Improved soldering of components with different thermal masses by hybrid printing of solder pastes
In this work are shown the results of an improved Pin in Paste (PiP) technique. With the purpose of soldering of SMD and PTH (Pinthrough Hole) components, the PiP technique is making use of solder paste printing. The problematic PtP soldering due to thermal shading of areas under large size connectors, which results from their big thermal masses, has been considered. The actual temperatures under such connectors differ from the optimum thermal profile needed for the particular solder paste which profile can be monitored by a KIC instrument. A solution of this problem is suggested by making use of a solder paste, whose optimum thermal profile corresponds to the actual thermal profile under the connector components. For that purpose are used two different kinds of solder pastes. The proposed method, named Hybrid Printing, of two kinds of SMD pastes is based on the special design of a rakel which allows depositing of more than one kind of solder paste within one processing step. The effect of the use of Bi containing, Pb containing and Pb-free solder pastes has been investigated. The application of hybrid printing in a real production line has been demonstrated. The results of the X-ray and metallographic assessment of the fabricated solder joints are presented.
Acceptance Testing Of Low-Ag Reflow Solder Alloys
Since the implementation of the European Union RoHS directive in 2006, the electronics industry has seen an expansion of available low-silver lead (Pb)-free alloys for wave soldering, miniwave rework, BGA and CSP solder balls, and, more recently, solder pastes for mass reflow. The risks associated with the higher processing temperatures of these low-silver (Ag between 0-3 wt%) solder alloys, such as potential laminate or component damage, increased copper dissolution, and reduced thermal process windows may present manufacturing challenges and possible field reliability risks for original equipment manufacturers (OEMs). In order to take advantage of potential cost reduction opportunities afforded by these new alloys, while mitigating manufacturing and reliability risks, the company has defined test protocols [1-4] that can be used for assessing new Sn-Ag-Cu(SAC), Sn-Ag, and Sn-Cu alloys for general use in electronics. This paper describes initial test results for low-silver alloys using these solder paste alloy assessment protocols for BGAs and leaded components, and the impact of the alloys on printed circuit assembly process windows. Specific pass/fail criteria for acceptance of an alloy are not included, however, as they may vary across industry segments. The assessment evaluates wetting behavior, solder joint thermal fatigue and mechanical shock reliability, intermetallic formation, general physical joint acceptability, and copper dissolution. The variables include multiple component types: two BGA components with the same paste/ball alloy combinations, and numerous leaded components that include common component platings. Surface mount (SMT) process temperature windows are typically constrained on the low end by the ability to melt solder and form acceptable joints, and on the high end by the maximum process temperatures of other materials, such as components. These two constraints have led to a process window of approximately 25°C when soldering with more conventional, Sn3.0Ag-0.5Cu paste. Low-silver SMT alloys have been found to reduce the thermal process window even further. Background During the industry transition from Sn-Pb to Pb-free solder in the early 2000s, significant work was performed by the iNEMI consortium to develop assembly process limits to both produce acceptable solder joints as well as protect other materials from damage. This work was originally presented at ECTC 2005 [5]. This presentation defined the characteristics of the acceptable process window for process alloys in the range of SAC305-SAC405 (Sn 3.0-4.0Ag 0.5Cu). It mandates that each and every solder joint has to reach at least 230°C during reflow to produce acceptable and reliable joints. In addition, this previous work [6] defined the minimum peak temperature for SAC305 as 230-232°C, but theorized that low-Ag alloys may only increase that temperature by 5-7 degrees Celsius. Solder alloys used in Surface Mount Technology (SMT) will generally require lower liquidus temperatures than those alloys used in wave soldering. SMT process temperature windows are typically constrained on the low end by the ability of the solder to melt and form good joints, and on the high end by the maximum exposure temperatures of the other materials, such as components or laminates. These two constraints have led to a process window of approximately 25°C with more conventional, Sn-3.0Ag-0.5Cu paste. Low-silver SMT alloys have been found to reduce the thermal process window even further. While the liquidus temperature is the theoretical minimum peak temperature that any solder joint must reach to start the wetting process, it is well known that this temperature is not always sufficient to produce acceptable joints on the various component plating surfaces. In addition, non-eutectic alloys remain pasty for several degrees and will often not produce acceptable joints until fully molten. It is therefore important to determine a minimum peak temperature at which a specific low-Ag alloy will repeatedly produce acceptable joints across the range of likely component sizes, platings, and PCB surface finishes. This minimum peak, or superheat, temperature is often 10-15°C higher than the liquidus point of the alloy. Superheat is defined as the temperature necessary to dissolve native oxides of metals, which is higher than the liquidus temperature. For low-silver reflow alloys with liquidus temperatures of 225-228°C, this produces a theoretical minimum peak of 240°C or higher. Extending the time above liquidus is sometimes used to improve solder joint formation but can cause increased intermetallic formation. 