Technical Library | 2020-09-23 21:37:25.0
The need to minimise thermal damage to components and laminates, to reduce warpage-induced defects to BGA packages, and to save energy, is driving the electronics industry towards lower process temperatures. For soldering processes the only way that temperatures can be substantially reduced is by using solders with lower melting points. Because of constraints of toxicity, cost and performance, the number of alloys that can be used for electronics assembly is limited and the best prospects appear to be those based around the eutectic in the Bi-Sn system, which has a melting point of about 139°C. Experience so far indicates that such Bi-Sn alloys do not have the mechanical properties and microstructural stability necessary to deliver the reliability required for the mounting of BGA packages. Options for improving mechanical properties with alloying additions that do not also push the process temperature back over 200°C are limited. An alternative approach that maintains a low process temperature is to form a hybrid joint with a conventional solder ball reflowed with a Bi-Sn alloy paste. During reflow there is mixing of the ball and paste alloys but it has been found that to achieve the best reliability a proportion of the ball alloy has to be retained in the joint, particular in the part of the joint that is subjected to maximum shear stress in service, which is usually the area near the component side. The challenge is then to find a reproducible method for controlling the fraction of the joint thickness that remains as the original solder ball alloy. Empirical evidence indicates that for a particular combination of ball and paste alloys and reflow temperature the extent to which the ball alloy is consumed by mixing with the paste alloy is dependent on the volume of paste deposited on the pad. If this promising method of achieving lower process temperatures is to be implemented in mass production without compromising reliability it would be necessary to have a method of ensuring the optimum proportion of ball alloy left in the joint after reflow can be consistently maintained. In this paper the author explains how the volume of low melting point alloy paste that delivers the optimum proportion of retained ball alloy for a particular reflow temperature can be determined by reference to the phase diagrams of the ball and paste alloys. The example presented is based on the equilibrium phase diagram of the binary Bi-Sn system but the method could be applied to any combination of ball and paste alloys for which at least a partial phase diagram is available or could be easily determined.
Technical Library | 2018-04-05 10:40:43.0
The miniaturization of microchips is always driving force for revolution and innovation in the electronic industry. When the pitch of bumps is getting smaller and smaller the ball size has to be gradually reduced. However, the reliability of smaller ball size is getting weaker and weaker, so some traditional methods such as capillary underfilling, corner bonding and edge bonding process have been being implemented in board level assembly process to enhance drop and thermal cycling performance. These traditional processes have been increasingly considered to be bottleneck for further miniaturization because the completion of these processes demands more space. So the interest of eliminating these processes has been increased. To meet this demand, YINCAE has developed solder joint encapsulant adhesives for ball bumping applications to enhance solder joint strength resulting in improving drop and thermal cycling performance to eliminate underfilling, edge bonding or corner bonding process in the board level assembly process. In this paper we will discuss the ball bumping process, the reliability such as strength of solder joints, drop test performance and thermal cycling performance.
