Dec05
Both solder joint bridging and open solder joint happen at same BGA?

Is that possible both solder joint bridging and open solder joint defect happen at same BGA (Ball Grid Array) or CSP (Chip Scale Package) device?BGA cry down(電路板則因為TAL(Time Above Liquidous)過長,與迴焊爐的上下爐溫溫差過大,兩相交互作用下形成電路板板邊下彎,造成了所謂的哭臉曲線)

Is that possible both solder joint bridging and open solder joint defect happen at same BGA (Ball Grid Array) or CSP (Chip Scale Package) device?

It is hard to see both solder joint bridging and open solder joint defect happen at same BGA device in the general situation, but it is still possible. There is a case that BGA substrate and PCB has a big deviation of CTE (Coefficient of Thermal Expansion) then make the BGA substrate with smile-up curve (concave) deformation and PCB with cry-down curve (convex) due to longer TAL (Time Above Liquidus) or big temperature deviation between above and below heater in the reflow oven.

It is possible to see both solder short and open happen at same BGA device if this kind of smile-up and cry-down curve happens at same time. Below picture show the BGA substrate with smile-up and PCB with cry-down curve. The balls locate in the center of BGA are almost touch with others together due to them are squeezed and become flat and wider. The balls locate at both ends of BGA had been elongated.

The balls locate in the center of BGA are almost touch with others together due to them are squeezed and become flat and wider. The balls locate at both ends of BGA had been elongated.

Below three actions can fix this kind of issue:

  • Reducing the slop of heating of reflow oven.
  • pre-heating the BGA device to eliminate the stress.
  • or applying higher Tg substrate material.

Besides, there are some reasons may cause the open solder for BGA.

  • Oxidized PCB solder pads or BGA solder balls. Sometimes, the wetted PCB or BGA will bring a similar symptom.
  • Expired solder paste.
  • Insufficient solder pastes volume printing.
  • Poor reflow temperature profile setting. Reflow temperature raise too fast will make smile-up and cry-down curve deformation on both PCB and BGA.
  • PCB design issue. If there is via-in-pad or call via-on-pad design on the BGA pads and without filling, then solder volume will hard to control and result in Insufficient solder due to the solder will flow into the via during reflow. Via-in-pad may also cause the voids/bubbles inside the BGA ball, via outgassing, swelled BGA ball.
  • HIP/HoP (Head-In-Pillow) or NWO (Non-Wet Open). HIP and NWO defects usually occurs when the BGA substrate or PCB deformation in the poor reflow heating process. When the solder paste is melted in the reflow zone, the BGA solder ball does not touch with the liquid solder due to board is warpage. In the cooling zone, the deformed PCB or BGA substrate will back to its position but the melted solder also become solid. Both solder on the PCB pad and BGA ball contact together with sold phase show the phenomenon of two balls.

    BGA head in pillow soldering defect

There are some methods to analyses the open solder for the BGA balls.

1. Microscope. Applying the microscope only can inspect the outer row of BGA solder ball. Applying fiberscope may have chance to inspect 2nd row of ball.

2. X-Ray inspector. The X-Ray inspector can find out solder short and hard to detect the solder open. It is recommended to apply 2.5D X-ray or 3D CT X-ray to turn a certain angle to inspect there are HIP or HoP issue or not?

3. Red Dye Penetration. Applying the non-destructive inspection solution before red dye since it is destructive test. Red dye can check the solder short and open solder.

4. Cross-section. This is also a destructive test solution. It focusses on the certain position to check open, short and IMC (Intermetallic Compound).


Related articles:
Why thinner PCB will need carrier for reflow process?
Improve BGA solderability by partial increase solder paste volume
What is Step-up & Step-down Stencil? Selective Solder Paste Volume Increasing Solution

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