PCB manufacturers introduce the reasons that affect the performance of double-sided PCB tin plates

　　The PCB manufacturer's editor will elaborate on the key factors that affect the performance of double-sided tin coated PCB boards. The term 'performance' here mainly refers to welding reliability, electrical performance, and long-term reliability. Tin spraying is one of the commonly used surface treatment processes for double-sided panels, and its performance is influenced by a combination of the following factors:

　　1、 The characteristics of the tin layer itself

　　Thickness and uniformity of tin layer

　　Insufficient thickness: During storage or multiple reflow soldering processes, the tin layer will form intermetallic compounds with the underlying copper. If the tin layer is too thin, the "fresh" tin that can be soldered will be depleted prematurely, resulting in decreased solderability, incomplete solder joints, or virtual soldering.

　　Uneven thickness: It can easily lead to poor solderability of local solder pads or the formation of "tin nodules" on large solder pads/traces, causing coplanarity issues (such as uneven QFP and BGA pin heights).

　　Flatness and smoothness of tin layer

　　Poor flatness: Surface unevenness (especially in large areas of copper foil) can affect the installation and welding quality of fine pitch components (such as 0.4mm pitch QFP), which may cause bridging or virtual welding.

　　Surface oxidation and pollution: If the cooling, cleaning or storage environment is not appropriate after tin spraying, a thick oxide layer will quickly form on the surface of the tin layer, seriously hindering the wetting and spreading of the solder, which is the primary cause of solderability failure.

　　Control of Tin Copper Alloy Layer

　　Tin spraying is a high-temperature process where molten tin reacts with copper to form a brittle layer of tin copper intermetallic compound at the interface.

　　Excessive thickness of the alloy layer: It will reduce the mechanical strength of the solder joint and is prone to cracking under thermal or mechanical stress. At the same time, it will consume more tin layers, affecting solderability.

　　Process control, such as tin pot temperature and immersion time, directly affects the thickness and morphology of the alloy layer.

　　2、 Influence of substrate and manufacturing process

　　Substrate material

　　Heat resistance: When tin spraying, the board needs to be immersed in molten tin at about 250 ° C. If the glass transition temperature of the substrate (such as FR-4) is low or the thermal expansion coefficient is not matched, it may cause the board to warp, delamination, or a decrease in the bonding strength between the copper foil and the substrate.

　　Moisture absorption: If the PCB is not fully baked and dehumidified before tin spraying, the moisture inside the board will quickly vaporize at high temperatures, which may cause "board explosion" or "hole blowing" phenomena, that is, delamination and bubbles may occur inside the hole wall or substrate, and in severe cases, through-hole fracture may occur.

　　Quality of solder mask layer

　　Alignment accuracy and adhesion: The solder mask window is precisely aligned with the solder pad. If there is a deviation, it may cause tin to climb up the solder mask or contaminate the edge of the solder pad. Insufficient adhesion of the solder mask layer at high temperatures can cause wrinkling and peeling.

　　Heat resistance: able to withstand high temperatures from tin spraying without discoloration, embrittlement, or loss of insulation.

　　Hole quality

　　Copper thickness on the hole wall: High temperature tin spraying will cause thermal stress on the copper layer on the hole wall. If the electroplated copper layer is too thin or uneven, thermal shock may cause hole wall fracture, resulting in interlayer interconnect failure.

　　Tin blocking problem in the hole: If the process control is improper during tin spraying, molten tin may flow into non plug holes and solidify to form "tin plugs" after cooling. This will hinder subsequent plug-in welding, and the solder plug may crack the hole wall during thermal expansion and contraction.

　　3、 Design and storage factors

　　PCB design layout

　　Copper area balance: If the distribution of copper foil on both sides or the same side of the board is extremely uneven (such as one side having a large area of copper plating and the other side having only fine wires), severe warping will occur due to uneven thermal expansion during high-temperature tin spraying and cooling processes. Warping can affect SMT mounting and soldering accuracy.

　　Pad and routing design: When isolated small pads are connected to large copper sheets, the large copper sheets dissipate heat quickly, which may result in insufficient temperature of the pad during tin spraying and poor tin layer coverage.

　　Storage conditions and time

　　Environment: High temperature and high humidity environment will rapidly accelerate the oxidation of tin layer and the moisture absorption of substrate. The ideal storage environment is constant temperature (<25 ° C), constant humidity (relative humidity<60%), and sealed to prevent dust.

　　Shelf life: The solderability of tin coated plates will naturally degrade over time. Usually, its effective "shelf life" is about 6-12 months (depending on storage conditions), much lower than that of immersion or ENIG processes.

　　summary

　　The factors that affect the performance of double-sided spray tinplate are a full chain system from "material ->process ->design ->management". among which

　　Process control is the core, directly determining the thickness, flatness, alloy layer state, and hole quality of the tin layer. The selection and pretreatment of substrates are fundamental and crucial for the structural integrity of PCBs at high temperatures. Reasonable design is a guarantee that can prevent congenital problems such as warping and uneven heat dissipation. Strict storage management is the key to extending its performance life, mainly to resist oxidation and moisture absorption. Therefore, in order to obtain high-performance and highly reliable double-sided tinplate, fine control should be carried out on all the above links.