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Tips for automated PCB assembly

From:    Author:JAGUAR Automation Equipment Co.,Ltd.    Publish time:2010-04-30 15:40    Clicks:1487

 A recent trend in the manufacture of printed circuit boards (PCBs) has been the increasing use of surface mount technology (SMT).
Many products have been redesigned or modified specifically for SMT use, and amongst the design criteria employed, applications using pick and place, high temperature resistance from reflow ovens and specific packaging for automated handling and size are key attributes.
Connector products provide special challenges to SMT due to the demands of their functionality (e.g. the mechanical forces generated in tightening screws and inserting and removal of plugs), and size and packaging. Until now connectors have been mounted in a secondary operation usually using the older wave soldering process, involving PCB manufacturers in additional costs, which we would all like to avoid.
As a manufacturer of both connection products and PCB’s Phoenix Contact has made considerable progress in developing solutions to the challenges of SMT.
A solution originally developed by the company is through hole reflow (THR) or “pin-in-paste” for high temperature applications.
Pin-in-paste
The process supplements SMT and reflow soldering, thereby eliminating the previously necessary wave soldering stage. For THR components to be used in the reflow process they have to be specially modified and some specific rules observed in the PCB layout.
Requirements
PCB mounting components that can withstand high temperatures are ideal, suitably packaged for the application of pick and place machines they can be directly inserted onto the PCB and finally soldered in the oven.
At the same time the components are required to meet the normal requirements of mechanical stability, electrical performance, colour etc. which are expected from conventional connectors.
Reflow soldering
To create the temperature profile required by reflow soldering many types of forced convection and vapour phase ovens are used. The use of infrared ovens is not recommended as they tend to raise the temperature of components too quickly, causing damage to or even destroying the components. Equally, unnecessarily long exposures – cycle times of more than five minutes, or exceedingly high momentary temperatures over 230?C should be avoided.
High temperature resilient plastics
The material must be able to withstand the temperatures associated with the reflow process, typically 185?C for a few minutes with a short exposure of a few seconds at 230 to 260 ℃.
It is worth noting that EU directives concerning the use of lead free solder will impact on the plastics used in connection products since most lead free solder pastes are expected to have liquid temperatures some 30?C higher than current lead based solders. It is very likely that connection products developed for SMT will be appropriate for lead free solders.
Component geometry
In addition to the normal challenges of connector design there are additional aspects of component geometry to be considered in THR designs. Firstly minimising the amount of plastic material coming into contact with the solder paste can be achieved by using an integral spacer. Our experience is that a minimum spacing of 0,3 mm is required. Secondly the design must not prevent heat reaching the solder points, as this will lengthen the process time exposing the components to risk of degradation.
Component volume also has to be taken into account since the larger the connector is the more heat it will absorb also the solder points may also be shadowed by the connector itself leading to incorrect soldering. These considerations together with packaging place limitations on the size of connectors that can be adapted to THR usage.
SMT is suitable for high volume manufacture and aims to eliminate costly manual placement of components by using pick and place machines, vacuum pipette handling normally being preferred to mechanical systems. The design of the connector needs to provide suitable flat suction surfaces. These surfaces have to be large enough to generate sufficient force to retain the component and to allow the high speeds used in automatic assembly. A further restraint on component design is the operational height of the assembly machine.
Solder pins
The solder pins differ from conventional pins. If they are too long they will take up to much of the solder paste which then does not flow back to the solder point, leading to poor mechanical and electrical performance. The minimum pin length is the thickness of the PCB giving the added advantage of allowing the possibility of placing components on both sides of the PCB. Square solder pins are ideal for SMT applications due to their excellent soldering properties.
PCB layout
Through plated holes are recommended and give reliable results, to allow for a pick and place machines accuracy; the holes should be 0,1 to 0,25 mm larger than the diagonal dimension of the pin. The nominal voltage of the components sets the maximum diameter of the external solder ring and it should not be less than 0,3 mm.
Solder paste application
Independent of the process used, the PCB holes should be completely filled with solder paste ideally with a hemispherical hump on the underside of the board and to provide as much solder paste as practically available. The amount of paste applied to the solder ring varies with the method used. The screen printing process which is the current norm together with standard stencils provides a 150 µm thickness of paste for all components, this may be insufficient for all SMT devices. Possible remedies include multi-level stencils however it is possible to apply the correct amounts of solder by varying the printing machine’s parameters. Where the requirements vary considerably a dispenser can be used that allows different amounts of solder paste to be applied.
Packaging
The choice of packaging medium is directly governed by the assembly method adopted for small quantities and mounting by hand conventional bulk packaging is sufficient. For large volume production the use of tube, tape or tray feeders can be employed, the packaging being tailored to the application.A recent trend in the manufacture of printed circuit boards (PCBs) has been the increasing use of surface mount technology (SMT).
