Traian Cucu

PCBs with different core materials assembling in vapor phase soldering technology

The increasing demand for high integration with continuous decreasing dimensions, maintaining quality and low cost for electronic modules brought on the market new types of printed circuits boards (PCBs), having different material core as substrate. Taking into account only rigid PCBs, glass for applications requiring high levels of cleanliness (medical apparatus) or dedicated to work in high humidity environmental conditions. In the practice, the EMS companies become to have difficulties for assembling the PCBs with glass core determined by high differences of the thermophisical properties compare to classical FR4, being hardly to adjust the SMT lines parameters. In this paper it is analyzed the influence of the PCB substrate thermophysical properties over lead-free solder joints quality and reliability as results of two soldering technologies, classical infrared-convection (IRC) and vapor phase (VPS). The experiments emphasize the differences regarding soldering thermal profile determined by the glass core of the PCB substrate. The optimum soldering solution for this PCB type will be provided. The results are undeniably that PCB support material is very important for the statement of the thermal profile of the process.

4 P soldering model for solder joints quality assessment

The reliability of microelectronics components and assemblies it should be considered as expression of solder joints functionality. The functions are in close connection with the microstructure of the solder joints, which are the result of the soldering Process temperature gradient action over the trinomial solder alloy/Paste, electronic components terminals/Pin and PCBs Pads finishes. Consequently the solder joints quality can be correctly evaluated taking into consideration not only the severally intrinsic parameters of the trinomial elements, Pad-Paste-Pin, but also interrelate the complex reaction at the interface between them and interdependence with the soldering Process parameters. Therefore the solder joints can be considered result of a complex function of 4P elements defined before. In the paper, a 4P Soldering Model will be proposed in order to characterize the solder joints quality, emphasizing dynamics of Pad-Paste-Pin-Process elements synergistically interactions. The different pad finishes (NiAu, ImmAg, ImmSn, HASL, OSP) realized on PCBs having different core material, (FR4, Glass, Al, Cu, Al2O3), the components terminals/pins/balls having different finishes, the different solder paste type and the reflow soldering process (infrared-convection/vapour phase/LASER) with their typically parameters are the 4P Soldering Model variables. Practical experiments were designed in order to study 4P Soldering Model elements, Pad-Paste-Pin-Process, interactions.

Lead-free electronic system integrated in a Vapour Phase Soldering equipment prototype

The purpose of this paper is to present the applied research work, performed in POLITEHNICA University of Bucharest, Center for Technological Electronics and Interconnection Techniques (UPB-CETTI) in the frame of one European project and two diploma projects, oriented to the development of a VPS (Vapour Phase Soldering) equipment using ecological, lead-free, electronics and tailored to small and medium size electronics companies (SME) needs. The VPS technology seems to be today the most appropriate for many EMS companies, especially for SMEs. The lead-free electronic system developed in UPB-CETTI is in the stage of integration in a VPS equipment prototype, which is under construction in these months. The system can be embedded also in plotters, cutter plotters or milling/drilling equipment for PCB manufacturing.

Investigation on the efficiency of thermal relief shapes on different Printed Circuit Boards

As known, thermal relief is a technique used to separate soldering pads or plated through-hole vias from large copper planes in order to reduce soldering dwell time by providing thermal resistance during the process, thus minimizing the heat transfer to the plane. However, during normal operation, the same thermal resistance limits the heat transfer for power component dissipation. The narrower the thermal pad, the greater temperature increasing during power dissipation. Hence, the dimensions of the thermal pads are critical. The work presents the results of a practical approach not for a hole via or a pad for an ordinarily component connected to a large copper area, but a medium power Surface Mounting Device, SMD, using the Printed Circuit Board, PCB, as heat sink. There are taken into consideration different thermal pads and there are investigated by temperature measurements with thermocouples and thermovision. The variables are the thermal pad dimensions, the area of the radiant surface, the base material of PCB, and the reflow soldering process. It is analyzed the effect of the thermal pad versus the temperature on the soldering pad and beyond the thermal pad and on the case of the device during reflow soldering process and during operation

PCBs thermophysical properties in lead-free assembling process assessment

As elements of interconnection structures at level two in electronic packaging, the pads could be considered as a particular solution into a virtual space defined by the materials, geometry, surface finishes and substrate [1, 2]. The assembling processes emphasize the substrate contribution over thermal mass and heat transfer [3]. Each of the pad particular solution defined in the design stage will have unique values of thermophysical properties (THP) [2]. In the paper it will be presented the results of the THP measurements for different types of PCBs having particular solutions defined by the pad finishes, geometry and substrate materials using ANTER equipments (FlashLine 3000 thermal diffusivity system and Unitherm1161V dilatometer type). Finally, scientific and practical conclusions shall be drawn in order to improve the quality, reliability and increase the energetic efficiency of the reflow soldering process.

Solder paste shelf life extending approach for prototyping and small series activities

In case of prototyping and small series it can be identified a common practical issue for EMS (Electronic Manufacturing Services) companies, especially in case of prototyping and small series activities, regarding solder paste loss percent, production costs and solder joints reliability consequences. Although the solder pastes producers assure minimum 8 hours stencil life-STL (the warranted solder pastes in printing period usage) for printing or dispensing in the recommended ambient conditions: temperature 22° to 28°C and 30%–70% relative humidity, and over six months shelf life-SHL (the shelf warranted storage period in the determined ambient conditions), the solder pastes loss percent goes over 20% in these cases. Process engineers allow to use solder pastes after SHL to and over STL limits [2], in order to reduce loses and implicit costs, taking (or not) into consideration possible solder joints reliability implications. Taking into consideration the solder pastes, as part of 4P Soldering Model for solder joints reliability analyze, in the paper will be presented the study done to find solution for prolonging SHL over producer data, maintaining their properties. The work will be used to develop acceptance methods for solder paste usage after SHL.

