Tutorial #73
George Riley
April 2007

INTRODUCTION

Re-working a high-density board is in many ways far more challenging than populating the original board. Starting with a blank board allows an optimal sequence of component placements and reflows. Reaching into a densely-populated board to remove and replace a component without disturbing its neighbors is not like finding the proverbial needle in a haystack ― it is like removing and replacing that needle, without disturbing any of the hay.

Technology advances, such as smaller components, finer pitch, chip-scale packages, and stacked die, offer solutions to designers, manufacturers, and packaging houses – but create problems for rework. The fundamental challenge is shrinking geometry. Replacing an 0201 device on the order of a single grain of rice, re-balling with 100µm solder spheres, separating stacked die at the right place, all require exceptional control and placement accuracy.

Technology regressions, such as lead-free solder, further challenge rework processes. Since lead-free solders do not “self-align” components through surface tension, initial placement accuracy must be better controlled.

Unfortunately motion control and placement accuracy, difficult as they may be, are not the only challenges. Reworking generally requires precisely controlled, spatially limited heating and cooling in both the removal and replacement steps. It’s useless to carefully replace a component if you’ve just fried its four neighbors. Lead-free solder again exacerbates the problem. Because lead-free process temperatures of 220°C and higher are perilously close to the maximum temperature limit for many components, rework temperatures must be more closely controlled and repeatable.

Component replacement may also require precise paste dispensing, onto pads approaching 0.250mm diameter, without the bare-board advantage of direct stenciling onto the board pads.

CHALLENGING REWORK TASKS

Reworking 0201 Passives

Figure 1 shows the relative size of small passives, compared with a common pin. An 0201 device is too small for manual rework without machine assistance. Inaccurate positioning, faulty solder paste printing, or machine vibration and shock may cause rework failures such as rotated, tombstoned, billboard, broken or missing components.

Figure 1. 0402, 0201, and 01005 passives compared to common pin.

Proper production replacement requires thermode nozzles that conduct heat through the tip to pick up, relocate, or replace rotated and other positional errors. A vacuum nozzle removes tombstoned and billboard components, and can clean residual solder from the pads. Fresh solder paste must be dispensed onto pads as small as 0.250 mm. (0.010″) in diameter.

A single system that integrates all of these capabilities can sequentially remove, paste, and replace a component. Alternatively, each process step may be performed sequentially at multiple locations on the module before moving to the next process step.

Replacing Single Balls

Single-ball reballing replaces one or more defective solder spheres in an area-array component. As shown in Figure 2, optical inspection can identify defective balls: misplaced or deformed, wrong size, ball bridges, or blank positions. Melting the solder and removing it with a vacuum nozzle clears the site for a replacement sphere. The replacement sphere is dip-fluxed, aligned and reflowed. Gas flow is controlled so that neighboring balls are not affected. Small-diameter spheres (< 100µm) require special handling tools. At these dimensions, a high-resolution optical system and precise placement equipment are essential.

Figure 2. Ball array showing some common defects.Reworking Stacked Die

Reworking stacked flip chip may require separating die from one another, or separating the complete stack from the substrate. The gas flow rate, direction, and temperature must be controlled to separate the stack at the desired location.

Hot air or nitrogen is applied directly to the point being reworked, while observing the relative position of the tool and die stack through a high magnification, low incidence angle camera. Nozzle design and air channel location determine the separation point. Removing residual solder from a stacked package may allow the package to be reused.

ADVANCED REWORK EQUIPMENT REQUIREMENTS

High-Resolution Fixed Optics

Bump diameters below 100 µm require placement accuracies of less than 25 µm (1 mil). Optical magnification should exceed 200X. However, high magnification alone is inadequate. To correct linear and rotational errors, all rework systems use beam splitters to superimpose images of both the component and the substrate. The placement accuracy of moveable beam splitters is inherently limited by the mechanical motion of the splitter. Fixed optical systems eliminate motion errors, providing placement accuracy better than needed for SMT component rework.

Thermal Management

The simplest industry-standard thermal control monitors the air/gas mixture provided to the die/package by placing a thermocouple in the hot-gas stream. This signal varies the heater current in an attempt to match the desired profile. While this is adequate for ordinary profiles, it can be inadequate for the high temperatures of lead-free rework, and for the dimensional constraints typical of close-packed designs.

Better thermal control under these conditions introduces “cold” air or gas, typically nitrogen, into the hot-gas stream in a mixing chamber just before the gases reach the component. Mixing gases optimizes tracking of the pre-set temperature, and when combined with mass-flow-control of gas volumes, produces an effective heat delivery system.

Integrated Dispensing

Stenciling systems developed for large SMDs are inadequate for high density substrates and 0201 components. An auger pump with fine (Type VI) solder paste can dispense a 0.250-mm (0.010² ) diameter bump. However, the time required for serial bumping makes it impractical for reworking large array packages. The preferred solder paste application method is direct component printing in an integrated system. Figure 3 shows a properly printed small module.

Figure 3. Paste-printed small module.

THE FINETECH SOLUTION

Finetech, long a leader in high-accuracy flip chip die placement systems, has applied their patented technology to create a family of integrated, modular rework systems that meet all of the requirements described above for reballing and for high-precision rework.

The FINEPLACER® systems integrate all of the modules within an open architecture that minimizes temperature variations from machine to machine. The architecture combines the process steps, allowing a single platform to perform the complete rework cycle. Open architecture readily accommodates changes to meet new requirements.

Finetech’s patented fixed optical system supplies both the high magnification and excellent stability for precise placement, repeatable from device to device and from machine to machine.

Finetech’s mixed-gas thermal controller results in a highly responsive system ¾ reducing over and under-shoot to barely-perceptible levels. Heater lifetime is also increased and system-to-system reproducibility is greatly enhanced (±2ºC).

Finetech’s “Micro Rework” re-balling application package may take less than three minutes for a complete single ball repair, including solder removal, ball pick and place, fluxing, and soldering.

Figure 4. Direct application of solder paste.

Finetech’s “Direct Component Printing Module” precisely aligns component pads with the stencil. As shown in Figure 4, fresh solder paste is applied directly to the component. The component is then automatically transferred to the reflow arm and placed on the substrate. The “Direct Component Printing Module” can be added to present FINEPLACER® systems, providing a complete rework system to directly paste today’s finest components – and tomorrow’s.

FOR MORE INFORMATION

Finetech provides a family of high-accuracy rework and repair systems and bonding platforms.