speed, convection rate and nitrogen use (using the same reflow oven for both). Both assemblies use nitrogen, but that is where the similarities end. In general, the ETO's panel zone temperatures were set higher, and the oven's convection rate was set at high versus medium for the EMPF-009. The belt speed for the ETO was 5"/min. slower than for the EMPF-009 board. All the factors had to be modified to take into account the difference in thermal mass between the two assemblies.

Of interest is the Zone 7 temperature setting. The ETO's temperature setting in Zone 7 is 270[degrees]C, 10[degrees]C less than the maximum panel temperature setting of the oven in question. If the ETO assembly were larger and more densely populated, this oven might not have the thermal capacity to solder the unit. Reducing belt speed may provide more thermal capacity, but it would reduce hardware output.

With the increase in temperature, and temperature exposure time due to the slower belt speed, component reliability may be compromised. Components with low thermal masses may reach temperatures for which they are not rated. Higher exposed temperatures can increase a component's moisture sensitivity, per JSTD-020A. Component delamination, popcorning and electrical failures can occur when components are exposed high temperatures during reflow. (Figure 2, online).

Since Pb-free solders require a reflow soldering peak temperature 20[degrees]C to 40[degrees]C higher than its their SnPb counterpart, this oven may not be able to support a Pb-free ETO production run.

Developing a thermal profile for power circuits has the same challenges as with circuit boards. Typically, a thermal profile is broken up into four components (Figure 3, online). These components are:

Initial (preheat) ramp. This zone starts the soldering process as the hardware's temperature increases from ambient. As the temperature increases, the solder paste loses its volatiles. Too fast a ramp rate will cause paste to splatter, resulting in solder balls. A prohibitively high ramp rate could also result in delamination or popcorning of the board and plastic components. An acceptable ramp rate is 2[degrees] to 4[degrees]C/sec., as specified on the manufacturer's data sheet.

Flux soak zone. During this period, the temperature gradient of components with different thermal masses is reduced. This permits the solder flux and activators within the solder paste to clean the surfaces to be soldered, promoting good solder wetting.

Reflow zone: This is also known as the "thermal spike," where the hardware's temperature is driven 20[degrees] to 40[degrees]C above the solder's melting point for 30 to 90 sec. By this time, all the solder joints are in liquid form and the solder joint forms. Depending on the component's weight, the solder joint's surface tension can snap the component back into proper alignment.

Cooldown ramp. This is where the temperature is lowered below the solder's melting point. As with the preheat ramp, an acceptable ramp rate is 2[degrees] to 4[degrees]C/sec.

Failure to create a proper thermal profile for either of these assemblies could have caused several problems. For instance, soldering may not occur when the temperature is too low, or inadequate preheat may lead to solder balls. Too high a profile could result in solder residues being baked onto the assembly, becoming difficult to remove.

With proper preparation and process development, one can achieve the same quality levels with a high-density assembly as they could with a less dense piece of hardware. High-power PCBs have a higher thermal mass than their digital circuit counterparts due to their heavier construction. A critical activity when developing the process is creating the thermal profile to support the higher density electrical hardware.

The caveat is, too much heat applied results in higher soldering temperatures that can cause hardware delamination and electrical failures. To prevent these types of failures, component and board preparation, such as baking the hardware prior to soldering to remove moisture, might be required.

Table 1. Thermal Masses of Common Materials

Material  Thermal Mass

FR-4       337.8 [e.sup.-6] J/[mm.sup.3] C
Plastic    873 [e.sup.-6] J/[mm.sup.3] C
Ceramic   2964 [e.sup.-6] J/[mm.sup.3] C
Copper    3428 [e.sup.-6] J/[mm.sup.3] C