Hazards and causes of welding bubble!

1. Cavities and their hazards;
Voids do great harm to solder joints. Statistical analysis shows that the failure related to voids accounts for
20% of PCBA failure;
There are two kinds of hazards of cavities:
1. Reduce the effective welding area, weaken the welding strength and reduce the reliability.
2. Pushing solder leads to short circuit between solder joints.

2. Voids in standard solder joints can be observed by slicing, X-ray and other means. Cavity is generally determined by using X-RAY image, and the acceptance standard is generally for the bubbles in the BGA solder ball. IPC-A610D requires that the porosity area shall not exceed 25% of the sphere area from the top view.

IPC-7095A has a detailed definition of the bubble acceptance standard in BGA solder balls

3. Cavity generation mechanism
The general reason for void formation is that gas is generated when solder is melted.
Gas source: water vapor:
(1) Flux reacts with metal oxides (SnO/CuO) to produce water
２ＲＣＯＯＨ  ＋  ＳｎＯ（ＲＣＯＯ）２Ｓｎ  ＋  Ｈ２Ｏ↑
(2) Water generated by esterification of organic acids in Flux
ＲＣＯＯＨ  ＋  Ｒ´ＯＨＲＣＯＯＲ´  ＋  Ｈ２Ｏ↑
(3) Be affected with damp
Organic matter cracking:
Flux residual organic matter is cracked to produce gas at high welding temperature.
3. Causes of cavity
Insufficient flux activity, three members (pin, solder, PCB PAD)
Water absorption, oxidation, PAD design (via on disk), hole breaking, surface treatment
Reflow time, Kekendal phenomenon, insufficient flux activity
The flux residue in the solder paste is not discharged from the molten solder, which is cracked at high temperature to form bubbles.

Oxidation:
1. Make the soldering reaction more violent and form more bubbles;
2. The oxidation is not easy to be completely removed, and the wetting speed is slow, which is unfavorable to the discharge of bubbles;
3. Bubble concentration due to weld refusal.
During SMT, the soldering tin is covered on the via, and the air inside the via cannot escape.
This kind of bubble has been accepted in the international standard (J-STD-001D)<area less than 25%>.

PTH perforation
During wave soldering, blowing out the broken hole on the PTH hole wall is called blowing hole.
The hole breaking of PTH is generally related to drilling, copper plating and other processes, because PCB substrate needs
After many wet processes, it is inevitable to inhale water vapor and chemical substances from the hole, which may release a large amount of gas under high temperature.

surface treatment
The oxidation prevention of the surface treatment layer is not in place, resulting in many cavities during welding.
Organic surface treatment such as OSP will produce voids due to the cracking of organic membranes.

The bare copper plate will generate a lot of bubbles due to oxidation

Reflow time
Effect of reflux time on bubble generation:
1. Longer reflow time is beneficial to bubble escape;
2. Too long reflow time will aggravate flux cracking;
3. PAD is re oxidized to form more bubbles.

Kirkendal phenomenon
Some tiny holes in the IMC of the solder joint become larger and larger over time, and eventually form a thin seam, leading to the fracture of the solder joint. This phenomenon is the Kirkendall effect.

Kokendal hole mechanism: unequal atomic diffusion

Kirkendall equation
There are two mechanisms for the formation of Kekendal holes:
1. Base Cu diffusion:
After welding, the Cu6Sn5 IMC layer of the solder joint is scallop shaped. In the subsequent aging, IMC will grow due to the continuous diffusion of the Cu bottom into Sn. The Cu diffusion will create vacancies at the interface between Cu and IMC, and these vacancies will gather to form holes.

2. Growth of Cu3Sn IMC layer:
When the welding is just completed, there is little or no Cu3Sn IMC in the solder joint
During aging, the following reactions will occur, leading to the growth of Cu3Sn IMC:

3. The seventh cause of cavitation
Kirkendall phenomenon
Prevention of Kekendal phenomenon:
1. Plating Ni barrier layer on copper bonding pad:
Kerkendal holes generally appear in the solder joints of the Cu substrate, because Cu expands in the solder
The diffusion speed is relatively fast, and the diffusion also continues at room temperature; the diffusion speed of Ni base in solder is slow, and the diffusion basically stops after welding

2. Add Cu to solder:
Adding a small amount of Cu to the solder can effectively restrain
Diffusion of Cu base in soldering tin, thus preventing the formation of Kekendal holes.

4. Case of void induced solder joint failure