BGA-焊接指导

A better understanding can reduce fears, reduce costs, and guarantee process control

Principle of ordinary hot air BGA rework system is: the use of very fine heat flow together to a surface mounted device (BGA) pins and pads, the solder melts or make the solder reflow, to complete the removal or welding functions. Removing the simultaneous use of a rubber suction nozzle is provided, suck the BGA up gently when all the solder melting.

Hot air BGA rework system heat flow is through a variety of different

specifications and sizes can be replaced by hot air gun to achieve. As the hot air flow is around out from the heating head, it does not damage the BGA as well as the component substrate or the surrounding while it can easily remove or welding BGA.

The differences between the different manufacturers repair system mainly lies in the different heating source, or hot air flow in different ways. Some nozzles are that the hot air flow in BGA devices around and bottom, some will only spray over BGA. Considering from the angle of protection device, choosing to air flow in BGA devices around and bottom is better. To prevent PCB warping also needs choose with rework system preheating function of PCB at the bottom.

Process Yield Problems

In some cases, particularly on larger PCB assemblies, failure may still occur giving low rework yields when all of the conditions for rework are apparently correct. The graph below shows a trace of a profile taken from under an A97 PBGA on where the thermocouple trace appears to be normal – hitting the desired pre-heat, soak and reflow temperatures. In this case with either solder paste or flux we were achieving a very low pass rate after rework.

The second graph shown below is the final profile that was used and has the original trace over-laid (In Green). As you can see, the 2 traces are very similar but the heating has been done in a totally different way. In this case we have reduced the top air temperature significantly while increasing the underboard heat to the whole board.

The route of the yield problems were down to the plastic BGA warping during reflow due to an excessive top air source temperature.

The following is an explanation of the kind of faults that can occur when everything appears to be set correctly (ie. Good screen print, no board damage, good placement) but the yields are low.

Warpage

One of the biggest problems is caused by either board or component warpage during reflow. Even minor deformation can lead to defects such as unsoldered joints and solder shorts. A normal PBGA stands off the PCB by about 0.020” and lifting by even 0.005” across the device is enough to cause an open circuit. Another factor is that the larger the device, the more prone it will be to these problems. Even if the board/component survive the process with no apparent defect, then the joint will be constantly under strain as the board returns to it’s normal shape causing long term reliability problems. Fig 1 Unsoldered Joints

Figure 1 shows unsoldered joints caused by PCB warpage, this can also be caused by

component warpage. The joints in the centre of the component have lifted above the PCB and were not able to make the solder joint. This is more typical when using only flux as there is no paste to make up the void area. Another characteristic is that the opens will generally be towards the centre of the device. Fig 2 Solder Shorts

Figure 2 shows solder shorts. In this case the deformation has caused the extra solder paste and the solder balls to combine into solder shorts. This is more typical with solder paste, and is normally characterised by shorts on the device periphery, especially in the corners when the device becomes “dog eared” during excessive reflow temperatures.

Solution to Problem

The solution to the problem of warpage is generally related to the profile settings, particularly insufficient underboard pre-heat temperature combined with too high a top temperature. Some boards are however badly balanced thermally, that also can lead to problems.

On large PCBs underboard support can also be a factor with the PCB sagging under it’s own weight. Note – downward pressure should not be applied by either the reflow head or the vacuum as this will exacerbate any problems.

Component De-lamination

Another cause of solder shorts is de-lamination of the component substrate during reflow. Plastic packages are generally Hydroscopic, meaning that they absorb moisture during

exposure to the atmosphere. If the device is then rapidly heated, the moisture expands creating a cavity inside the device. This effect is known as “pop-corning”.

This defect is characterised by a blister on the underside of the component due to the internal expansion. Under X-Ray inspection you would normally see solder shorts towards the centre of the device. Fig 3 Solder Shorts

Blistered substrate

Figure 3 shows the effect of “pop-corning”

Solution to Problem

The solution to this problem is to slowly pre-bake the components and boards prior to reworking them. This will allow any moisture to escape slowly prior to assembly.

Users should also ensure that only correctly stored devices are used for both assembly and rework (The components are generally supplied in a dry pack that should be re-sealed after opening).