AN824
Vishay Siliconix
Document Number: 71990
06-Jan-03
www.vishay.com
3
Board pad design. The landing-pad size for MICRO FOOT
products is determined by the bump pitch as shown in Table 3.
The pad pattern is circular to ensure a symmetric,
barrel-shaped solder bump.
TABLE 3
Dimensions of Copper Pad and Solder Mask
Opening in PCB and Stencil Aperture
Pitch Copper Pad
Solder Mask
Opening
Stencil
Aperture
0.80 mm 0.30 " 0.01 mm 0.41 " 0.01 mm
0.33 " 0.01 mm
in ciircle aperture
0.50 mm 0.17 " 0.01 mm 0.27 " 0.01 mm
0.30 " 0.01 mm
in square aperture
ASSEMBLY PROCESS
MICRO FOOT products’ surface-mount-assembly operations
include solder paste printing, component placement, and
solder reflow as shown in the process flow chart (Figure 5).
FIGURE 5. SMT Assembly Process Flow
Stencil Design
IIncoming Tape and Reel Inspection
Solder Paste Printing
Chip Placement
Reflow
Solder Joint Inspection
Pack and Ship
Stencil design. Stencil design is the key to ensuring
maximum solder paste deposition without compromising the
assembly yield from solder joint defects (such as bridging and
extraneous solder spheres). The stencil aperture is dependent
on the copper pad size, the solder mask opening, and the
quantity of solder paste.
In MICRO FOOT products, the stencil is 0.125-
mm (5-mils)
thick. The recommended apertures are shown in Table 3 and
are fabricated by laser cut.
Solder-paste printing. The solder-paste printing process
involves transferring solder paste through pre-defined
apertures via application of pressure.
In MICRO FOOT products, the solder paste used is UP78
No-clean eutectic 63 Sn/37Pb type3 or finer solder paste.
Chip pick-and-placement. MICRO FOOT products can be
picked and placed with standard pick-and-place equipment.
The recommended pick-and-place force is 150 g. Though the
part will self-center during solder reflow, the maximum
placement offset is 0.02 mm.
Reflow Process. MICRO FOOT products can be assembled
using standard SMT reflow processes. Similar to any other
package, the thermal profile at specific board locations must
be determined. Nitrogen purge is recommended during reflow
operation. Figure 6 shows a typical reflow profile.
0
50
100
150
200
250
0 100 200 300 400
Thermal Profile
Time (Seconds
FIGURE 6. Reflow Profile
Temperature (_C)
PCB REWORK
To replace MICRO FOOT products on PCB, the rework
procedure is much like the rework process for a standard BGA
or CSP, as long as the rework process duplicates the original
reflow profile. The key steps are as follows:
1. Remove the MICRO FOOT device using a convection
nozzle to create localized heating similar to the original
reflow profile. Preheat from the bottom.
2. Once the nozzle temperature is +190_C, use tweezers to
remove the part to be replaced.
3. Resurface the pads using a temperature-controlled
soldering iron.
4. Apply gel flux to the pad.
5. Use a vacuum needle pick-up tip to pick up the
replacement part, and use a placement jig to placed it
accurately.
6. Reflow the part using the same convection nozzle, and
preheat from the bottom, matching the original reflow
profile.
