Boyi Wu

Failure mechanism and mitigation of PCB pad cratering

Pad cratering of printed circuit board (PCB) is becoming a prevailing issue encountered in the PCB assemblies which is accelerated when switching to leadfree process. These units with pad cratering may not fail during functional test as and raise potential failure in the field. This paper uses both experimental and finite element analysis (FEA) approaches to understand the pad strength and pad stresses. An extensive mechanical test by pin pull tests are performed on PCB materials. Cohesive elements are employed to simulate the bonding at the interfaces of pad, laminate and fibers. The results from FEA show that the laminate cracking can be successfully simulated. Guidelines of testing for materials selection are then outlined to mitigate the PCB pad cratering effectively.

Investigation on PCB related failures in high-density electronic assemblies

This paper presents an investigation on field returned open and short failures related to printed circuit board (PCB), including via hole crack, prepreg crack and insufficient circuit etching. After an experimental study with cross section, time domain reflectometry (TDR), and finite element (FE) modelling, it was found that weak plating and corrosion induced via hole crack was a major root cause of interconnect open failures. Such a failure was always mistakenly treated as solder joint open. Except for via hole crack, PCB prepreg crack was another contributor for open failures. The prepreg material for some lot code in production was in poor quality in terms of long-term durability. Dye and pry and cross section investigation showed that for some field returned units, the pad-to-prepreg interface was the weakest adhesion part in the entire solder joint interconnects. Crack propagated across the whole support and hence pad cratering occurred upon overstress. For the circuit short problem, insufficient etching of copper traces and pads was found to be a dominant contributor in harsh environment for a long time. Particularly, corrosion induced dendrite growth brought a high risk of circuit short of insufficiently etched conductors for the fine-pitch lead-free application. In summary, it is important to monitor and control the incoming quality of incoming PCBs so that the the risk of high field return rate and high cost for repair can be minimized.

Investigation on PCB pad strength

In this paper, a pin pull test is introduced as an effective way to assess the PCB pad strength. The pad strength is evaluated by different level of strain, pad type, PCB life phase and accumulation effect of various stations. Finite element analysis (FEA) is employed to analyze whether the PCB material affects the pad strength. It is found that PCB pad strength is sensitive to the rework process, product station and thermal cycling. Meanwhile, it is confirmed that the pad strength will degrade with the increasing strain loaded on PCB. A suitable and safe strain limit related to PCB pad strength should be established for each PCBA manufacturing process. The pad cracking issue may be also improved by selecting pad type or changing a suitable material of PCB. Optimal design should be considered to minimize the risk of pad crack in PCB production. To contain the PCB cratering issue, the pin pull test described in this work is shown effective and low cost and could be used in the production line for either process monitoring or incoming quality control.

A novel soldering method to evaluate PCB pad cratering for pin-pull testing

Abstract The pin-pull test has attracted attention because it can detect a high percentage of laminate cracking failures. In this study, a novel pin-pull test method is proposed to simplify pin soldering and the pin-pull test. The test method utilizes existing tools to perform the soldering and to collect quantitative data on the strength of printed circuit board (PCB) pad. The proposed procedure can be implemented with low cost and be adopted easily using any material or surface mounted technology (SMT) test laboratory. The procedure only requires a common tensile tester with regular copper pins and an external heat element to solder the pin to the pad. The impact of PCB pad design, manufacturing processes (including reflow and rework), thermal cycling, and long-term field usage on pad cratering are explored systematically based on the proposed method. The test result indicates that the high number of traces attached to the pad may significantly increase pad strength such that resistance to pad cratering is improved. Furthermore, pad strength dramatically declines when PCB pads are subjected to thermal stress or are used in the field for long periods.

Theoretical analysis on the element diffusion during thermomigration

With the miniaturization trend and functional demand in high-density microelectronic packaging, the thermomigration in flip chip solder joints due to the joule heating becomes a serious reliability issue in recent years. A majority of researches have been devoted to understanding the failure mechanism of the thermomigration accompanied by the electromigration behavior; and based on the microstructure observation, the thermomigration in eutectic SnPb and other lead free solders in terms of SnBi, SnCu and SnAgCu has been investigated and the dominate diffusion element in each solder bulk has been identified according to the microstructure evolution. However, only a few works are focused on the theoretical analysis for this novel migration process. In this study, according to our previous research of the thermomigration in the composite SnPb solder bump of the flip chip system at low ambient temperature, the tendency of the each diffusion element in this composite SnPb solder is certificated based on the diffusion theory. It is proved that both Sn and Pb atoms in the solder bulk tend to migrate from the hot side to the cold side during thermomigration, and the different microstructure phenomenon appeared in the other studies, such as Sn and Pb atoms are detected to be the dominate diffusion element when the ambient temperature is higher and lower than 100°C respectively, is mainly due to the different diffusion rate of them. Furthermore, this conclusion can be set up as the universal law for the diffusion process under a certain thermal gradient, which indicates that no matter what kind of atoms they are in the solder bulk, all of them have the tendency to move from the hot zone to the cold zone due to the asymmetrical temperature distribution. On the other hands, the important thermomigration parameter, the molar heat flux Q∗of Sn atoms in the SnPb solder alloy is calculated to be 19.1 kJ/mole.

