PCBA Tech - Process monitoring of PCB surface coating and SMT

The use and choice of PCB surface coating in SMT processing

The choice of PCB surface coating technology for SMT soldering in SMT processing mainly depends on the type of final assembled components, and the surface treatment process will affect the production, assembly and final use of the PCB.

1. SMT processing technology PCB solderability surface treatment is divided into 3 categories according to the purpose:

1, for welding: the surface of copper must be protected by a coating (plating layer), otherwise it is easy to oxidize;

2, connection: such as gold fingers, Ni-Au electroplating or Ni-Au electroless plating;

3. Wire Bonding process: Electroless Ni-Au plating.

2. The basis for the selection of PCB solderability surface coating:

When selecting PCB solderable surface plating in SMT processing, it is necessary to consider the selected soldering alloy composition and the application of the product.

1. Solder alloy composition

The compatibility of PCB pad coating and solder alloy is the primary factor in choosing PCB solderable surface coating (plating). This directly affects the solderability and connection reliability of the solder joints on both sides of the pad. For example, Sn-Pb hot air leveling should be selected for Sn-pb alloy, and hot air leveling for lead-free metal or lead-free solder alloy should be selected for lead-free alloy.

2, reliability requirements

Products with high reliability requirements should first choose the same hot air leveling as the solder alloy. This is the best choice for compatibility. In addition, high-quality Ni-Au (ENIG) can also be considered, because the connection strength of the interface alloy Ni3Sn4 between Sn and Ni is the most stable. If ENIG is used, the Ni layer must be controlled to >3μm (5-7μm), the Au layer ≤lμm (0.05～0.15μm), and the solderability requirements must be put forward to the manufacturer.

3, manufacturing process

When choosing PCB solderable surface coating (plating) layer, the compatibility of PCB pad coating layer and manufacturing process should also be considered. Hot air leveling (HASL) has good weldability, can be used for double-sided reflow welding, and can withstand multiple welding. However, since the surface of the pad is not flat enough, it is not suitable for a narrow pitch. OSP and immersion tin (I-Sn) are more suitable for single-sided assembly and one-time soldering process.

Process monitoring of SMT patch processing

11% of SMT quality problems are caused by design, 27 are caused by process, 31% are caused by process materials, and 31% are caused by process control. It can be seen that design for manufacturability (DFM), process optimization, process control, and supply chain management (procurement and management of process materials) are very important for achieving high quality.

SMT process monitoring:

Process monitoring is an important activity to ensure quality and production efficiency. Take SMT's key process reflow soldering as an example. Although the reflow soldering furnace is equipped with a temperature sensor (PT) and a furnace temperature control system to control the furnace temperature, the set temperature of the equipment is not equal to the actual temperature of the solder joints on the assembly board. Although the display temperature of the furnace is controlled within the temperature control accuracy range of the equipment, but due to the difference in the quality of the assembly board, the number of layers, the assembly density, the number of assembly boards entering the furnace, the conveying speed, the airflow, etc., the assembly board entering the furnace The temperature curve will also fluctuate randomly. In the current patch processing and assembly density is getting higher and higher, the assembly board is getting more and more complicated, and the lead-free process window is very narrow, a few degrees of temperature change may also affect the welding quality. Therefore, continuous monitoring of the reflow soldering process is necessary.

Process monitoring requires technicians to have good measurement knowledge, statistical knowledge, causal analysis capabilities, and in-depth understanding of equipment performance.

Because of the many variables on the production line, equipment, personnel, materials, etc. have their own many variables, which influence and restrain each other to varying degrees every day. How to take sufficient effective monitoring without affecting production and increasing production costs is a difficult task.

At present, software and equipment capable of continuous monitoring are becoming more and more popular, such as reflow soldering process control tools, using AOI software technology to achieve process control, etc. However, the automatic monitoring and feedback of process parameters requires a large investment, which is currently not realized by most domestic SMT companies. In this case, we are required to formulate some practical and effective specifications and systems, adhere to the implementation of the specifications, and achieve process stability through manual monitoring and monitoring.