Illumination system (the projection lighting that lets a solder paste inspection camera measure deposit height and volume)

01

Laser line or fringe projection, and the shadow effect

Jean-Marc Peallat of Vi Technology describes laser triangulation SPI as a laser trace whose deviation gives height, combined with a 2D image. Its drawbacks are "mainly linked to the lack of resolution leading to a poor accuracy", and with one laser "the exact value of the volume cannot be calculated": the single angled beam and the camera leave a blind spot on the far side of each deposit, the "Shadow effect", where paste is not counted. "The error in the measurement value varies from shape to shape but can be up to 48% in the case of round apertures", and laser profiling is also affected by PCB colour and finish. [1]

Moiré topography instead projects modulated interference fringes and measures height and volume by moving the observation grating; Peallat lists its drawbacks as limited depth of field "(more or less compensated by Z movement)", sensitivity to noise and vibration in the grating process, and cycle time. [1] [2]

02

One projector or two, depth of field and warped boards

Peallat notes that "The MoirØ methodology is also subject to “shadow effect” if only one source of grating is used", and that "Often, most of the systems using MoirØ have 2 modes of operation": one source for speed at reduced accuracy, two sources for better accuracy at slower speed. Inspecting field of view by field of view with individual Z adjustment means "the references won’t be the same" across a BGA array, which can make height or volume readings differ. [1]

The scanning approach he describes, flying absolute height profilometry, scans the board with a top-down camera and "2 structured light sources (from each sides)", eliminating the shadow effect; with "a 6mm depth of field of view" it measures the board profile during the scan and compensates for warpage over the whole board, referencing height around each pad without Z adjustment. [1]

Give the smallest apertures and deposits, the board size, thickness and expected warpage, and the cycle time; ask whether the quoted accuracy is with one or two projectors.

03

Phase-shift measurement at known intervals

Chrys Shea, writing in Circuits Assembly, explains the stripes as a Moiré pattern and "the method of measuring them at known intervals" as phase shift interferometry: "By taking several measurements at known intervals, the height of the surfaces can be calculated mathematically." Koh Young's system divides each deposit "into tiny segments as small as 10 µm", measures their location and height, builds a 3D model and compares it with a theoretical one, with a gage R&R "of less than 10%" in the article. [3]

The article places the need where 0.5 mm area array devices arrived and inspection had to read 0.010-inch deposits accurately and fast enough for production. [3]

04

SPI illumination details

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Deposits: [smallest aperture; paste height range]
Board: [size; thickness; warpage]
Method: [laser line; Moiré; phase shift]
Projectors: [one; two; direction]
Depth of field: [mm]
Resolution: [XY µm; height]
Repeatability: [GR&R; conditions]
Cycle time: [s per board]

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