Technical Library | 2020-10-27 02:07:31.0
For companies that choose to take the Pb-free exemption under the European Union's RoHS Directive and continue to manufacture tin-lead (Sn-Pb) electronic products, there is a growing concern about the lack of Sn-Pb ball grid array (BGA) components. Many companies are compelled to use the Pb-free Sn-Ag-Cu (SAC) BGA components in a Sn-Pb process, for which the assembly process and solder joint reliability have not yet been fully characterized. A careful experimental investigation was undertaken to evaluate the reliability of solder joints of SAC BGA components formed using Sn-Pb solder paste. This evaluation specifically looked at the impact of package size, solder ball volume, printed circuit board (PCB) surface finish, time above liquidus and peak temperature on reliability. Four different BGA package sizes (ranging from 8 to 45 mm2) were selected with ball-to-ball pitch size ranging from 0.5mm to 1.27mm. Two different PCB finishes were used: electroless nickel immersion gold (ENIG) and organic solderability preservative (OSP) on copper. Four different profiles were developed with the maximum peak temperatures of 210oC and 215oC and time above liquidus ranging from 60 to 120 seconds using Sn-Pb paste. One profile was generated for a lead-free control. A total of 60 boards were assembled. Some of the boards were subjected to an as assembled analysis while others were subjected to an accelerated thermal cycling (ATC) test in the temperature range of -40oC to 125oC for a maximum of 3500 cycles in accordance with IPC 9701A standard. Weibull plots were created and failure analysis performed. Analysis of as-assembled solder joints revealed that for a time above liquidus of 120 seconds and below, the degree of mixing between the BGA SAC ball alloy and the Sn-Pb solder paste was less than 100 percent for packages with a ball pitch of 0.8mm or greater. Depending on package size, the peak reflow temperature was observed to have a significant impact on the solder joint microstructural homogeneity. The influence of reflow process parameters on solder joint reliability was clearly manifested in the Weibull plots. This paper provides a discussion of the impact of various profiles' characteristics on the extent of mixing between SAC and Sn-Pb solder alloys and the associated thermal cyclic fatigue performance.
Technical Library | 2019-09-24 15:41:53.0
This paper focuses on three different coating material groups which were formulated to operate under high thermal stress and are applied at printed circuit board manufacturing level. While used for principally different applications, these coatings have in common that they can be key to a successful thermal management concept especially in e-mobility and lighting applications. The coatings consist of: Specialty (green transparent) liquid photoimageable solder masks (LPiSM) compatible with long-term thermal storage/stress in excess of 150°C. Combined with the appropriate high-temperature base material, and along with a suitable copper pre-treatment, these solder resists are capable of fulfilling higher thermal demands. In this context, long-term storage tests as well as temperature cycling tests were conducted. Moreover, the effect of various Cu pre-treatment methods on the adhesion of the solder masks was examined following 150, 175 and 200°C ageing processes. For this purpose, test panels were conditioned for 2000 hours at the respective temperatures and were submitted to a cross-cut test every 500 h. Within this test set-up, it was found that a multi-level chemical pre-treatment gives significantly better adhesion results, in particular at 175°C and 200°C, compared with a pre-treatment by brush or pumice brush. Also, breakdown voltage as well as tracking resistance were investigated. For an application in LED technology, the light reflectivity and white colour stability of the printed circuit board are of major importance, especially when high-power LEDs are used which can generate larger amounts of heat. For this reason, a very high coverage power and an intense white colour with high reflectivity values are essential for white solder masks. These "ultra-white" and largely non-yellowing LPiSM need to be able to withstand specific thermal loads, especially in combination with high-power LED lighting applications. The topic of thermal performance of coatings for electronics will also be discussed in view of printed heatsink paste (HSP) and thermal interface paste (TIP) coatings which are used for a growing number of applications. They are processed at the printed circuit board manufacturing level for thermal-coupling and heat-spreading purposes in various thermal management-sensitive fields, especially in the automotive and LED lighting industries. Besides giving an overview of the principle functionality, it will be discussed what makes these ceramic-filled epoxy- or silicone-based materials special compared to using "thermal greases" and "thermal pads" for heat dissipation purposes.
Technical Library | 2023-05-22 16:49:42.0
Our customers' issues • Apertures are getting smaller • Paste does not release as well • Contaminates the bottom of the stencil • Increases defects / reduces yield Insufficient solder Bridging Solder balls on surface of PCB Flux residue • Requires more frequent cleaning • Reduced efficiency (wasted time) • Increased use of consumables (cost) USC fabric (use "cheap" fabric to reduce cost) Lint creates more defects Cleaning chemistries (use IPA to reduce cost) IPA breaks down flux and can create more defects
Technical Library | 2014-03-06 19:04:07.0
Over the last few years, there has been an increase in the rate of Head-in-Pillow component soldering defects which interrupts the merger of the BGA/CSP component solder spheres with the molten solder paste during reflow. The issue has occurred across a broad segment of industries including consumer, telecom and military. There are many reasons for this issue such as warpage issues of the component or board, ball co-planarity issues for BGA/CSP components and non-wetting of the component based on contamination or excessive oxidation of the component coating. The issue has been found to occur not only on lead-free soldered assemblies where the increased soldering temperatures may give rise to increase component/board warpage but also on tin-lead soldered assemblies.
