PCBA Tech - PCBA: How to judge the problem of BGA dropped parts?

I believe that many people have always had a question, how to judge that the BGA drop is caused by the PCB factory's manufacturing process? Or is it caused by a defect in the manufacturing process of the circuit board? Or is it a PCB design problem? As long as many PCB designers encounter the problem of BGA parts falling, they often insist that it is caused by poor soldering on the manufacturing end, which makes many PCB factory engineers unable to argue.

Although most of the reasons for the BGA drop are related to the poor manufacturing process of the factory or parts, the reliability of the circuit board and structural design cannot be ruled out.

How to judge whether the BGA drop is a PCB factory process or design problem? How to judge the BGA drop is a PCB factory process or design problem?

Before truly analyzing the problem, it is necessary to understand the environmental conditions in which the problem occurs, because the same undesirable phenomenon that occurs under different conditions may often come from different root causes. Generally, the drop of BGA usually occurs in the product.

During the drop test, or if the customer accidentally falls from a height, if no external force is applied to it, it will fall. It is almost certain that 99% of the products or parts are produced by problems, generally nothing more than It is caused by vacancy, false welding or oxidation of the welding surface of the parts, and the board with ENIG surface treatment may also be caused by the black pad phenomenon (Black Pad) caused by too much phosphorus.

Secondly, we must understand the nature of the problem. First, think about which section will the part break and fall under normal conditions? Of course, it is cracked from its most fragile place. If the reflow soldering of SMT is not soldered well, it should be broken between the BGA ball and the solder pad of the circuit board. Of course, it will happen in this place. The fracture may also be caused by the oxidation of the solder balls of the BGA or the oxidation of the solder pads of the circuit board; if the design of the solder pads is too small to withstand too much falling impact, it will cause the solder pads on the circuit board The phenomenon of being pulled down, in fact, this phenomenon may also be caused by the poor pressing of the PCB manufacturer; in addition, there may be too many voids in the solder ball, which causes the solder ball to be broken in the middle of the solder ball due to insufficient strength .

Just because there are various possible reasons for the BGA falling, when we analyze such a problem, it is best to check the circuit board and the part surface of the BGA at the same time to confirm where its fracture surface is located, and also to check the fracture surface. The shape is judged to be caused by strong pulling by external force or poor welding, so a high-power microscope must be prepared, and if necessary, SEM/EDA (Scanning Electron Microscope/Energy-Dispersive X-Ray spectroscopy) must be prepared to analyze the elements on the fracture surface. Composition.

When checking the BGA drop, the corresponding single solder ball and solder pad should be used as the unit, and the BGA solder ball and PCB solder pad should be checked at the same time, so that a complete answer can be obtained.

You can check the fracture surface of the BGA first to see if the solder balls are still on the BGA. If there is no accident, the solder balls should remain on the BGA. If the solder balls are not on the BGA, it may be the solder ball of the BGA part itself. bad.

If the solder ball of the BGA is intact, check whether the fracture surface of the solder ball is rough or uneven. If it does, it means that the solder ball eats well, and the fracture should be caused by an external impact. If the broken tin surface is smooth and arc-shaped, it is likely to be non-wetting or cold soldering. The solder surface on the PCB pad that you want to be correct should also appear A smooth arc shape.

If the fracture surface occurs in the middle of the solder ball, check to see if there is a partially concave smooth surface on the fracture surface. This usually means that there are bubbles in the solder ball, and the bubbles are also one of the possible causes of solder fracture., Just like a hollow beam and column, its supporting force will be weakened. There are many reasons why solder balls can produce bubbles. One part is that the reflow profile has not been adjusted properly, and another part is from the white-purpose designer who insists on designing through holes on the solder pads because they only care about saving space in this way. They don’t know this. It will cause serious consequences such as poor soldering, less tin, air bubbles, etc., and then the manufacturer must pay for it.

Related reading: Vias-in-pad processing principle

The last breaking point is that the solder pad of the PCB is pulled down, which means that the connection between the solder pad and the PCB becomes the weakest place. This can happen. Generally, it may come from the design of the solder pad of the PCB is too small, or the PCB is too small. Insufficient bonding strength may also be caused by repeated high-temperature maintenance of the PCB board, which weakens the bonding strength. If the problem comes from the previous item, the solution can be to increase the size of the solder pad, and you can also use a green paint (solder mask). Cover the outer ring of the solder pad, which can also reinforce the strength of the solder pad. After all the methods have been tried and the problem cannot be solved, consider underfill (bottom filling), because underfill is really troublesome. After the reflow, the test is completed to add the filling and baking actions, which is inconvenient for maintenance.