Cause analysis of bad welding in BGA repair table

January 30, 2023

In the process of welding a BGA using a BGA repair table, the BGA welding may be poor due to manual operation or mechanical equipment. Only relevant adjustments are made according to known conditions, so we analyze the causes of welding defects.

1. Poor bridging of BGA is easy to occur

When BGA repair table is welding BGA components, the welding ball is connected to the welding ball, resulting in a short circuit, that is, welding spot bridging, which can be detected by X-ray.

The main reasons for tin connection are: poor solder paste printing, patch is not allowed, uneven or excessive flux, automatic welding BGA repair platform welding pressure is too large, BGA corner warping or other causes of arch.

2, BGA air welding, at this time should choose the appropriate welding ball

In the standard process, solder paste is usually applied to the PCB pad prior to reflow soldering. Therefore, using the same size ball when using the BGA repair table will result in insufficient tin. At this time, large welding balls should be used to ensure the mechanical strength of the solder, BGA pad and PCB pad height.

For example, the standard 0.8mm spaced BGA pad should use a 0.4mm ball, but the industrial design uses the solder paste process, so in order to obtain sufficient tin, a 0.45mm ball can be used.

3, BGA cold welding, will lead to electrical performance failure or dysfunction, so that the welding ball can not be completely melted

When welding the BGA, the BGA repair table did not reach a high enough temperature, which caused some balls to not melt completely, and the paste and the BGA ball were not fully wet. The ball is still in a state that cannot reach full melting, that is, the mixture that is still in the liquid solidification state is cooled, and the surface of the ball is rough and uneven, and there is no rule.

When using 2D/X - ray detection, the image has a blurred edge outline with a burr - like irregular projection along the perimeter. Even if the welded joint is cold welded, the mechanical strength of all the solder joints is reduced, resulting in electrical failure or malfunction.

4, BGA solder joints, most of which are caused by external causes of BGA in production due to storage, storage and delivery of poor, leading to surface metal surface oxidation, vulcanization and pollution (grease, sweat, etc.) or surface weldability coating quality problems, resulting in the loss of weldability. After welding, there is no strong alloy bond between pad and pad, resulting in continuous and reliable electrical signals, which is mainly due to external causes.

Usually, BGA components present during debugging use a force press signal, and when there is no signal after the force has disappeared, we consider this a typical "welding" phenomenon.

5, component deformation, this situation can be directly seen from the appearance

In BGA welding, the various materials inside the components have different coefficients of thermal expansion, which causes the packaging to warp after the internal materials are heated during heating. Or the parts are wet inside and not preheated before heating, which causes the internal moisture to evaporate and expand during welding heating, thus causing the popcorn phenomenon of the parts.

Therefore, it is possible to preheat the welding first and reduce the peak temperature as much as possible while maintaining the quality of the welding. The device is stored and installed to enhance water management.

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