Soldering PCBs or Printed Circuit Boards in a reflow oven depends on factors like solder composition, flux type, component density, board speed, and temperature profile. A typical reflow oven has four zones—a preheat zone, a soak zone, a reflow zone, and a cooling zone. Setting a successful reflow profile involves adjusting the above factors to achieve proper and uniform soldering on all components across the board. In this article, we, at PCB Trace Technologies Inc., discuss the various ways to achieve this.
Reflow Profile for Leaded vs. Lead-Free Solder
As long as the electronic industry was using the tin/lead eutectic solder with a composition of 63% tin and 37% lead, soldering was rather a simple process. Eutectic solder has a melting point of 183 °C, and achieving good soldering was possible with a peak temperature of 220 °C. Therefore, by keeping the temperature across the entire board between 195 °C and 225 °C, a variation of 35 °C, it was possible to achieve excellent soldering results.
Things changed with the implementation of lead-free solder, which is available in different configurations. A typical SAC solder, consisting of tin/silver/copper alloy, has a melting point above 220 °C. However, some components, being temperature sensitive, can suffer damage beyond temperatures of 230 °C.
Therefore, to accommodate such components, it is necessary to maintain the peak temperature of lead-free assemblies within a range of 230-245 °C, giving a variation of only 15 °C, a 60% drop from assemblies using the tin/lead solder. With a mixture of large and small components on board, it is necessary to reduce the process window even further, as large components have a high thermal mass, while smaller components are often temperature sensitive.
Soldering large components with their high thermal mass requires exposing them to higher peak temperatures for a longer period. On the other hand, smaller components, most of them temperature-sensitive, require lower temperatures and a shorter duration. This reduction in the process window requires the process controls to be rather tight, while maintaining a narrow temperature bandwidth across the board.
Meeting such requirements is rather tough for assembly houses, and soldering complex boards requires them to spend time and effort developing their reflow solder profile.
Types of Solder Profiles for SMT Boards
A proper reflow profile is an important factor for achieving defect-free, high-quality solder joints during an SMT reflow process. Typically, two types of reflow profiles are popular with PCB manufacturers:
Standard Reflow Profile
This profile usually has a flat soaking zone, with a temperature spread of 150-160 °C. The soaking zone raises the temperature of the PCB and its components closer to the soldering temperature. The soaking zone follows the preheat zone, and it also has a fast ramp-up rate. The final ramp-up rate from soaking to the reflow zone is also high.
Optimized Reflow Profile
The optimized profile has a slower ramp-up rate both at the preheating zone and the reflow zone. Although this profile offers a more gradual temperature build-up on the board before soldering, it can expose the PCB and its components to a higher overall heat input.
Different reflow profiles can create different defects and issues. For instance, a high ramp-up speed at the preheating zone will lead to thermal agitation, causing the solder paste to slump. It can also induce defects like solder balling, bridging, solder beading, tombstoning, and micro-cracking on some components. These issues are more common in boards with high component density and with a mix of large and small components. Therefore, for boards with BGAs, it is advised to use an optimized reflow profile that has a slower preheat ramp-up rate.
Failure Analysis of BGAs
BGA packages are popular as they pack numerous I/O pins into a relatively small area. Therefore, any failure of a BGA device can be highly expensive. Some ways in which BGAs fail are as follows:
- BPS or Black Pad Syndrome
This failure mechanism is unique to ENIG or electroless-nickel immersion-gold finished PCBs. During the IG deposition, phosphorus concentrates at the EN/IG interface in the EN layer. The presence of phosphorus retards the normal wetting process of solder during the reflow process, creating a weak interface that can fail in the presence of thermal stress, shock loading, or mechanical bending.
BGAs tend to have a larger TCE or thermal coefficient of expansion mismatch with the PCB, as they have a larger die size compared to other plastic molded components. This leads to higher warpage stresses in the BGA solder joints.
- Brittle Fracture
Even if there is no BPS failure, the BGA substrate can fail due to brittle fracture. This is mainly caused by mechanical shock, such as during a drop.
- Die Cracking
BGAs can fail due to their dies cracking because of thermal expansion mismatch or mechanical bending.
- Warpage
Die cracks can result from a mismatch in TCE figures between the PCB and the BGA package. If the warpage is excessive during reflow, some balls of the BGA may not contact the solder paste, resulting in insufficient wetting and dry joint formation. The heat during reflow can cause either the BGA package or the printed board to warp. Even a small deformation can cause defects like solder shorts and open solder joints. A regular BGA is typically separated from the board by about 0.02”, and a lift of even 0.005” is enough to cause an open circuit. The larger the BGA chip, the more it will be prone to such problems. Even if there is no apparent defect, the joint can be constantly under strain as the hot board returns to its original shape, leading to long-term reliability problems.
