Analysis of the reasons for copper foil detachment in PCB four layer spray tin plate

　　Copper foil detachment (also known as copper foil bubbling or peeling) is a serious defect in PCB manufacturing, which refers to the separation of copper wires, solder pads, or copper skin from the insulating substrate (such as FR-4) on the PCB circuit board. The reason for copper foil detachment on this PCB with four layers of tin spraying board is caused by multiple factors, which can be summarized as follows:

　　1. Substrate and pre-treatment issues

　　Poor quality of laminated board: The substrate (semi cured sheet or core board) itself has poor resin curing, moisture, or surface contamination such as oil and oxidation, resulting in insufficient initial bonding force with copper foil.

　　Insufficient roughness of copper foil: Before lamination, the surface of copper foil usually needs to be roughened (such as blackening, browning) to increase surface area and mechanical anchoring effect. If not handled properly or the process parameters do not meet the standards, it will seriously affect the bonding strength between copper foil and resin.

　　Unclean pre-treatment: Before pressing or graphic transfer, the copper surface is not thoroughly cleaned, leaving dust, fingerprints, and previous process chemicals, forming an isolation layer that hinders the effective bonding between copper and resin.

　　2. Compression process issues

　　Improper pressing process parameters: Poor temperature, pressure, and time control during the lamination process. Insufficient fluidity and curing of resin due to low temperature or short time; Uneven or insufficient pressure makes it impossible to eliminate interlayer bubbles and achieve tight adhesion. This will inevitably lead to insufficient interlayer bonding force, especially between the inner copper foil and the semi cured sheet.

　　Interlayer alignment and flatness: Inaccurate alignment or uneven surface of multi-layer boards can result in uneven local pressure and weak bonding in certain areas.

　　Bubbles and voids: Failure to effectively expel air or volatile substances during compression results in the formation of bubbles or voids between layers, which become stress concentration points and delamination initiation points during later heating or stress.

　　3. Strong thermal shock during tin spraying (hot air leveling) process

　　This is a unique and crucial triggering factor for tin plate spraying.

　　High thermal stress: During tin spraying, the PCB needs to be briefly immersed in a high-temperature (usually 230-260 ℃) molten tin lead or lead-free tin solution, and then leveled by high-temperature hot air. This process causes severe thermal shock to the PCB.

　　Mismatch in coefficient of thermal expansion (CTE): The CTE of copper, resin, and glass fiber in PCB varies greatly. The sudden and intense heating and cooling result in significant shear stress between the materials. If there is a weak point in the interlayer bonding force itself, this thermal stress is sufficient to directly cause the copper foil to be "pulled" away from the substrate, forming bubbles or peeling.

　　Multiple heating: If there are too many tin spraying times (such as rework) or insufficient preheating before tin spraying, the thermal shock effect will be doubled, greatly increasing the risk of detachment.

　　4. Mechanical and physical stress

　　Improper splitting operation: Mechanical vibrations and stresses generated during V-cut or milling of the splitting plate may be transmitted to weak bonding areas, causing or expanding delamination.

　　Subsequent assembly stress: During processes such as plug-in, soldering (especially wave soldering), screw locking, and testing probe crimping, mechanical forces or local thermal stresses applied may become a "straw that breaks the camel's back".

　　5. Design and environmental factors

　　Unreasonable design: Large areas of copper skin (especially unbalanced copper or grid treatment) and uneven distribution of copper thickness are more prone to stress concentration due to significant differences in expansion and contraction when heated.

　　Environmental and storage impact: PCBs are stored in high temperature and high humidity environments for a long time, and moisture may seep into the edges or defects of the board, reducing interlayer bonding. In the subsequent high-temperature process (such as welding), water vaporization and expansion generate pressure, leading to "bursting" and accompanied by copper foil detachment.

　　summary

　　For the four layer tin plate, the detachment of copper foil is not caused by a single factor, but by the combined effect of "internal factors" and "external factors". Internal factors "refer to the inherent or acquired weak points in the bonding force between layers inside the PCB (due to materials, pre-processing, lamination processes, etc.). The "external factors" refer to the thermal shock caused by the tin spraying process and various stresses during subsequent assembly. The thermal stress of tin spraying is often the direct trigger for exposing and intensifying internal bonding defects. To avoid this problem, systematic control must be carried out from multiple aspects such as material selection, process control, process optimization (especially pressing and tin spraying parameters), and reasonable design.