Technical Library | 2021-08-25 16:34:37.0
As the traditional eutectic SnPb solder alloy has been outlawed, the electronic industry has almost completely transitioned to the lead-free solder alloys. The conventional SAC305 solder alloy used in lead-free electronic assembly has a high melting and processing temperature with a typical peak reflow temperature of 245ºC which is almost 30ºC higher than traditional eutectic SnPb reflow profile. Some of the drawbacks of this high melting and processing temperatures are yield loss due to component warpage which has an impact on solder joint formation like bridging, open defects, head on pillow.
Technical Library | 2019-09-19 00:28:48.0
The symbiotic relationship between solder masks and selective finishes is not new. The soldermask application is one of the key considerations to ensure a successful application of a selective finish. The selective finish is the final chemical step of the PCB manufacturing process, this is when the panels are at their most valuable and are unfortunately not re-workable. Imperfections are not tolerated, even if they are wholly cosmetic. Quality issues often manifest themselves in the form of a 'ping pong' conversation between the fabricators, the soldermask suppliers and the selective finish suppliers. Without tangible evidence these discussions are difficult to resolve and the selective finish process is usually regarded as responsible. This paper will focus on the chemical characteristics and use them to predict or identify potential issues before they occur rather than specifically name 'critical' soldermasks. It is also the intention of this paper to address the potential of a soldermask to react to common yield hiking practices like UV bumping and oven curing. It is hoped that this awareness will help fabricators to ensure maximum yields by asking the right questions. 'Critical’ soldermasks impact all selective finishes. In this paper, practical experience using immersion tin will be used to highlight the relationship between 'critical' soldermasks and some of the issues seen in the field. The paper will include a novel approach to identify re-deposited volatiles after the reflow.
Technical Library | 2014-10-02 20:10:07.0
Sn3.0Ag0.5Cu (SAC305) is the most popular near eutectic lead-free alloy used in the manufacturing processes. Over the last several years, the price of silver has dramatically increased driving a desire for lower silver alloy alternatives. As the results, there is a significant increase in the number of alternative low/no silver lead-free solder alloys available in the industry recently.In this paper, we'll present the performance and process capability of various low/no silver alloy solder pastes. Data from printability, wetting test, slump test, solder ball test, voiding, etc… will be discussed and compared with the control SAC305 solder paste. Benefits and concerns of using low/no silver alloy solder paste materials will also be addressed.
Technical Library | 2022-10-31 17:30:40.0
This paper presents a quantitative analysis of solder joint reliability data for lead-free Sn-Ag-Cu (SAC) and mixed assembly (SnPb + SAC) circuit boards based on an extensive, but non-exhaustive, collection of thermal cycling test results. The assembled database covers life test results under multiple test conditions and for a variety of components: conventional SMT (LCCCs, resistors), Ball Grid Arrays, Chip Scale Packages (CSPs), wafer-level CSPs, and flip-chip assemblies with and without underfill. First-order life correlations are developed for SAC assemblies under thermal cycling conditions. The results of this analysis are put in perspective with the correlation of life test results for SnPb control assemblies. Fatigue life correlations show different slopes for SAC versus SnPb assemblies, suggesting opposite reliability trends under low or high stress conditions. The paper also presents an analysis of the effect of Pb contamination and board finish on lead-free solder joint reliability. Last, test data are presented to compare the life of mixed solder assemblies to that of standard SnPb assemblies for a wide variety of area-array components. The trend analysis compares the life of area-array assemblies with: 1) SAC balls and SAC or SnPb paste; 2) SnPb balls assembled with SAC or SnPb paste.