- Solder Voiding
This is a frequently reported problem for BGAs. An inadequate reflow profile is typically responsible for void creation, which can entrap flux, resulting in separation of the ball from the BGA substrate pad. The large mass of the BGA can lead to reflow problems as it tends to lag behind other components on the assembly board in terms of peak temperature.
The lead-free solder paste’s chemistry uses flux that can outgas inside solder joints when the solder is in a molten state.
Extensive trial and error are necessary to achieve improvement in the soldering process. Optimizing the profile is paramount in achieving flawless reflow results. As a starting point, engineers refer to the suggested reflow profile from the solder paste manufacturer. However, these manufacturers supply only the basic time/temperature information. For an optimized reflow, engineers must take into account the board and component characteristics, thereby dictating the maximum temperature and a proper ramp rate.
- ∆T Across the Component
A large ∆T or difference of temperature across the BGA component can often be a cause of production defects. The temperature at the point of reflow determines how concurrent the reflow is, that is, how quickly the solder changes from its solid to the liquid state. For instance, for a solder joint turning from solid to liquid at its melting temperature of 217 °C, with a ∆T of 10 °C, and a ramp rate of 0.5 °C/Sec, the total time to achieve reflow is about 20 seconds. On the other hand, for the same ∆T of 10 °C, if the ramp rate is changed to 2.5 °C/Sec, the total time to achieve reflow will reduce to 4 seconds, producing a more simultaneous reflow.
Reflow Profile Settings for BGAs
Establishing a profile for a new board requires using thermocouples at multiple locations on the board and the component. For instance, thermocouples must be attached at the top, bottom, and corners of the component. It is also necessary to check the temperature at different locations on the board. This is to ensure the board reaches the minimum reflow temperature necessary to solder the large components, while at the same time, it does not exceed the threshold temperature that can damage smaller and more heat-sensitive components. For BGAs, the minimum reflow temperature allows the solder balls to melt and form solder joints. The right temperature profile focuses on the following areas:
- Soak Zone
The temperature in the soak zone must neither be a slow soak nor a fast soak. This will avoid oxidation due to too slow a soak or trapping the volatiles with a fast soak. A proper soak profile helps to equally distribute the temperatures across the PCB, while for specific solder pastes, it helps to vaporize volatile materials for fully cleaning and activating metallic surfaces. Long soak times and higher soak temperatures should be avoided as they can cause the paste to prematurely lose activity, resulting in solder balling and wetting. Reducing the soak time can decrease the potential of defects such as tombstoning.
- Ramp to Spike
This is the rate at which the temperature ramps up from the end of the soak zone to the peak or spike of the actual soldering point. Typically, two schools of thought are popular, one favoring a faster ramp to a low peak, and the other favoring of slower ramp to a high peak. More manufacturers use the high peak.
Many SMT processes use a linear profile as the starting point for ramping to the spike. This allows adequate time for activating the solder paste and cleaning the metallic surfaces, while reducing the risk of premature activity loss.
- Ramp Rate
This is another important factor, deciding the rate and the period during which activation happens, thereby releasing the volatiles.
- TAL or Time Above Liquidus
The time above the liquidus improves the wetting, reducing the chances of entrapment of flux.
- Peak Temperature
This is another area where a minor change can drastically affect the profile. For instance, a high peak temperature can damage LEDs and plastic parts.
For any thermal profile setting, the aim is to find a time/temperature setting that achieves reliable solder joints for the total assembly.
- Cooling
BGAs, with their large mass, may crack or warp due to the differential cooling effects after soldering. This mainly happens due to the difference in temperatures between the top surface and the bottom side of the BGA, and between the BGA and the PCB surface.
The cooling rate must be optimized with a minimal temperature difference between the top surface of the BGA and its solder joint area, which should ideally be less than 7 °C during the beginning of the cool-down phase after the soldering. In this critical region, the balls are not yet completely solidified to the board, and are typically in the range 180-160 °C. It is preferable to divide the cooling section into multiple zones, each with its different temperature, for efficient cooling.
Conclusion
According to PCB Trace Technologies Inc., the fastest and easiest way to optimize the thermal profile of a PCB with BGAs is with proper knowledge. The engineer must analyze the PCB layout to know the position of ground planes, locate the top and bottom components with different heat tolerances, and so on. They must also be aware of the oven’s limitations and capabilities.
