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Solder paste inspection (SPI) traditionally uses resist height or a pseudo-pad height to calculate solder volume on a pad. However, as features are increasingly miniaturized and compressed on the PCB, resist thickness differences are making SPI measurements inaccurate. Toshiyuki Matsuoka, Anritsu Precision Co. Ltd. and Tim Cappoen, Seika Machinery Inc., discuss a new sensor technology that combines two optical systems to measure solder paste deposits from the true pad.
In recent years, 3D SPI after solder paste printing has been more widely adopted into inspection regimes for electronics assembly. In general, 3D solder paste inspection machines measure the solder volume, area, and height based on the resist reference or use the pseudo pad reference method. Trends like higher component placement density and smaller components have led to much thinner stencils at solder paste print. Resist thickness differences of PCBs now affect the process significantly. Therefore, solder volume measurement based on pad reference (and not resist reference), is becoming a requirement. Newly developed sensors for paste inspection systems* can reference from the pad or copper layer.
Solder Volume Measurement
In the case of resist reference, the solder that is lower in height compared to the resist surface will not be calculated into the volume measurement. Since the resist thickness will vary some from PCB to PCB, this method cannot measure true solder volume. The height of the pad surface within the PCB will be even, regardless of the existence or nonexistence of the resist layer. Therefore, it is possible to measure true solder volume using the pad surface as the reference plane.
These examples show how solder deposits are measured using resist reference vs. pad reference.
Solder area can be measured as the solder cross section offset in the height direction. Solder volume is the sum of the upper portion of the cross section face and the cross section area multiplied by the offset. When using the resist as reference, the area equivalent to the resist thickness is not recognized as solder. When using the pad as the reference, the area that is not recognized as solder can be minimized.
Figure 1A. An example of SPI using the resist reference method.Figure 1B. The same inspection area, measured with the pad reference method.
In the example shown in Figure 1, the solder is 100-µm high and has a 250-µm bottom face diameter. This is considered at 100% coverage. Using the resist as reference (Figure 1A), 30% will not be recognized as solder if the resist thickness is 20 µm, and the cross section face is set at a designated value. When using the pad as reference, the unrecognizable volume can be suppressed to 0.2% (Figure 1B). Therefore, using pad surface reference, we can measure the real volume (absolute value) of the printed solder.
Accurately Measuring Solder with 3D SPI
Laser measurement methodology. Triangulation displacement uses a laser beam to measure solder volume without contact. With the triangulation method, each pixel can obtain distance images consisting of height data. This distance image, which calculates the solder volume, is a 3D profile of the printed solder. Since the laser method precisely focuses and emits a laser beam for measuring, it offers very high resolution. Additionally, each sensor is calibrated in accordance with a traceability system, enabling accurate, absolute solder volume measurement. Two types of optical systems are built into multi-angle (MA) sensors, which function in the following ways.
The specular reflection sensing method has the system receive light by diagonally focusing the measurement beam. Since a position sensitive detector (PSD) is used for the photo diode, ultra-high-speed data sampling is possible, minimizing tact times. Displacement data and light volume data can be obtained, enabling a judging function based on scattering solder light volume data from the shiny resist surface and scattering light. In addition, displacement data of the resist surface is obtained when adopting resist reference measurement.
The scattering sensing method is a method to acquire scattering light by focusing a vertical laser beam on the PCB. The scattering sensing method is suitable for measuring objects that scatter light like solder. Also, by setting sensing systems on both sides of the emitted laser beam, shadowing is prevented, enabling a true view of the solder profile.
Figure 2. Hybrid optical system of MA sensor.
Multi-angle (MA) sensors. The newly developed MA sensor is a hybrid triangulation method (specular reflection sensing method + scattering sensing method) built into one sensor (Figure 2). Since the specular-reflection and scattering light received, respectively, are measured by exclusive optical systems, resist surface, pad surface, silk surface, solder, exposed base material, etc. are accurately and automatically judged in accordance with each base material. This allows the SPI system to automatically recognize the reference plane. These sensors make preparations such as reference plane definition or resist thickness measurement unnecessary. Additionally, measurement of absolute value, repeatability and directivity is possible, bringing repeatability variation below 1%.
Figure 3. Pad surface reference measurement is unaffected by the resist thickness on the PCB under inspection.Conclusion
With the hybrid optical system of the MA sensor, it has become possible to measure solder using pad reference. When resist exists on the pad surface, the vertically emitted beam passes through the resist surface and measures the pad surface reference under the resist. With a pad surface reference measurement, SPI is unaffected by the resist thickness.
* The sensors were developed for the SOLLEAD-MA (Multi Angle) SPI machine from Anritsu Precision and are patent-pending.
January 2010, SMT