Failure strength study of silicon die and LCD glass by FEA and experiment

Measurement of failure strength of brittle material is always a challenge. Silicon die and LCD glass are two major brittle components in electronic products. Both of them may fail when subject to a large stress during manufacturing, assembling and in field. In order to assess the risk of failure, both 3-point bending (3PB) and ball-on-ring (BOR) are employed to measure the flexural strength of silicon die and glass in this paper. The samples are taken from actual consumer digital products. It is found that the results from two different testing methods are quite different, which include the values of strength and the failure modes. The failure mechanism and location are also studied using finite element analysis (FEA). Two different cracking modes come out of the test results for both silicon die and LCD glass. One is cracking into many small pieces and this is the main cracking mode. The other is cracking into two half.

Investigation of thermomigration in composite SnPb solder joints

Due to the increasing functional demand and miniaturization in high-density microelectronics packaging, thermomigration in flip chip solder joints owing to Joule heating becomes a serious reliability issue. In recent years, a series of researches have been devoted to examining the failure mechanism of thermomigration in eutectic SnPb and lead-free solder joints. However, only a few studies were focused on the thermomigration phenomenon at room temperature. Also, the investigation of thermomigration in solder joints of commercial products was insufficient.

A novel pin-pull tester and its application on PCB pad cratering assessment

The pin-pull test is appealing because it can obtain a very high percentage of laminate cracking failures. In this paper, a pin-pull tester using a promoted soldering method is designed and manufactured. The improved pin soldering process is provided based on a method proposed in previous study. First, the requirement to form solder ball on the printed circuit board (PCB) pad is canceled, and in addition the activation flux is utilized in the material preparation step instead of applying in soldering process. Second, A solder ball on the pin is required enough weight such that the liquid solder ball can drop down from a short distance during soldering step; herein, the enough weight is defined as that the gravity of liquid solder is bigger than the upward wetting force produced by the liquid solder to the heating pin. The software for the testing procedure has been developed for the tester too. Finally, the developed pin-pull tester is utilized to assess the strength susceptibility among three types of PCB pads. Following the promoted process, the pin can be soldered on the pad easily and effectively while the flux coating is activated. The manufactured pin-pull tester, assisting with the promoted testing procedure, can perform the testing successfully. The capability to perform the pin-pull testing automatically is the unique advantage of this developed tester. The test results indicate that large number of traces attached to the pad may significantly increase the pads strength such that resistance to the pad cratering is improved. The pin-pull tester is expected to be improved continuously, and used in many phases, such as pad designing, PCB material out quality control (OQC) & Incoming quality control (IQC) and quality control in product line.

Failure mode of SAC305 lead-free solder joint under thermal stress

With the progress of the lead-free in electronics industry, the reliability of SAC 305 lead-free solder joint is particularly important. According to typical components (BGA) suffering from thermal cycling and thermal aging test, present the fatigue life of the solder joint. Check the solder joint cracking at different cycling. Cross section and metallographic microscope are performed to inspect the microstructure of the lead-free solder joint. Study the solder joint failure analysis under thermal cycling and thermal aging. Guide subsequent reliability test and failure analysis.

Effects of Ni/Ag coating on the wettability of Sn-3Ag-0.5Cu alloy on Cu substrates at different temperatures

With the development of high density interconnection and fine pitch technology, good wettability effectively guarantee the reliability of solder joins. Hence, it is essential to carry out researches on wetting behavior between molten solders and metal substrates. The effect of Ni, Ag coating and temperature on wetting behaviors of Sn-3Ag-0.5Cu solder on Cu substrate after reflowing at 523 K, 553 K and 583 K were studied in this paper. The results showed that when temperatures are constant, Ag coating are favorable to the wetting of Sn-3Ag-0.5Cu alloy on Cu substrates. Because the diffusion rate of Ag itself to Sn-3Ag-0.5Cu solder is much faster than Cu and deeper grooves in scallop-like IMC provide a more convenient access for Cu atoms to dissolve into molten solders. These two factors lead to faster IMC formation rate and better wettability due to the reaction products are the diffusion frontier. While the slower diffusion of Ni to molten solder and none groove-like IMC formation lead to slower growth rate of IMC and worse wettability of Sn-3Ag-0.5Cu alloy on Cu/Ni substrates than pure Cu substrates. In addition, an increase in temperature promotes the substrate atoms diffusion and reaction rate with Sn-3Ag0.5Cu alloy, decreases the viscosity as well as surface tension of melting solder and contributes to better wettability significantly.