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Solutions for Advanced Rework Challenges
December 31, 1969 |Estimated reading time: 9 minutes
The electronic world is getting smaller, more crowded and vertically integrated. Central to these challenges is the need to provide a rework and repair process. This article focuses on several complex rework processes in a lead-free environment.
By Chris Underhill
Disney was right; it’s a small word after all. Our “electronic” world is getting smaller, more crowded, vertically integrated and has a higher value per unit area than ever before. Central to these challenges is the need to provide a rework and repair process that will:
- Deliver heat only where it’s needed. It does no good to rework one component, only to disturb several others during the process.
- Demand high accuracy for rework systems - as area-array features fall below 4-mil, sub-10-µm placement accuracy will be commonplace.
- Provide reproducibility from system to system. Surface mount processes are being stretched to their limits; and errors will multiply. Rework volumes will increase, multiple units will be necessary - all running identical profiles and expecting identical results.
- Accomplish all of the above in a lead-free environment.
Small Passive Rework
An 0201 device is smaller than a fine soldering needle tip (Figure 1). As a result, the simplest form of manual rework (without machine assist) is not a realistic production process. To provide some idea of the size of an 0201 component, a single unit weighs less than 0.2 mg and is essentially the same weight as a small grain of sand. Failures that occur during automated small passive placement/reflow are generally due to inaccurate positioning, faulty solder paste printing and influences of vibration and shock. These result in common failures such as rotated, tombstoned, billboard, broken or missing components.
Figure 1. 0201 thermodic nozzle.
While the price of such passive devices is commensurate with their size, they are integrated into more expensive modules and circuits. Their replacement requires:
- Thermodic nozzles that use primarily heat conduction through the tip to pick up, reposition and replace rotated and other “non-flipped” errors.
- A tool (vacuum nozzle) to remove tombstoned and billboard faults, as well as residual solder from the pads.
- Integrated paste dispenser to deposit fresh solder paste on pads as small as 0.250 mm (0.010") in diameter.
By integrating these processes into one platform, the complete cycle can be performed on an array of components sequentially removing, pasting fresh solder paste and replacing the component, or carrying out the same step on each module before moving to the next process step.
Single-ball Reballing
Single-ball reballing is a technique used to replace a defective solder sphere in an area-array type component (BGA, CSP or flip chip), in which the sphere connection is defective and, if replaced, restores a scrap component to good health (Figure 2).
Figure 2. Solder ball on pick-up tool.
Due to the array characteristics of the devices, visual identification shows the location of the defective ball. The defect is removed by melting the solder and removing it using a vacuum nozzle. After fluxing a replacement sphere by dipping into a flux tray with the proper flux depth, the sphere is aligned to the solder pad and reflowed. It is key to control the gas flow to ensure that neighboring balls are not disturbed.
Things get tenser as the sphere diameter approaches 100 µm. Anti-static pick-up tools, together with a “puff” of air, are needed to release the ball from the tool tip. At such dimensions, both accuracy and optical resolution play a more important role, and material handling problems become just as important as rework and repair.
Because this is a serial process, it is unlikely to appeal to large area-array packages with hundreds of I/Os. However, as the ball count and diameter decrease, where replacement packages may not be available or prohibitively cost effective, the process offers a simple and immediate solution. Furthermore, this technique is an add-on to an otherwise established process, adding significant incremental value at small cost outlay.
Stacked Die Rework (Flip Chip)
Stacked die rework provides the ability to rework stacked packages that are being adopted in more applications than just the memory module arena. The process can separate die from one another, or the complete package from the substrate/interposer. The problem here is the ability to control the airflow rate, direction and temperature gradients to such an extent that the “stack” separates in the required location.
Figure 3. Stacked die rework nozzle.
The process requires special tooling to apply hot air or nitrogen directly to the bond requiring rework. Using a high-magnification camera at a low-incidence angle to observe the position of the tooling relative to the die stack becomes advantageous. The ability to remove the uppermost package or one from lower down depends on the nozzle design and, in particular, the location of air channels with respect to the solder interface between the die (Figure 3). Removing residual solder from a stacked package allows the package to be reused or studied for failure analysis identification. For some applications, vacuum and hot-gas flow are not sufficient to overcome the bond between devices. Under these circumstances, it may be necessary to use a “gripper” device built into the nozzle.
Integrated Dispense
The 0201 component size and higher-density substrates have driven the need for integrated paste-dispense capability. It is not possible to apply solder paste using a stenciling process aimed at larger SMDs. With typical pad sizes of 0.3 × 0.3 mm (0.012 × 0.012"), precise solder paste volume is needed. For 0402 rework, a Type-V solder paste is typically used, while Type VI is recommended for 0201 devices. Using an auger-style pump, it is possible to dispense a 0.250-mm (0.010") diameter bump with a Type-VI paste. Furthermore, integrating this capability into the rework system removes the need for additional expensive equipment, in a process (rework) still viewed as an “expense” rather than a “value” process.
