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Surface Mount Technology

icon4 Surface Mount Technology

Surface Mount PCBs

Modern electronic assemblies are now designed using surface mount components and technology. Successful electronic assemblers have had to optimise their methods to achieve an efficient and high quality SMT process.

Surface Mount Assembly Using Machinery

Use of leaded or through hole parts have become rarer, and are now discouraged by assemblers. It is far more efficient to machine assemble an electronic circuit if it is completely populated with surface mounted components. Leaded parts are laborious to fit. They need lead trimming and bending, hand placement, further trimming and hand soldering. SMD parts are simply machine picked and placed from a reel. Parts such as connectors which have not been available in SMD form can now be machine placed.

The latest model SMD placement machines are capable of placing a wide variety of part sizes from the smallest type 1005 SMD parts (dimension 1.0 mm x 0.05 mm) up to large SMD connectors (dimension 30 mm x 10 mm). There are now very few parts not available in SMD form and for which an SMD placement machine cannot handle the placement. Newbury Electronics Ltd. is equipped with such machines.

With careful use of parts, it is quite practical to design products which are completed in one pass down an automatic SMD assembly line without any extra hand assembly work other than to snap the circuit board into a moulded enclosure.

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icon5 SMT PCB Assembly

Surface Mount Soldering

A key process in surface mount assembly is the design of the solder paste stencil. It is paramount that the quantity of solder paste deposited on each pad is repeatable and precisely controlled. We consider this to be so important that we precision cut our stainless steel stencils by laser using our own in-house industry standard laser cutting machine. Modifications and improvements to production stencils can be made at low cost within hours as and when required, minimising delays and waiting time.

Surface Mount Soldering Process

The solder paste printing process has advanced considerably in the last few years. We use modern printing machines incorporating automatic load and unload to reduce handling. Automatic camera fiducial registration ensures perfect placement of solder paste every print. Automatic under stencil cleaning minimises cross contamination of surplus solder paste between prints, a major potential cause of faults. Automatic optical inspection (AOI) of the solder paste print checks the height and size of each pad printed.

Our inspection (AOI) and capability is geared towards surface mount technology. AOI machines are equipped with both vertical downward looking cameras and also four sideways viewing cameras pitched at 0, 90, 180 and 270 degrees to permit the identification of “lifted” legs on gull wing type components. This pernicious fault arises where a QFP leg is minutely bent upward so that solder joint is not quite made during the reflow process. The sideways looking AOI cameras can detect the slight gap under the leg in this situation. Another useful feature on the AOI is a laser measuring device which calculates the height of BGA packages at each of their four corners. Poor fusing of a BGA‘s solder balls will cause the BGA to “capsize” slightly. Comparison of the height measurements can detect this.

Our Takaya flying probe test machine is particularly capable with surface mount technology. The machine measures resistance, capacitance and inductance through four lead screw driven probes. We develop test programmes from the pick & place machine programmes which give the location of the parts, and the BOM which identifies the technical measurement that is applied to confirm that the value of the parts placed are correct. We also use Diode and transistor checks.

The electrical connection from a PCB pad through to the internal lead frame of a surface mount IC can be verified by injecting a low power RF signal into the PCB pad and detecting the signal through a machine driven moveable miniature aerial placed over the surface of the body of the IC.

Surface mount assembly presents challenges to manufacturers which must be met through astute selection of appropriate production equipment and rigorous implementation of process control.

For a PCB assembly quote, use our online quote form or contact us if you’d like to discuss your requirements.

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