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Precise manufacturing process is saving valuable PCB real estate

Automated dispensing enables efficient RF shield edge interconnect (SEI)
Precise manufacturing process is saving valuable PCB real estate

Radio frequency (RF) shielding is a fact of life for compact electronic communications devices to protect sensitive circuits. The process of attachment of these shields to the PCB assembly has varied with time, and manufacturing engineers have always sought a better, more cost-efficient and easier method of accomplishing that addition. Complicating matters has been the ongoing need toward increasing miniaturization of the internal PCB, while at the same time responding to the consumer-driven demand for ever increased functionality. And as process engineers do not want the component solder joints to see liquidus temperatures a second time, the subsequent reflow for the metal sheet attach is performed using low-temperature melting alloys.

Michael L. Martel and Rajiv L. Iyer, Speedline Technologies, Inc.

A new process has been developed for RF shielding on using automated solder paste dispensing. The process is known as shield edge interconnect (SEI). This process, configured by Speedline Dispensing Technology, relies on extremely accurate dispensing of solder paste on copper traces located along the outer edge of the PCB. The result is a robust process solution for the SEI method in which proprietary closed loop dispenser, pump, vision and software technologies enable a high-volume manufacturing process for huge throughput. Additionally, SEI designs enable parts to be processed through underfill before placing of the RF shield. The SEI method allows more complete use of valuable PCB real estate and provides that saved space to be utilized for added components on mobile consumer products such as smart phones and tablets.
The drive to miniaturization and high density packaging in PCB assembly has driven complex designs with reduced PCB real estate and footprints with variable SMT pad dimensions <500μm. This has driven OEMs to look to PCB edges as a potential area for shield placement, by routing copper traces along the outer edge of the PCB. With SEI, components are located 300 to 400 microns away from the edge pad. Speedline has developed an innovative new approach to the SEI method utilizing precision automated solder paste dispensing for SMT RF shield attachment. The process requires high-precision dispensing capability enabled by specific hardware (dispenser and pump), machine vision and intelligent software configurations. A singular advantage of this approach is that it enables underfill routines to be performed before placing the shield metal pieces onto the PCB, rather than through perforations in the metal sheet, as design limitations in the past required.
Implementing SEI
The key to a successful SEI placement process is to uniformly dispense a controlled amount of solder paste accurately on the edge copper traces such that 50% of the solder volume is on the top of the edge pad, and the remaining 50% solder volume is deposited on the side trace of the edge pad for good bonding between the metal sheet and the substrate edge. Dispensing in close proximity to the edge creates a unique challenge, which is demanding. This challenge is met using a precisely controlled traditional auger pump, considered the best choice for solder paste dispensing, followed by software technology in process development of edge detection (of the PCB) for high process accuracy as well as strict control of dispensed line width.
SMT line for SEI assembly
The inherent simplification of the one-step shield attachment process also creates a complete metallic (solder) bond surrounding the entire PCB that makes the RF shield highly reliable. Low temperature lead-free (no-clean) solder alloys are used to attach RF shield metal pieces. This is because an SMT PCB assembly that includes these shields must ‘see’ two reflow passes; the first to attach the SMT components, the second to solder bond the RF shields. Although the first pass will typically involve higher-temperature lead-free solder (e.g., SN100C @ 227°C.) to form the solder joints for the component leads, process engineers do not want those solder joints to see those liquidus temperatures a second time due to the potential for oxidation degradation, de-wetting and a number of other reliability problems. The second pass reflow for the metal sheet attach is then achieved using low-temperature melting alloys, such as a Sn/Bi/Ag alloy with a melting temperature point below 140°C. See the figure that shows a typical flow scheme whereby the PCB is assembled with components and reflowed in the standard manner prior to shield attach. Following those process steps, the low-temperature solder paste for metal piece attach is dispensed, then the shield is put in place and the assembly is reflowed well below 140°C.
Technology considerations for SEI
Key process control parameters essential to achieve successful SEI include the following:
  • Precise control of solder volume dispensed per pad/interconnect area
  • PCB edge sensing accuracy – absolutely critical for this atypical dispense application
  • Ability to dispense varying solder paste types (including types 3 through 6)
  • Ability to dispense pastes via varying needle sizes (IDs);
  • Precise line (solder bead) width dispense accuracy
  • Repeatable consistency of deposition over high volume/throughput.
Pump capability
Establishing pump capability is key in developing a robust process for SEI. The Camalot Model 635 servo drive (SD) pump, an auger-based system, was chosen for this application because it has demonstrated the capability to deliver a reliable process in the solder paste dispensing arena. It is well suited for line type applications with viscous materials, and incorporates a patented positive shutoff/no drip design. Capability analysis (Cpk) of the pump is conducted on line widths measured by dispensing different solder volumes or bead widths. Data is statistically validated with variability plots of multiple line widths (presented in figure). These results have shown 635 SD pump capability with tolerance of 10% over average or target given by the customer.
Dispenser platform
The dispense platform chosen was the Camalot Prodigy, a recently-introduced dispensing platform with the required performance specifications and capability of incorporating the pump and vision systems needed. The dispensing system has an accuracy of ±35 microns at 3-Sigma, and repeatability is ±10 microns at 3-Sigma. This is supported by high acceleration speeds of 1.5g in motion to dispense packaging materials at independent locations. Accuracy and speed are supported by linear drive technology, refined motion control architecture, and a rigid, frame design plus a robust XY-gantry system.
Software Capability
Software capability is critical to effectively program dispense locations based on the design of the PCB and dispense requirements. ‘Line dispense’ commands with parameters aligned to the auger pump capabilities are used to design the dispensing process. With the SEI process, the coverage area is very unique to traditional solder paste dispensing. This is where a newly-developed feature known as ‘Edge Find’, based on vision algorithm, is used to accurately locate the edge pads of the PCB and ensure that dispensing is accurate with optimum solder paste coverage.
Vision system and Edge Find requirement
A state-of-the art vision system was designed and developed to provide the advanced vision capabilities required by SEI. The camera system used is equipped with digital vision enhancement with sub-pixel definition. These hardware enhancements maximize the accuracy of the fiducial locations, components and pad edges, to provide additional features including visual inspection for automated deposition monitoring.
The primary objective of ‘Edge Find’ or ‘Edge Def(ine)’ is to allow the vision system to find the edges of the PCB accurately. To execute this feature we use a ‘Locate Edge’ command where the user is required to teach the edge pads of interest. This ‘Locate’ command for edge find uses same options as used for our standard fiducial, but instead we teach the edge of the copper pad as the model the user will access the ‘Locate’ template for Edge Find in the program. A sample template showing how the template appears with respect to a PCB edge pad. The vision system and software have the ability to distinguish the edge pad from the metal sheet if there is a shield present outside the edge pad and accurately dispense on the edge of the pad. The edge of the PCB is traced by teaching the position of the edge pad on the PCB (see next figure). If the edge pad is located on the left-hand edge of the PCB, the user selects ‘Right Edge 2’ to look for the second edge coming from the right to left side of the PCB. If the edge pad is located on the top edge of the PCB, the user selects ‘Bottom Edge 2’ to look for the second edge coming from the bottom to top side of the PCB. Similarly, different edge pads at different locations can be taught by the user, using the Locate Edge feature in the process program. It is highly recommended that this feature always be taught in pairs. The software has the capability of teaching multiple edge pads to have higher accuracy and account for manufacturing variability such as shape, size and discoloration.
Combined effect – software and advanced vision
The combination of the advanced vision system for edge pad recognition with algorithms developed within the software enables a 9 or 13 microns (µm) per pixel resolution, necessary for the vision to detect edge pads and other pads as small as 50 microns in width or diameter. The combined technology takes into account the variability of the edge pad shapes where less than ideally-shaped edge pad could still be detected by the vision system accurately so as to dispense the right volume of solder paste. This constitutes a key difference between the stencil printing and automated dispensing processes where dispensing can accommodate the natural variability inherent in the edge pad traces due to PCB fabrication tolerances.
Line width measurement and adjustment
Line width measurement and line width adjustment are very critical to SEI. Line width adjustment is an advanced form of process control that has the ability to monitor the accuracy of the amount of material dispensed and make adjustments automatically if the line widths are found to be out of tolerance or specification limits.
Conclusion
Shield edge interconnect (SEI) application using automated solder paste dispensing is made a robust process that is of course dependent on strict process control. Implementation is qualified by specific capabilities in the dispenser, pump, vision system and software tools relative to line width control and edge definition capabilities. It creates a sufficiently durable RF shield connection to be suitable for mobile communication device assembly delivering products of high quality with a multitude of functionality.