Figure 4. Integrated dispenser.
It is fair to say that such a “serial” solder paste process in which one dot is applied at a time has its limitations. However, with automation applied to the process, array-style packages can be handled. Additionally, epoxy, ACP and underfill materials can be dispensed similarly, providing the tool with additional capabilities (Figures 4 and 5).
Figure 5. Solder paste dispense for 0603 IDC (side-by-side).
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Lead-free Rework
Lead-free is not an incremental issue - it is the standard protocol going forward. With the move to lead-free alloys, precise thermal management becomes even more critical. Process temperatures in excess of 220°C are reaching the maximum-allowed limit for most components and must be carefully regulated. The system must be able to manage this limit in a repeatable way.
Due to the “non-self-aligning” disadvantage of lead-free solders, initial alignment and subsequent placement of package to substrates becomes more demanding. No longer will the physics of surface tension of the melted solder bumps that cause the chip to align itself automatically to the substrate take care of “sloppy” alignment. Lastly, the quality of the final solder joint is enhanced in a reducing/inert environment. Nitrogen, introduced into the rework profile prior to reflow, enhances surface brightness, improves wetting capability and positively affects solder joint integrity.
Thermal Management
Various thermal management philosophies are used among rework system manufacturers. The simplest is a standard proportional control in which the air/gas provided to the die/package through the nozzle is monitored by a thermocouple placed in the hot-gas stream. This “proportional” signal is fed back to the heating element, reducing or increasing the current in an attempt to “track” the desired profile. This can adequately manage standard profile demands, but is hard-pressed to meet JEDEC profiles, especially at elevated temperatures.
An alternative concept uses “cold” air or gas, typically nitrogen, introduced into the hot-gas stream in a mixing chamber placed just before the air passes through the nozzle onto the component. This mixture results in a highly responsive system - reducing over and under-shoot to barely-perceptible levels. Tracking of the pre-set temperature is optimized and, when combined with mass-flow control of gas volumes, produces an effective heat-delivery system. Heater lifetime is also increased and system-to-system reproducibility is enhanced (±2ºC).
The Accuracy Issue
With conventional C4 collapse-and-align advantages gone, basic system accuracy is an issue. As bump diameters edge below 100 µm, accuracies better than 25 µm (1 mil) are required. To achieve this, optical magnification should exceed 200× - an 0201 package magnified to 12 × 6 cm and an 80-µm solder sphere to 1.6 cm when viewed on a monitor. However, possessing this magnification does not guarantee successful placement. For that, the system design is the determining factor, and a large part of the argument is the use of a moveable or fixed-beam splitter. Beam-splitter technology is common to all rework systems: images of both component and substrate are superimposed and adjustments made to accommodate linear and rotation errors. It is at this point that the two approaches differ. Moveable optics generally limit placement accuracy, whereas fixed optical systems preserve placements beyond those needed for SMT component rework.
Typical handheld/mobile devices contain active, passive and odd-form components, sometimes with RF-shield cans. Component proximity challenges the ability of the tool to remove and replace a component and do so without creating additional rework. In these situations, the hot air/gas must be delivered in a localized area, with minimal lateral-temperature dispersion. This can be achieved by controlling airflow rate, minimal nozzle thermal mass and nozzle design aimed at each component.
Conclusion
It is not difficult to predict the direction that rework system manufacturers must follow - track dimension size, device proximity and vertical integration within a system architecture that minimizes temperature variations machine-to-machine. Advantageously, it should possess an open architecture capable of integrating the various process steps to ensure that a single platform can perform the rework cycle.
As assembly processes leading to the need for rework are automated, certain segments of the rework market will also seek an automated solution. Following test and AOI, the rework/repair station will be required to accept databases, defining and detailing the failure location and device type. They will also be expected to handle product arrays of “like” faults. Removal, surface conditioning, paste/flux dispense and device replacement will be sequenced automatically.
This applies more to OEM situations whose defective product is identified and corrected prior to distribution. Such situations will result in the separation of the types of rework solutions - those aimed at board-by-board repair in which each repair differs from the job before, a new board or different component and those that address volume-production needs. Fulfilling both needs will be the domain of a few progressive manufacturers who recognize the need to develop processes in tandem with advances in packaging design and assembly integration. Satisfying only the former application will be the domain of many standard array-package-oriented systems where technology demands are more lenient on the rework system design.
As the 40th anniversary of Moore’s Law passes, designers must satisfy the ever-growing need for speed, bandwidth, functionally and whatever else is introduced into the marketplace. As a result, rework and repair is one technology with a predictably busy future. To err is human, even for machines.
Chris Underhill, general manager, Finetech, Inc., may be contacted at (480) 893-1630; e-mail: chris@finetechusa.com.