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Non-contact Surface Profiling Improves Board Quality
December 31, 1969 |Estimated reading time: 1 minute
By Ciaran Murphy, Taylor Hobson Ltd.
PCBs are present in almost all products with built-in electronics. Fundamentally, a PCB consists of a network of copper tracks and planes on interconnecting layers. The tracks terminate at pads on the outer layers. When components are mounted onto the PCB, solder joints connect component pins to the pads, and the tracks connect the pins of different components. Correct operation of a PCB depends on the integrity of the tracks and the solder joints. Non-contact 3-D measurements carried out using a dedicated raster-scanning system can help assure this integrity.
The most common PCB fabrication method is to plate copper over a substrate and remove unwanted copper using a photoresist and chemical etching. This process leaves behind the copper tracks. The PCB is then drilled to assure that the requirement for interconnection between different layers has been met.
Measurement provides valuable feedback on pre-etching, post-etching, and drilling quality. For example, over-etching may be unacceptable on a controlled impedance board. Track-width measurements must be made to verify conformity to specifications.
PCB Assembly After PCB fabrication, components are mounted onto the board and fastened using solder joints. Typically, solder paste is deposited on the PCB pads using a screen printer. Components are then placed on their pads using automatic placement systems.
Surface mount devices (SMDs) can be placed on both sides of a board. Solder joint problems often are considered the most common source of end-of-line defects. If too little paste is deposited, the solder joint is weak, which can create an open circuit; too much paste can create a short circuit. Solder joint failure is likely to lead to product failure. Solder paste area, height, and volume are the best indicators of solder joint quality.
Following measurement on a dedicated raster-scanning system, these parameters can be determined using 3-D software. The 2-D profile of a solder paste deposit also can be used to determine if it is bad, good, or borderline. In a bad or borderline case, the profile helps identify problems on the screen printer (i.e. non-balanced squeegee, clogged stencil, lack of paste). The 3-D analysis software allows for the extraction of 2-D profiles from the original 3-D data set so additional parameters of height, spacing, and area may also be assessed. SMT
Ciaran Murphy, business development manager, Taylor Hobson Ltd., may be contacted at (44) 116 246 3167; CMurphy@taylor-hobson.com.