PCB surface treatment: horizontal tin spraying

- Jul 26, 2018-

Main functions of tin spraying:
   
Oxidation of bare copper surface;
Carbon dioxide keeps soldering tin;
   
Other surface treatments include: hot melt, OSP, chemical tin, chemical silver, chemical nickel gold, electroplated nickel gold, etc.But with the best performance of the pewter spray board;
   
Vertical tin spraying has the following disadvantages:
Uneven heating up and down of the plate, after the first in and out, easy to appear plate bending defects;
The thickness of tin on the welding disk is uneven. As a result of hot wind blowing and gravity, the lower edge of the welding disk produces solder sag, which makes the welding of SMT surface mounted parts not easy to be fixed and stable. It is easy to cause the deviation of parts after welding or the phenomenon of monument stoning
The soldering disk on bare copper has a long contact time with hole wall and soldering tin on the soldering plate, generally more than 6 seconds. The copper solubility increases rapidly in the soldering furnace, and the increase of copper content will directly affect the soldering tin of the soldering plate.
   
Horizontal tin spraying can greatly overcome the above defects. Compared with vertical tin spraying, it mainly has the following advantages:
The contact time between solder and bare copper is relatively short, about 2 seconds, and the IMC is thin with a long shelf life.
Ammonia has a short wetting time of about 1 second;
Torque plate is evenly heated, with good mechanical performance and less warping.
Horizontal tin spraying process:
Pre-cleaning treatment -- preheating -- flux coating -- horizontal tin spraying -- hot air knife scraping -- cooling -- after cleaning treatment
   
1.Pre-cleaning treatment:
The main purpose is to clean the micro-etched copper surface. The micro-etching depth is generally 0.75 to 1.0 microns. Meanwhile, the attached organic pollutants are removed to make the copper surface truly clean and effectively contact with the molten tin, and the IMC is rapidly generated.The evenness of microcorrosion will make the copper surface have good soldering tin.Hot air dries quickly after washing.
   
2.Preheat and flux coating
The pretropical zone is generally an infrared heating tube about 1.2 meters long or 4 feet long. The transmission speed of the plate depends on the size, thickness and complexity of the plate.'60mil(1.5mm) board speed is generally between 4.6-9.0m /min.The surface temperature of the plate reaches between 130 degrees and 160 degrees.Preheating can effectively prevent the metal part of the preheating section from rusting or burning out because of dropping to the flux before applying the flux.
   
3.Soldering tin: the soldering tin content in the soldering tank is about 430 kg, which is 63/37 eutectic alloy. The temperature remains around 260 degrees.In order to avoid the contact between soldering tin and air and the formation of oxide scum, a layer of ethylene glycol oil is intentionally floating on the defrosting surface of soldering furnace.The rolling transmission speed of the plate is about 9.1m/min through the transmission wheel. There are three rows of up and down rollers in the tin furnace area, and the residence time is only about 2 seconds.The span between the front and rear rollers is 6 inches, and the length of the rollers is more than 24 inches.The distance between the upper and lower air knives is 15-30mil, and the moon between the upper and lower air knives and the vertical direction is 2-5 degrees tilted, which is conducive to blowing off the tin pile in the hole and the tin pile on the plate surface.
 
4.Factors related to the setting of hot air pressure: the thickness of the plate, the distance between the welding disc, the appearance of the welding disc, and the thickness of the tin dip (in vertical tin spraying, in order to prevent the scratches between the air knife and the deformed plate, the distance between the blade and the plate is quite wide, so it is easy to cause the unevenness of the tin surface of the welding disc).
   
5.Cooling and cleaning treatment: cool air is first used to blow from bottom to top on the air bed of about 1.8m, while floating the board surface, cooling the lower surface first, and then blowing from top to bottom with cold air in the bearing area of about 1.2m rotating wheel.Cleaning treatment can remove the flux residue without too much thermal shock
   
6.Horizontal tin spraying thickness can be divided into three types:
The thickness of tin can be determined by microsection: after fine polishing, the IMC thickness between copper and tin alloys can be found by microetching method. The simple preparation of microetching potion is: the volume ratio of hydrogen peroxide to ammonia 1:3 is 10-15 seconds.The thickness of the interface alloy is usually 6 inches at a time and 1.8 inches at two times.Tin spray thickness can be measured by X-ray fluorescence thickness meter.Flatness flatness of a plate is mainly plate bending (upwarping in the long direction of the bow board) and board twist (upwarping in the diagonal direction of the board).The size of the board changes
   
7.The relationship between the tin spray thickness and the air knife: the tin thickness that can be retained on the welding disc is affected by two forces.Due to the high pressure of the air knife, the thickness of tin will also be reduced at last. The surface tension of the welding disc with small shape is usually relatively large, which can withstand the pushing and scraping of the hot air knife, so the thick tin can be left behind.The larger shape of the welding disc, with less surface tension, the hot air knife will scrape off more tin, leaving only a small tin crown cresb at the end of the welding disc.
  
8.Tin thickness on the through-hole wall: the heat sink effect of a heat sink of a heat dissipation seat will be caused if the inner flat ring is introduced or extended on the hole wall, making the molten tin on the spray easier to be cooled and solidified and the solid tin layer thicker.The tin thickness at the corner of the hole is about 0.75 microns and 30 micrometers.The reduction of pore diameter is about 18 to 30 microns, the most significant reduction is in the center of the hole, where the tin layer is the thickest.
   
9.IMC, Flatness, and the board size change: the thickness of the IMC a tin to 6 inches, the three data is the inspection level suitable is the best tool to spray tin temperature curve, the variation of all related with the temperature, good eta phase of Cu6Sn5 IMC namely, soldering performance good, malignant epsolon Cu3Sn phase, good pretreatment is conducive to good alloy layer generated;The Cu3Sn of the malignant epsolon phase is positively correlated with the time of tin spraying and the thickness of tin spraying.
The flatness of the plate is mainly affected by the following factors: a.C. how much water does class absorb from the air;Tin boards before three hours bake at 100 degrees, good dimensional stability, baking volatilization of volatiles of inside can remove half;The tin should be baked before spraying, and the aging plate should be baked before assembling, so as to reduce the generation of blow hole when the air out through hole will blow hole.
   
10.Comparison of horizontal and vertical IMC thickness
Project horizontal tin spraying vertical tin spraying
6- 8 inches at a time 12.6-17.7 inches
Micron 0.32-0.447 0.152-0.27