5D solder paste inspection—merits beyond 3D technology
3D techniques Laser triangulation and phase shiſt moiré are the most commonly utilized techniques to measure solder paste height in 3D SPI, and both methods have strengths and weaknesses. Common for both methods is to mea-
X Scan X Figure 2. Basic principle of a laser triangulation for solder paste height measurement.
• Data with traceability to down- stream inspection and test
Modern manufacturing trends are increas- ingly abandoning repairs in favor of pre- vention through improved process control. Tis is particularly the case in automotive electronics. Te typical printing problems causing end-of-line defects can be detected by SPI and therefore offer significant opportunity for permanent yield improve- ment and reduced cost to manufacture.
2D techniques In the first automated solder paste inspec- tion applications,
conventional AOI
machines were used to verify correctness of the solder paste deposit aſter print. Specially developed algorithms were able to measure important 2D parameters such as coverage, offset, shape, smearing, slump- ing and bridging of the paste by using the stencil’s Gerber data as a programming input file. Defects from those parameters can all be detected by a 2D analysis in combination with tolerances set onto the
CCD Camera
Gerber reference. Later, the 2D SPI inspection evolved to
better algorithms and better capturing sys- tems. Te use of RGB lighting systems in combination with different lighting angles dramatically improved the detection capa- bilities by isolating the solder paste from any type of PCB background, such as PCB color, copper, gold, solder resist, silkscreen, tracks and vias. Aſter isolation of solder paste is successful, reliable 2D measure- ments can be obtained. 2D inspection methodologies are line scanning (line sensor) in greyscale or color or area scan- ning (field sensor) in greyscale or color. Using 2D technology, two parameters
cannot be obtained: height and volume of the solder paste deposit. 3D technology is needed to capture these parameters. Te additional benefit of 3D versus 2D
is oſten questioned, as 2D will capture most of the defects appearing in the printing process. On the other hand, what could be the added value to measure 3D parameters such as height and volume?
CCD Camera Ɵ1A ?
sure only the height of the deposit. By inte- gration of the height data of each XY pixel, the corresponding area and volume data is calculated. To obtain accurate height data of the
deposit, which is typically between 50 µm and 300 µm in a normal printing process, it is crucial to measure the height from the correct reference. Tis zero reference is typically the surface of the pad onto where the paste is printed. Because the solder paste is covering the pad aſter printing, the pad surface cannot be used as reference for the height measurement. To create a decent accuracy, usually the pad surrounding is used as zero reference, and this also com- pensates for inevitable board warp. If the board warp is significant and the pads are not horizontal, the zero reference can be obtained by applying a 3D reference plane that follows the board warp over the pad to be measured. By using the surrounding of the pad
for zero referencing, a correction has to be applied, because the surrounding of the pad has a height offset compared to the pad surface. To obtain this height offset, usu- ally a golden board is used to capture the offset data of each pad. Te use of a golden board in the programming stage to obtain height offsets, only gives accurate results if the production boards are identical to the golden board (same batch) and different surface layers onto the PCB are not varying in thickness (stable PCB manufacturing
Reference Plain Figure 3. Moiré principle for golden board scanning.
Reference Plain Figure 4. Moiré principle on a deposit.
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Global SMT & Packaging – February 2011 – 9
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