1. Current status
As we all know, since the printed circuit board cannot be reassembled after assembly, the cost loss caused by scrap due to micro void is the highest.Although eight of the PWB manufacturers noticed the defect due to customer return, the defect was mainly raised by the assembly manufacturer.Solderability issues have not been reported at all by PWB manufacturers, only three of which have mistakenly attributed the "shrinkage" problem (i.e. solder filling only half the hole depth after wave soldering) to the silvering layer on the har-plate with a large internal heat sink/surface.Further studies by OEM have confirmed that this problem is entirely due to solderability problems arising from circuit board design and is not related to silver sinking process or other final surface treatment.
Ii. Root cause analysis
By analyzing the root cause of the defect, the defect rate can be reduced to the minimum through the combination of process improvement and parameter optimization.The giovanni effect usually occurs under the crack between the solder mask and the copper surface.In the process of silvering, the gap of the crack is very small, which restricts the supply of silver ions at this place of silvering fluid. However, copper in this place can be corroded into copper ions, and then silvering reaction takes place on the copper surface outside the crack.Since ion conversion is the source of silver sinking reaction, the attack degree of copper surface under crack is directly related to silver sinking thickness.Any of the following causes will cause cracks: excessive lateral erosion/development or poor bonding of the solder film to the copper surface;Uneven electroplated copper layer (where the hole mouth is thin copper);There are obvious deep scratches on the base metal copper under the solder mask.
Corrosion occurs when sulfur or oxygen in the air reacts with the metal surface.Silver reacts with sulfur to form a yellow Ag2S film on the surface. If the sulfur content is high, the silver sulfide film will eventually turn black.Silver is contaminated by sulfur in several ways, air (as described above) or other sources, such as PWB wrappers.Silver reacts with oxygen in another process, usually oxygen reacts with copper under the silver layer to form dark brown cuprous oxide.This defect is usually due to the high speed of silvering and the formation of a low density silvering layer, which makes the copper in the lower part of the silver layer easy to contact with the air, so the copper will react with the oxygen in the air.Loose crystal structures have large interstitial Spaces between grains and require a thicker silvering layer to achieve antioxidant.This means that a thicker layer of silver is deposited during production, which increases production costs and increases the chance of weldability problems, such as micro-voids and poor welding.
Exposure of copper is usually related to the chemical process before silvering.This defect appears after the silver sinking process, mainly because the residual film which is not completely removed before the process hinders the deposition of the silver layer.The most common is the residual film brought by the resistance welding process, which is caused by the impurities in the developing liquid, namely the so-called "residual film", which hinders the silver precipitation reaction.The mechanical treatment process is also one of the reasons for producing copper exposure. The surface structure of the circuit board will affect the evenness of contact between the board and the solution. If the solution circulation is insufficient or too much, an uneven silvering layer will also be formed.
Ion fouling on the surface of the circuit board can interfere with its electrical properties.These ions mainly come from the silvering fluid itself (remaining in the silvering layer or under the resistance welding film).Different silver solutions have different ion content. The higher the ion content is, the higher the ion pollution value is under the same water washing condition.The porosity of silvering layer is also one of the important factors affecting ion pollution. The ions in residual solution are easy to exist in the silver layer with high porosity, which makes it more difficult to wash, and finally leads to the corresponding increase of ion pollution value.After washing effect also can affect ion pollution directly, washing water is insufficient or water quality is unqualified can cause ion pollution to exceed standard.
The microcavity is usually less than 1mil in diameter, and the cavity above the metal interface compound between the solder and the welding surface is called microcavity, because it is actually a "plane cavity group" of the welding surface, thus greatly reducing the welding binding force.The root cause of microvoids in the surface of OSP, ENIG and silvering silver is not clear, but several factors have been identified.Although all microcavities of the silvery layer occur on the surface of thick silver (thickness > 15 nm m), microcavities occur in not all thick silver layers.When the copper surface structure at the bottom of the sunken silver layer is very rough, it is easier to create micro-voids.The occurrence of microcavities also seems to be related to the type and composition of organics deposited in the silver layer.In view of the above phenomenon, OEM, EMS, PWB and chemical suppliers have carried out several simulation studies on welding, but none of them can completely eliminate the micro-voids.