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www.us-tech.com
March, 2013
Considerations for Reduced Cost Micro Array Component Rework
By Ed Zamborsky, Eastern Territory Sales Manager, OK International Inc. M
icro ball grid array packages (µBGA) rapidly are becoming the package of choice when
designing new printed circuit board (PCB) assemblies. These packages generally offer high, first-pass process yields with significant real estate sav- ings over their predecessors. Process or design errors occasionally require the need for rework. Because rework requires component removal, special considerations must be taken into ac- count. These include excessive heat and improper removal as well as the fact that site-cleaning techniques have the potential to cause damage to com- ponent pads, adjacent components or the PCB itself. In short, proper rework requires sound practices, proper tools and quality training. The list below provides advan-
tages to using µBGA packages:
1. Higher I/O (input/output) Den - sity. As the drive to reduce board size continues, the need for greater I/O density intensifies. The fine-pitch µB- GA package offers several times the amount of I/Os per square inch of PCB space than the widely used BGA.
2. Mechanically Robust Design. The µBGA can be handled without concern for bending and misaligning the leads. This reduces component packaging costs and pre-assembly defect rates of components.
3. More Stringent Placement Tol - er ances. Standard BGAs typically
offer a much larger pitch between terminals, making them more forgiv- ing in terms of placement accuracy. The µBGA and land grid array (LGA) with their smaller ball size and lower pitch require more accurate place- ment. Typically, a machine with a placement accuracy of 1 mil is re- quired. Likewise, the use of lead-free solders presents additional chal- lenges, including decreased occur-
to PCB, the LGA’s center ground pad provides a direct path for heat to dis- sipate into the board. This reduces the thermal resistance of the inter- connections, thus minimizing the likelihood of damage to the chip and internal wire bonds.
5. Ease of Stencil Printing. The LGA makes the stencil printing process a requirement for rework since these components are supplied
X-ray inspection post reflow, which is time consuming and expensive. Sec- ond, the use of µBGAs involves a high- er level of complexity in terms of sig- nal trace routing on the PCB, which increases the cost of the board. Final- ly, despite the high first-pass yields of the µBGA and LGA, defective solder joints still occur for various reasons. As a result, the rework process be- comes more involved because solder joints are hidden and cannot be touched up. The component must be completely removed from the board. This requires rework equipment, offer- ing a higher level of process control.
Solder Paste or Flux Deposition With lead-free µBGAs, solder
Array site with component removed.
rences of self-centering during reflow and the inability to rely on the sur- face tension of the solder self-center- ing the device as in the past with standard, leaded BGAs.
C
4. Direct Thermal Path. In terms of heat dissipation from component
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without solder. An LGA stencil plate must be designed to meet the re- quirements of the pattern and be fit- ted to the rework machine. However, there are some disad-
vantages of the µBGA and LGA pack- ages. First, solder joint is validated by
balls melt at 217°C and typically con- tain adequate solder volume to create an acceptable joint. Current industry trends are to apply paste flux during the rework operation; this will reduce the need for custom solder paste plates and provide the necessary flux for proper attachment. This can be best accomplished with a flux dip plate that ensures a consistent application of paste flux to the bottom 30 percent of the solder ball. Conversely, LGAs require appli-
cation of the correct thickness of sol- der paste, since the components are supplied tinned without adequate solder for connection to the PCB. In rework, solder paste is applied with a single component stencil directly to Continued on page 32
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