1 Pb-free does not technically mean no Pb, as it allows Pb in concentrations up to 1000 ppm. When the phrase Pb-free is used in this paper, this is meant to indicate that the material is RoHS-compliant. Maximum component temperatures, which define the upper limit of a process window, are often defined by the supplier, but in-lieu of clear limits the J-STD-020[7] classification requirements, based on package size, are typically used. Table 4-2 of the J-STD-020 will define the maximum peak component temperature for any given package on a PCA (Printed Circuit Assembly), with typical PCAs containing a range of components. The range of maximum package temperatures is between 245°C and 260°C. Many PCA designs contain a wide range of component sizes creating complexity in reflow oven program generation. It is typical to use the largest component to set the maximum peak temperature for a reflow profile, as it will typically be the coolest part of the board. Using Table 4-2 to define the maximum temperature for a large package will create a minimum peak temperature as high as 245°C. An additional complication is the need to reach this minimum peak temperature at every solder joint across the entire PCA. Even with sophisticated reflow ovens, it is often difficult to keep the temperature delta across a complex or thermally massive PCA within 15°C. The data presented here (see Table 1) is based on four low-Ag alloys’ data submitted by multiple alloy vendors over two years. These alloys are commercially available, and have different dopant concentrations. The next section details the test methods for alloy evaluation. The subsequent section details the test results. Table 1: Liquidus temperatures of the alloys reviewed in this paper Alloys Evaluated Ag content Liquidus Temperature (°C) A 0.3% 226 B 0.3% 228 C 0.3% 227 D 1.0% 225 Test Methods The test protocol described below reflects the company’s standard evaluation procedure for reflow alloys for general electronics assembly use. For all of the tests the following were specified: industry standards and test boards where available, appropriate controls (an alloy currently used widely in production), test parameters and execution details. The tests address both reliability and manufacturability concerns when soldering with a low-Ag alloy. Manufacturing DoEs There were two manufacturing design of experiments (DoEs) as part of each SMT alloy evaluation. Both consisted of building boards with a low-Ag SMT alloy and a SAC305 control at defined process parameters (with multiple peak reflow temperatures and times above liquidus, TAL). Small test boards were selected for each DoE so that they could be processed with very small temperature deltas across the boards, eliminating this variable. This gave confidence in the DoE results on whether an alloy was able to form acceptable solder joints at each specified temperature and TAL. BGA solder joints were evaluated for two different sizes of BGA component (see test board in Figure 1). In this DoE, three parameters were evaluated: BGA solder joint formation (in cross-section) per IPC-A-610 [8], intermetallic compound (IMC) thickness, and Cu dissolution. Figure 1: BGA Manufacturing DoE Test Board. Boxed locations (U4, U9, PCB pad for U11) are thermocouple locations. The second DoE evaluated numerous leaded components (see test board in Figure 2). In this DoE, leaded solder joint formation per IPC-A-610 was assessed for the low-Ag alloy. Figure 2: Leaded component DoE test board. Numbered components are to be populated (1-5 to be cross-sectioned). Ref: SMTA Saber For the low-Ag alloy to be feasible for use in general PCA assembly, it must not dissolve excessive amounts of Cu, it must not form very thick IMCs that might be prone to fracture under mechanical stresses, and it must form acceptable solder joints within the OEM’s PCA processing window. The first two requirements (Cu dissolution, IMC thickness) were evaluated at a single process condition, 250 °C and 120 seconds TAL, processed three times. This process condition was selected as representative of the PCA process conditions of a complex product board that is reworked twice. This would be the worstcase conditions, from an IMC growth and Cu dissolution standpoint. The last requirement, forming acceptable solder joints, was the basis for the wide range of TALs and peak reflow temperatures in both DoEs; ranges beyond what would be considered a normal process window, but necessary to determine the processing cliffs where the low-Ag alloy fails to form acceptable solder joints. An example of the manufacturing DoE process conditions is in Table 2. Table 2: Example process conditions for manufacturing DoE. *Note that the liquidus temperature varies by alloy composition. Of the 15 process conditions assembled, a subset (green boxes) was selected for cross-sectioning and inspection to determine BGA solder joint formation per IPC-A-610, IMC thickness, and Cu dissolution. If questions arose on the data, additional process conditions (yellow boxes) may be evaluated. Wetting balance was evaluated for the low-Ag alloys (compared to SAC305) by following industry test method J-STD003[9]. A sample size of 10 coupons, per alloy, was used. A standard activated rosin flux #2 for Pb-free alloys was specified to keep different vendor test result
A New Vacuum Pressure Infiltration CFRP Method and Preparation Experimental Study of Composite
In order to prepare a carbon-fiber-reinforced polymer composite (CFRP) with ideal microstructure and properties, a new vacuum pressure infiltration CFRP method is proposed based on an analysis of existing CFRP preparation process methods. Research on composite material preparation systems was carried out by using this new method principle. The system mainly includes a fiber pre-forming module, a vacuum heating infiltration module, a hot-press curing molding module, and a data acquisition control module. Under the conditions of natural curing at 0 MPa + 6 h + 25 °C, vacuum heating curing at –0.05 MPa + 30 min + 80 °C, and hot-press curing at 0.7 MPa + 5 min + 50 °C, a two-dimensional (2D) CFRP with excellent microstructure and properties was successfully prepared. Observing the microstructure of the prepared composite material, it can be found that the inside of the composite material was sufficiently and uniformly infiltrated, and common preparation defects such as holes and delamination were effectively controlled. Through the performance test, the bending strength of the material reached 790 MPa.