Zusammenfassung
Mobile kompakte Kommunikationsgeräte überstreichen weite Frequenzbereiche bis in den Mikrowellenbereich. Die hier auf sehr engem Platz implementierten Hochfrequenzfunktionen müssen vor gegenseitiger schädlicher Beeinflussung und anderen Störeinflüssen geschützt werden (hohe EMV-Festigkeit). HF-Schaltungen sind in der Regel mit Schirmblechen abgeschirmt. Mit dem weiter entwickelten SEI-Prozess (Shield Edge Interconnect) lässt sich die Bestückung der Abschirmungen vereinfachen, noch präziser vornehmen und dabei wertvoller Platz auf der Baugruppenoberfläche einsparen, der dann für weitere Komponenten-Bestückung für noch mehr Funtionalität zur Verfügung steht.
Les appareils compacts et mobiles de communication couvrent de vastes plages de fréquences jusque dans les domaines des micro-ondes. Les fonctions à haute fréquence qui sont ici implémentées dans un espace très réduit doivent être protégées des influences nuisibles mutuelles et des autres influences parasites (forte résistances à la VEM). Les commutations HF sont en règle générales protégées avec des tôles de blindage. Avec le processus SEI (Shield Edge Interconnect) dont le développement a été poursuivi, l’implantation des blindages a été simplifiée, peut être effectuée de manière plus précise tout en gagnant à la surface de l’élément de construction, une place précieuse qui devient alors disponible pour l’implantation d’autres éléments afin d’assurer encore plus de fonctionnalités.
Current Issue
Titelbild EPP EUROPE Electronics Production and Test 11
Issue
11.2023
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