To solve poor soldering problem of lead free hot air solder leveling during SMT production, it is necessary to have a detailed understanding of the lead free hot air solder leveling during process. The following is lead free hot air solder leveling process. Lead free hot air solder leveling can be divided into vertical hot air leveling and horizontal hot air leveling.
Main role of HASL:
I. prevention of bare copper surface oxidation;
II. keep the solderability.
The process of hot air solder leveling:
Pre-cleaning → Preheat → Flux Coating → Vertical hot air leveling → Hot air knife scraping → Cooling → Post-cleaning
1. Pre- cleaning
Mainly micro-etched copper surface to be clean, micro-etch depth is generally 0.75-1.0um, at the same time removing the attached organic pollutants, make the copper surface really clean, effective contact with molten tin and rapidly generate intermetallic compound (IMC). Uniform micro-etched will result in good soldering of the copper surface. Hot air blow to dry board quickly after washing.
2. Preheat and Flux Coating
The preheat zone is generally an infrared heating tube about 1.2 meters long or 4 feet long. The board transmission speed depends on the size, thickness and complexity of the board, the speed of the 60mil (1.5mm) board is generally between 4.6-9.0m/min. Between the surface temperature of 130-160 degrees to coat flux, normally double-sided coating, can use hydrochloric acid as an activated flux. Preheating before flux coating can effectively prevent the copper from rusting or burning out due to flux dripping.
3. Tin Coating
The tin content in the molten tin cell is about 430 kilograms, they are pure tin or SN100C (solder alloy composed of eutectic), temperature is maintained at around 260 degrees. In order to avoid the contact of solder with air and generate oxidative scum, a layer of glycol oil is floated on the surface of molten tin in the solder pot. The selection of oil should be considered for compatibility with flux. Boards scroll through the transmission wheel at a speed of approximately 9.1m/min, there are three rows of up and down rollers in the tin furnace area, and the stay time is only about 2 seconds. The span between the front and rear rollers is 6 inches and the length of the rollers is 24 inches or more, so the upper limit of the board that can be processed is 24 inches.
4. Related factors of Hot air pressure setting:
Board thickness, pad spacing, pad shape, tin thickness (in order to prevent the air knife and board from scratching in the vertical spray soldering, the distance between the air knife and the board surface is relatively big, so it is easy causes the tin on PAD to be uneven.
5. Cooling and post-cleaning treatment:
The cold air blow from 1.8 meters air bed from the bottom upward, the board was floated, the bottom surface was cooled first, then use cold air was blown from top to bottom in the bearing area of about 1.2 meters. The cleaning process removed the flux residue. At the same time, it will not bring too much heat shock.