Thermophysical properties of some Low Temperature Lead-Free solder pastes dedicated to automotive applications

The maximum temperature of all parts of automotive electronics equipment is given by the location and can vary between 358 K and 478 K. RoHS EU Directive in the context of the manufacturing costs reduction, necessity led automotive industry to create new Low-Temperature Lead-Free (LTLF) solder pastes family. In the paper, the thermophysical properties of the alloys from ALPHA® CVP-520 (SnBiAg) and ALPHA® OM-535 (SBX02) test samples having as a source solder pastes were compared with similar from ALPHA® OM-6106 (Sn62) solder paste. The presented work extend the theoretical studies and experiments upon heat transfer in VPSP in order to optimize the technology for soldering process (SP) of automotive electronic modules and could be extended for home and modern agriculture appliances industry.

Multicriterial approach correlation of the solder joints functional tests results

The assembling of the electronic components on PCBs becomes a key issue in electronic packaging reliability. The solder joints resulting from the soldering process represent an important outcome for the PCB assembling. Moreover, the Lead-Free soldering requests, imposed by the RoHS Directive, can cause difficulties in obtaining a high quality/reliable soldering process. Considering the reliability of electronic modules as expression of solder joints functionality, could be emphasized the lack of tests over solder joints functionality on assembling line. Usually are realized optical inspections and, in precise situation, X-Ray analysis are done over assembled PCBs being followed by “in circuit” electrical functional tests. The defects type “hide” or “cold/dry joints” could be not found. In the paper are presented the results of the functional tests of solder joints realized in two soldering technology, infrared-convection and vapor phase. Regarding to thermal functionality, infrared analysis of temperature distribution on the surface of a populated PCBs using thermovision were done. A correlation among functionality, reliability and thermovision functional analysis will be realized based on the experiments results. In consequence, the authors emphasize thermovision as available in circuit indirect test method for “hide” defects of solder joints.

Low silver, lead free solder paste, new alloy developments

The compliancy with RoHS directives as well as the trend towards green technologies and green processes has been the main driver for the paste manufacturers. One direction that has proven to be worth while exploring is the one towards low melting point alloys that will enable energy savings during the manufacturing processes. Last but not least these emerging alloys will add an extra benefit to the economy of the production process, the elimination of wave soldering process. The solder paste used: OM520, 42%Sn/57.6% Bi/0.4%Ag, type 3 powder by IPC J-STD-005 [7], the study was completed on a FR2 PCB with OSP finish. A SAC305 paste — OM338PT, SAC305, type 3 powder by IPC J-STD-005 [8] — was used as benchmark for the measurements. The interconnection structure elements at level two in case of surface mount technologies are electronic components terminals — pin —, the dedicated metallic surface on the printed circuit board — pad —, and the solder joints. The solder joints are the result of the soldering process consisting of the temperature gradient action over the solder paste volume at the interface between pins and pads [1, 3]. The solder joint overall performance is described by the intermetalic formation at the interface of pad and solders volume as well as the pin and solder volume [2, 4]. The implementation of a lead free process needs to take in account the sum of all parameters that affect the soldering process and the interdependence between them [5]. In the paper will be presented the results of termocycling of the boards assembled with the two alloys for the BGA components using reflow process under air. Scientific and practical conclusions shall be drawn in order to prove the improvements brought by the bismuth alloy in reliability and energetic efficiency of the reflow soldering process.

Pin-in-paste DFM constraints in vapor phase soldering technology for optoelectronic components

The topical trends in the field of electronic equipments developing are a large integration on pcb support for different types of components and devices, including optoelectronic type, from small to medium power, in condition of reducing physical dimensions, in order to create new electronic products in short time at lower manufacturing cost. The condition for economical success for a product is to assure the product, even from the conception stage, with a high level of quality by reducing the product cost; to conclude, designing according with production possibilities by using Design For Manufacturing (DFM) concept. This desideratum depends on the conception and design of the product. According to DFM concept, a successful project assures design requirements for the system and finally for printed circuit boards (PCB), accomplishes the assembling technology constraints defined by international standards in the field of electronic packaging, such as IPC or Restriction of Hazardous Substances Directive. Active from July 1, 2006, the RoHS Directive 2002/95/EC adopted in February 2003 by the European Union, and adopted in Romania by HG - 992/2005, completed by HG - 816/2006, call forth important consequences in assembling technologies. In order to minimize manufacturing cost, Pin-In-Paste offers solutions for complete assembling of high complexity PCBs in Vapor Phase Technology using only one reflow machine avoiding overheating of the assemblies relatively to infrared reflow oven. Starting from RoHS consequences analysis, especially thermal profile, the paper presents the applied research performed in the assembling lines on VPS machine in order to define the design requirements for Pin-In-Paste dedicated stencils and PCBs, experiments result and conclusions regarding DFM requirements for lead-free assembling technologies of optoelectronic components. Finally, scientific and practical conclusions shall be drawn to configure the optimum implementation way for Pin-In-Paste in Vapor Phase Technology. The authors emphasizes that Vapor Phase Technology has all the conditions to become the disruptive technology of the moment.