5D solder paste inspection—merits beyond 3D technology
X
Figure 5. Dual multi-sampling laser technology. >150% Area/Volume ≈ 1
NG’s detected by both 3D and 5D
130% OK Volume = 80% 70% OK’s but with warnings 0% 70% 0% AREA Area spread (Max-Min)= 65% @ 80% Volume Area spread (Max-Min)= 28% @ 133% Volume
Figure 6. The relation between volume and area measurements is not linear, and defects can appear independently.
process). In reality both are rather uncom- mon. Laser triangulation is
amongst the
oldest technology to measure the height of solder paste deposits. Te laser projects a top-down laser
slit beam, and a camera is used to capture the reflected laser lines. Te laser head is moved in in defined pitches to cover the target area. Te triangulation method is used to calculate the height. To obtain a better repeatability,
two
lasers can be used in a 180˚ setup. Te height data from both measurements is averaged to reduce the effect of the deposit shadowing itself near steep edges. Moiré topography is a method of
tridimensional measurement by phase modulation. In the moiré method, lines are projected on an object as modulated
interference fringes, and by moving the observation grating that creates the lines, the height of the object can be measured. Te grated light is projected on a
target, and the resulting image is captured at known angles, using an area scan sensor and its height calculated with basic trian- gulation mathematics. Inspection of a large area is done by
merging a sequence of small area scans (the sensor’s field of view (FOV)), some- times with an optional Z-adjustment for focus to compensate for board warp. Te method becomes repeatable by incorporat- ing oversampling techniques. At the same time, variations due to color and reflection changes are reduced through the use of a white light source. However, the range of measurable height is narrow and particu- larly inconsistent at low heights (i.e. below
10 – Global SMT & Packaging – February 2011
Figure 7. True area, shape and volume measurement by combining 2D and 3D technology.
50 µm). Near the merge of 2 FOVs, cover- ing the same area (e.g. the print of a large BGA type), the focal references and opti- cal distortion can vary and result in differ- ences in height or volume measurements. Tis in turn can produce inaccurate indi- cations of solder paste quality, even when measurement results are very repeatable. Board warp further complicates the mea- surements and usually degrades accuracy and repeatability figures. Dual multisampling laser
systems
combine the advantages of moiré systems and laser. Dual multi-angle color lasers can accommodate different PCB colors and eliminate the shadow effects at the same time. Te multi-sampling technology of doing multiple measurements on each object detail boosts the resolution, accu- racy and the repeatability figures while maintaining the fast speed of a scanning
www.globalsmt.net 130% >150%
True Area and Shape
3D Blind Region
Volume = 133%
NG’s detected by 5D only
VOLUME
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