Technical Library | 2013-01-17 15:37:21.0
A problem exists with electroless nickel / immersion gold (ENIG) surface finish on some pads, on some boards, that causes the solder joint to separate from the nickel surface, causing an open. The solder has wet and dissolved the gold. A weak tin to nickel intermetallic bond initially occurs, but the intermetallic bond cracks and separates when put under stress. Since the electroless nickel / immersion gold finish performs satisfactory in most applications, there had to be some area within the current chemistry process window that was satisfactory. The problem has been described as a 'BGA Black Pad Problem' or by HP as an 'Interfacial Fracture of BGA Packages…'[1]. A 24 variable experiment using three different chemistries was conducted during the ITRI (Interconnect Technology Research Institute) ENIG Project, Round 1, to investigate what process parameters of the chemical matrix were potentially satisfactory to use and which process parameters of the chemical matrix need to be avoided. The ITRI ENIG Project has completed Round 1 of testing and is now in the process of Round 2 TV (Test Vehicle) build.
Technical Library | 2013-07-25 14:02:15.0
Bottom-termination components (BTC), such as QFNs, are becoming more common in PCB assemblies. These components are characterized by hidden solder joints. How are defects on hidden DFN joints detected? Certainly, insufficient solder joints on BTCs cannot be detected by manual visual inspection. Nor can this type of defect be detected by automated optical inspection; the joint is hidden by the component body. Defects such as insufficients are often referred to as "marginal" defects because there is likely enough solder present to make contact between the termination on the bottom-side of the component and the board pad for the component to pass in-circuit and functional test. Should the board be subjected to shock or vibration, however, there is a good chance this solder connection will fracture, leading to an open connection.
Technical Library | 2015-02-12 16:57:56.0
Electronic systems are known to be affected by the environmental and mechanical conditions, such as humidity, temperature, thermal shocks and vibration. These adverse environmental operating conditions, with time, could degrade the mechanical efficiency of the system and might lead to catastrophic failures.The aim of this study is to investigate the mechanical integrity of lead-free ball grid array (BGA) solder joints subjected to isothermal ageing at 150°C for up to 1000 hours. Upon ageing at 150°C the Sn-3.5Ag solder alloy initially age-softened for up to 200 hours. This behaviour was linked to the coarsening of grains. When aged beyond 200 hours the shear strength was found to increase up to 400 hours. This age-hardening was correlated with precipitation of hard Ag3Sn particles in Sn matrix. Further ageing resulted in gradual decrease in shear strength. This can be explained as the combined effect of precipitation coarsening and growth of intermetallic layer. The fractured surfaces of the broken solder balls were also investigated under a Scanning Electron Microscope. The shear failures were generally due to ductile fractures in bulk solders irrespective of the ageing time.
Technical Library | 2023-08-04 15:27:30.0
A designed experiment evaluated the influence of several variables on appearance and strength of Pb-free solder joints. Components, with leads finished with nickel-palladium-gold (NiPdAu), were used from Texas Instruments (TI) and two other integrated circuit suppliers. Pb-free solder paste used was tin-silver-copper (SnAgCu) alloy. Variables were printed wiring board (PWB) pad size/stencil aperture (the pad finish was consistent; electrolysis Ni/immersion Au), reflow atmosphere, reflow temperature, Pd thickness in the NiPdAu finish, and thermal aging. Height of solder wetting to component lead sides was measured for both ceramic plate and PWB soldering. A third response was solder joint strength; a "lead pull" test determined the maximum force needed to pull the component lead from the PWB. This paper presents a statistical analysis of the designed experiment. Reflow atmosphere and pad size/stencil aperture have the greatest contribution to the height of lead side wetting. Reflow temperature, palladium thickness, and preconditioning had very little impact on side-wetting height. For lead pull, variance in the data was relatively small and the factors tested had little impact.
Technical Library | 2024-04-08 15:46:36.0
A designed experiment evaluated the influence of several variables on appearance and strength of Pb-free solder joints. Components, with leads finished with nickel-palladium-gold (NiPdAu), were used from Texas Instruments (TI) and two other integrated circuit suppliers. Pb-free solder paste used was tin-silver-copper (SnAgCu) alloy. Variables were printed wiring board (PWB) pad size/stencil aperture (the pad finish was consistent; electrolysis Ni/immersion Au), reflow atmosphere, reflow temperature, Pd thickness in the NiPdAu finish, and thermal aging. Height of solder wetting to component lead sides was measured for both ceramic plate and PWB soldering. A third response was solder joint strength; a "lead pull" test determined the maximum force needed to pull the component lead from the PWB. This paper presents a statistical analysis of the designed experiment. Reflow atmosphere and pad size/stencil aperture have the greatest contribution to the height of lead side wetting. Reflow temperature, palladium thickness, and preconditioning had very little impact on side-wetting height. For lead pull, variance in the data was relatively small and the factors tested had little impact.
Technical Library | 2013-08-29 19:52:43.0
Au over Ni on Cu is a widely used printed circuit board (PCB) surface finish, under bump metallization (UBM), and component lead metallization. It is generally accepted that less than 3 wt.% Au in Sn-Pb solder joints inhibits formation of detrimental intermetallic compounds (IMC). However, the critical limit for Au content in Pb-free solder joints is not well established. Three surface-mount package platforms, one with a matte Sn surface finish and the others with Ni/Au finish, were soldered to Ni/Au-finished PCB using Sn-3.0Ag 0.5Cu (SAC305) solder, in a realistic manufacturing setting. The assembled boards were divided into three groups: one without any thermal treatment, one subjected to isothermal aging at 125°C for 30 days, and the third group aged at 125°C for 56 days...
Technical Library | 2018-07-11 22:46:13.0
For a demanding automotive electronics assembly, a highly thermal fatigue resistant solder alloy is required, which makes the lead-free Sn-Ag-Cu type solder alloy unusable. Sn-Ag-Bi-In solder alloy is considered as a high reliability solder alloy due to significant improvement in thermal fatigue resistance as compared to a standard Sn-Ag-Cu alloy. The alloy has not only good thermal fatigue properties but it also has superior ductility and tensile strength by appropriate addition of In; however, initial results indicated a sub-par performance in joint reliability when it is soldered on a printed circuit board (PCB) with Electroless Nickel Immersion Gold (ENIG) surface finish. Numerous experiments were performed to find out appropriate alloying element which would help improve the performance on ENIG PCBs. Sn-Ag-Bi-In solder alloys with and without Cu additions were prepared and then tests were carried out to see the performance in a thermal fatigue test and a drop resistance test.to investigate the impact of Cu addition towards the improvement of joint reliability on ENIG finish PCB. Also, the mechanism of such improvement is documented.
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 | 2012-12-13 21:20:05.0
First published in the 2012 IPC APEX EXPO technical conference proceedings. We investigated the micro-void formation of solder joints after reliability tests such as preconditioning (precon) and thermal cycle (TC) by varying the thickness of Palladium (Pd) in Electroless Nickel / Electroless Palladium / Immersion Gold (ENEPIG) surface finish. We used lead-free solder of Sn-1.2Ag-0.5Cu-Ni (LF35). We found multiple micro-voids of less than 10 µm line up within or above the intermetallic compound (IMC) layer. The number of micro-voids increased with the palladium (Pd) layer thickness. Our results revealed that the micro-void formation should be related to (Pd, Ni)Sn4 phase resulted from thick Pd layer. We propose that micro-voids may form due to either entrapping of volatile gas by (Pd, Ni)Sn4 or creeping of (Pd, Ni)Sn4.
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