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July, 2015


The Fundamentals of Closed-Loop Solder Paste Inspection


By Ondrej Simecek, Marketing Dept., Test Research, Inc., Taipei City, Taiwan


ity. It is one of the first automated inspection systems employed in any automated printed-circuit-board (PCB) production line, and can offer details about the integrity of soldered connec- tions. A SPI system receives PCBs from a screen printer and forwards inspected boards to an automated pick-and-place machine for placement of components upon solder deposits. Further along in electronic production, the entire PCB assembly is heated in a reflow oven until the solder paste melts and forms solder joints between solder pads and those placed compo- nents. The SPI system is a first step in ensuring the ultimate quality of a high-volume manufactured electronic product. SPI equipment can provide infor-


S


mation to help monitor and improve production quality. In a typical pro- duction environment, SPI collects


An interface for an SPC


software tool is integrated into the system’s main


inspection results window. Operators can use this SPC tool to collect and


analyze SPI results in any desired time frame.


information that can be used for both immediate adjustment and long-term statistical evaluation of production yields. It can help maintain high levels of statistical process control in any electronic manufacturing facility. SPC is a method of quality con-


trol (QC) that employs statistical data, approximations, and charts to analyze and improve a process, typi- cally a production process. In a pro- duction process, several key factors control and affect the final product quality level, including the choice of raw materials, process variations, operational procedures, and machine parameters. SPC is applied in the pro- duction process to collect data, meas- ure differences and variations, and to monitor and control those process variations to ensure a process pro- duces as many products as possible at the output that are within acceptable


Component placers may optimize placement of chips onto printed solder paste to use the auto-corrective effect of solder reflow.


By comparing historical inspec-


tion results in different categories, quality engineers can discover pre- liminary causes of particular defect symptoms. With aids from SPC and other QC tools, quality engineers can confirm the root causes leading to a particular defect symptom. By repeating SPC problem-solving tech- niques, an overall process improve- ment plan can be outlined and then


functions help quality engineers identify the root causes of solder- related problems, implement correc- tive actions and preventive solutions, and monitor and control the improv- ing process to manufacture better- quality products. Specific parameters detected by


the SPI system can be used as guid- ance inputs by manufacturing equip- ment to automatically adjust produc-


of placed components and actual printed solder paste.


Solder Paste Realigning Itself Due to the physical characteris-


tics of the reflow process, heated sol- der paste melts and forms a joint between a component and a solder pad underneath. While melting, sol- der paste will adhere and realign


Continued on page 55


Solder paste printer uses SPI offset information to gradually realign the stencil with the soldering pads and stabilize production quality.


window. Operators can use this SPC tool to collect and analyze SPI results in any desired time frame, i.e., discrete and continuous data by hour, day, shift, working order, batch number, machine number, and by other collec- tion parameters.


control limits (LCLs) in a given time- frame.


Fast Inspection Results SPI systems from TRI not only


provide accurate and fast solder paste inspection results, but the SPC


older paste inspection (SPI) can provide invaluable information about electronic production qual-


tolerance limits. SPC is a common tool used in the electronics manufacturing industry to monitor and control process variations and to improve final product quality in a continuous development loop.


Integrated Interface In an SPI system from Test


Research, Inc. (TRI), an interface for an SPC software tool is integrated into the system’s main inspection results


short-term, long-term, and preven- tive actions can be implemented as part of improving overall electronic manufacturing production quality. Other SPC functions such as


real-time monitoring charts (C- charts and P-charts) augment the SPC software package, and they are used as early-warning functions when sequential inspection results begin rising versus upper control limits (UCLs) or falling versus lower


tion parameters to maintain consis- tent high quality, cut production costs, and reduce operator workload. The closed-loop SPI systems


developed by TRI offer information feeds in two directions: a feedback loop to the solder paste printer and a feed-forward loop which sends in - spec tion results and skip-board infor- mation ahead through the produc- tion line. A typical example of infor- mation feedback used by the stencil printer is a detected printing offset, solder paste volume, and stencil cleaning warning, which triggers an automated cleaning process. Most solder paste printers are configured to periodically clean the stencil to maintain stable printing quality. However, a rigid cleaning schedule may result in excessive consumption of cleaning agents. By incorporating SPI volume data, the printer can dynamically extend the cleaning schedule to minimize cleaning costs and optimize the printing parame- ters while maintaining consistent quality. Production and inspection equip -


ment further along the production line may accept inspection results and skip-board data from the SPI system to adjust operation or skip defective boards from further processing. An automated process control (APC) link with a component placer may employ SPI offset data to match the position


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