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The aim of this paper is to evaluate using statistical methods how two soldering techniques – the convection reflow and vapour phase reflow with vacuum – influence reduction of voids in lead-free solder joints under Light Emitted Diodes (LEDs) and Ball Grid Arrays (BGAs).

Distribution of voids in solder joints under thermal and electrical pads of LEDs and in solder balls of BGAs assembled with convection reflow and vapour phase reflow with vacuum has been investigated in terms of coverage or void contents, void diameters and number of voids. For each soldering technology, 80 LEDs and 32 solder balls in BGAs were examined. Soldering processes were carried out in the industrial or semi-industrial environment. The OM340 solder paste of Innolot type was used for LED soldering. Voidings in solder joints were inspected with a 2D X-ray transmission system. OriginLab was used for statistical analysis.

Investigations supported by statistical analysis showed that the vapour phase reflow with vacuum decreases significantly void contents and number and diameters of voids in solder joints under LED and BGA packages when compared to convection reflow.

Voiding distribution data were collected on the basis of 2D X-ray images for test samples manufactured during the mass production processes. Statistical analysis enabled to appraise soldering technologies used in these processes in respect of void formation.

The purpose of this paper is to evaluate selected methods of reduction voidings in lead-free solder joints underneath thermal pads of light-emitting diodes (LEDs), using X-ray inspection and Six Sigma methodology.

On the basis of cause and effect diagram for solder voiding, the potential causes of voids and influence of process variables on void formation were found. Three process variables were chosen: the type of reflow soldering, vacuum incorporation and the type of solder paste. Samples of LEDs were mounted with convection and vapour phase reflow soldering. Vacuum was incorporated into vapour phase soldering. Two types of solder pastes OM338PT and LFS-216LT were used. Algorithm incorporated into X-ray inspection system enabled to calculate the statistical distribution of LED thermal pad coverage and to find the process capability index (C_pk) of applied soldering techniques.

The evaluation of selected soldering processes of LEDs in respect of their thermal pad coverage and statistical C_pk indices is presented. Vapour-phase soldering with vacuum is capable (C_pk > 1) for OM338PT and LFS-216LT paste. Convection reflow without vacuum with LFS-216LT paste is also capable (C_pk = 1.1). Other technological soldering processes require improvements. Vacuum improves radically the capability of a reflow soldering for an LED assembly. When vacuum is not accessible, some improvement of capability to a lower extent is possible by an application of void-free solder pastes.

Six Sigma statistical methodology combined with X-ray diagnosis was used to check whether applied methods of void reduction underneath LED thermal pads are capable processes.

This paper aims to investigate voiding phenomena in solder joints under thermal pads of light-emitting diodes (LEDs) assembled in mass production environment by reflow soldering by using seven low-voiding lead-free solder pastes.

The solder pastes investigated are of SAC305 type, Innolot type or they are especially formulated by the manufacturers on the base of (SnAgCu) alloys with addition of some alloying elements such as Bi, In, Sb and Ti to provide low-void contents. The SnPb solder paste – OM5100 – was used as a benchmark. The solder paste coverage of LED solder pads was chosen as a measure of void contents in solder joints because of common usage of this parameter in industry practice.

It was found that the highest coverage and, related to it, the least void contents are in solder joints formed with the pastes LMPA-Q and REL61, which are characterized by the coverage of mean value 93.13% [standard deviation (SD) = 2.72%] and 92.93% (SD = 2.77%), respectively. The void diameters reach the mean value equal to 0.061 mm (SD = 0.044 mm) for LMPA-Q and 0.074 mm (SD = 0.052 mm) for REL61. The results are presented in the form of histograms, plot boxes and X-ray images. Some selected solder joints were observed with 3D computer tomography.

The statistical analyses are carried out on the basis of 2D X-ray images with using Origin software. They enable to compare features of various solder pastes recommended by manufacturers as low voiding. The results might be useful for solder paste manufacturers or electronic manufacturing services.