Many products have been redesigned or modified specifically for SMT use, and amongst the design criteria employed, applications using pick and place, high temperature resistance from reflow ovens and specific packaging for automated handling and size are key attributes.
Connector products provide special challenges to SMT due to the demands of their functionality (e.g. the mechanical forces generated in tightening screws and inserting and removal of plugs), and size and packaging. Until now connectors have been mounted in a secondary operation usually using the older wave soldering process, involving PCB manufacturers in additional costs, which we would all like to avoid.
As a manufacturer of both connection products and PCB’s Phoenix Contact has made considerable progress in developing solutions to the challenges of SMT.
A solution originally developed by the company is through hole reflow (THR) or “pin-in-paste” for high temperature applications.
Pin-in-paste
The process supplements SMT and reflow soldering, thereby eliminating the previously necessary wave soldering stage. For THR components to be used in the reflow process they have to be specially modified and some specific rules observed in the PCB layout.
Requirements
PCB mounting components that can withstand high temperatures are ideal, suitably packaged for the application of pick and place machines they can be directly inserted onto the PCB and finally soldered in the oven.
At the same time the components are required to meet the normal requirements of mechanical stability, electrical performance, colour etc. which are expected from conventional connectors.
Reflow soldering
To create the temperature profile required by reflow soldering many types of forced convection and vapour phase ovens are used. The use of infrared ovens is not recommended as they tend to raise the temperature of components too quickly, causing damage to or even destroying the components. Equally, unnecessarily long exposures – cycle times of more than five minutes, or exceedingly high momentary temperatures over 230℃ should be avoided.
High temperature resilient plastics
The material must be able to withstand the temperatures associated with the reflow process, typically 185?C for a few minutes with a short exposure of a few seconds at 230 to 260℃.
It is worth noting that EU directives concerning the use of lead free solder will impact on the plastics used in connection products since most lead free solder pastes are expected to have liquid temperatures some 30?C higher than current lead based solders. It is very likely that connection products developed for SMT will be appropriate for lead free solders.
Component geometry
In addition to the normal challenges of connector design there are additional aspects of component geometry to be considered in THR designs. Firstly minimising the amount of plastic material coming into contact with the solder paste can be achieved by using an integral spacer. Our experience is that a minimum spacing of 0,3 mm is required. Secondly the design must not prevent heat reaching the solder points, as this will lengthen the process time exposing the components to risk of degradation.
Component volume also has to be taken into account since the larger the connector is the more heat it will absorb also the solder points may also be shadowed by the connector itself leading to incorrect soldering. These considerations together with packaging place limitations on the size of connectors that can be adapted to THR usage.
SMT is suitable for high volume manufacture and aims to eliminate costly manual placement of components by using pick and place machines, vacuum pipette handling normally being preferred to mechanical systems. The design of the connector needs to provide suitable flat suction surfaces. These surfaces have to be large enough to generate sufficient force to retain the component and to allow the high speeds used in automatic assembly. A further restraint on component design is the operational height of the assembly machine.
Solder pins
The solder pins differ from conventional pins. If they are too long they will take up to much of the solder paste which then does not flow back to the solder point, leading to poor mechanical and electrical performance. The minimum pin length is the thickness of the PCB giving the added advantage of allowing the possibility of placing components on both sides of the PCB. Square solder pins are ideal for SMT applications due to their excellent soldering properties.
PCB layout
Through plated holes are recommended and give reliable results, to allow for a pick and place machines accuracy; the holes should be 0,1 to 0,25 mm larger than the diagonal dimension of the pin. The nominal voltage of the components sets the maximum diameter of the external solder ring and it should not be less than 0,3 mm.
Solder paste application
Independent of the process used, the PCB holes should be completely filled with solder paste ideally with a hemispherical hump on the underside of the board and to provide as much solder paste as practically available. The amount of paste applied to the solder ring varies with the method used. The screen printing process which is the current norm together with standard stencils provides a 150 µm thickness of paste for all components, this may be insufficient for all SMT devices. Possible remedies include multi-level stencils however it is possible to apply the correct amounts of solder by varying the printing machine’s parameters. Where the requirements vary considerably a dispenser can be used that allows different amounts of solder paste to be applied.
Packaging
The choice of packaging medium is directly governed by the assembly method adopted for small quantities and mounting by hand conventional bulk packaging is sufficient. For large volume production the use of tube, tape or tray feeders can be employed, the packaging being tailored to the application.

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