Dual Preheater Rework Prevents Damage to Dense Lead-free Assemblies


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Advances in PCB and component technology have created higher demands on the equipment used to repair and rework array components. It is no longer possible for the operator to simply increase the reflow temperature and expect safe and controlled component removal. These uncontrolled operators can damage the latest lead-free assemblies by utilizing older and less-capable rework systems. Ed Zamborsky and Paul Wood, OK International, explain the new machine technologies for safe BGA rework.PCB Technology

Latest-generation PCBs are increasingly more complex as customer demand drives them to smaller and more compact footprints. One of the most common devices, the smartphone/PDA, has increased in functionality while becoming more user friendly and much smaller. To design these devices, the electronics industry has increased PCB thickness to accommodate the increased I/O density of BGA and stacked BGA devices. To control the thermal energy produced, PCB designers have been forced to increase the number and weight of the embedded ground planes. This results in high cost and high-density boards with very demanding rework applications. As an example, current 3G and 4G phones are being produced with up to 12 copper layers, while larger server boards can have up to 24 layers of copper.

This means that more heat is needed to reflow and rework these PCBs, while the temperature maximums for the component lids remain the same (about 260°C), which in turn come very close to the melting temperature of the internal materials.

Component Technology

The BGA solder ball pitch is constantly evolving downward. For example, a typical 35-mm2 BGA with 1.27-mm pitch has a ball count of 680. Today, that same package size with a pitch of 1.00-mm now has a ball count of 1156. This increase of package density requires 40-50% more heat capacity to safely rework the part. In the era of leaded solder, an operator would typically increase the top air temperature in an attempt to reflow the part. Today, with lead-free solder alloys, the reflow temperature is very close to the maximum component lid temperature. The simple operator solution of temperature increase is disastrous for the component under reflow. These high top-side temperatures can result in die interconnection damage, component delamination, and dog-ear warping of package corners (a cause of catastrophic bridging).

Package on package (PoP) components pack even higher densities into the component footprint. A simple 15-mm2 BGA usually has 196 balls. The PoP version, which has a smaller pitch and 3 layers, can have up to 1300 or more solder connections. These packages are very sensitive to top-side nozzle temperatures, so the only practical solution is to provide additional heating from the board underside.

New Packages, Old Solutions

With existing models of single preheater rework machines, the operator is forced to rework these difficult applications by increasing the bottom heaters to very high and sometimes maximum heater output temperatures. This high output usually causes the adjacent underside components to be very close to solder melt temperatures. To prevent the bottom-side components from falling off, the operator must raise the nozzle temperature even higher to achieve component solder ball melt, heating it to unsafe levels. This temperature maxing typically results in component rework processes that are not repeatable, generating very poor rework yields. In addition, this technique frequently overheats the top of the component with temperatures in excess of 260°C.

New Packages, New Solutions

The true solution for today’s more complicated lead-free components is a rework system with the versatility to allow different heating areas to be selected for the bottom of the PCB assembly. This dual-heater design enables an operator to heat the entire PCB during the preheating zones of the process and then automatically switch the heating area to concentrate exactly where it is needed to rework the selected component. This redirection of energy allows the part to be reflowed at lower and safer nozzle air temperatures. The component and PCB are therefore protected from excessive soldering temperatures, staying below the maximum component lid temperature.

Figure 1.

As seen in Figure 1, we want to achieve reflow of the solder balls within the range of 235–245°C, keeping the lid temperature below 260°C and the adjacent component solder joints well below the solder melt temperature of 217°C. Imagine a large, high-capacity heater positioned to heat the bottom of your PCB. Because of its size, you have no choice but to heat the entire board. However, in the center of this large heater resides a separate, smaller heater that can be activated to heat only a select portion of the board. This smaller heater is precisely positioned under the reflow nozzle, creating a virtual vertical reflow oven.

Key benefits of using dual-zone preheat include increased productivity and rework yields; a reduction of rework cycle times, which protects the component under rework from thermal damage; and better management of the narrow lead-free process window without reaching excessive peak temperatures that damage components, connectors, adjacent solder joints and the PCB substrate.

Figure 2.

As shown in Figure 2, during the reflow process, the large bottom heater (subzone) is utilized in preheat 1 (aqua) and preheat 2 (yellow) zones of the profile, while at the same time the top-side nozzle temperature is set well below the subzone temperature. This allows for controlled warming of the PCB without worries that the top-side nozzle temperature is damaging the component with excessive temperatures.

During the soak zone (gold) and peak zone (red), the system is programmed to switch the large preheater off and the smaller preheater on. This allows the overall board temperature to stabilize and the target area to safely reach reflow temperature. It can also be observed that the top-side nozzle temperature remains set below the bottom subzone temperature; this means that a greater portion of the reflow energy is provided by the bottom heaters. This energy shift is important for component safety, but it also allows the operator to achieve repeatable reflow of the most demanding lead-free assemblies.

Finally, in the cooling zone (blue) both bottom-side blowers are utilized while the top-side heater is bypassed, introducing room-temperature air through the reflow nozzle. This cooling zone creates a controlled cooling rate of approximately 4–6°C per second to achieve the best possible environment for solder joint formation.

Conclusion

Component packaging and solder technologies have evolved, but the BGA rework system has seen only minor improvements. The reliance on older and possibly outdated rework equipment has finally reached the point where that legacy machine is actually costing more in reduced throughput and damaged components. With dual-zone bottom heating technology on new-generation rework machines, we can meet the thermal demands of today’s complicated and high-value assemblies, while reducing the risks associated with rework.

Ed Zamborsky is the Americas product manager for OK International, and can be contacted at ezamborsky@okinternational.com. Paul Wood is the advanced product applications manager for OK International and can be contacted at pwood@okinternational.com.

SMT, February 2010

Read More Articles on BGA Rework:Rework and Repair: The Complete BGA Rework Process A Guide to Infrared (IR) Rework on BGAsReballing Rework: The Bright New Future

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