SMOBC&HAL is one of the most common surface coating methods used for board surface treatment. It is widely used in the production of circuits. The quality of sprayed tin directly affects the quality of welding soldering during subsequent customer production. And solderability; therefore the quality of sprayed tin becomes a key point for the quality control of circuit board manufacturers;
There are currently two kinds of spray tin: vertical spray tin and horizontal tin spray;
The main role of spray tin:
1 Prevention of bare copper surface oxidation;
2 maintain solderability;
Other methods of surface treatment include: hot melt, organic protective film OSP, chemical tin, chemical silver, chemical nickel gold, nickel plating gold, etc.;
Vertical spray mainly has the following disadvantages:
1 The board is unevenly heated from top to bottom, and the board is bent in and out first.
2 The tin thickness on the pad is uneven. Due to the blowing force of the hot air and the force of gravity, solder sinkers are generated at the lower edge of the pad, which makes the soldering of the SMT surface mount component less stable and tends to result in partial deviation of the soldered component. Move or erect phenomenon tombstoning
3 The contact time between the pad on the bare copper board and the hole wall and the solder is longer, generally more than 6 seconds. The amount of copper dissolved increases rapidly in the solder furnace. The increase of the copper content directly affects the solderability of the pad because of the The thickness of the IMC alloy layer is too thick, so that the shelf life of the board is greatly shortened;
Horizontal spraying of tin greatly overcomes the above drawbacks. Compared with vertical spraying of tin, it has the following advantages:
1 The contact time between tin and bare copper is short, about 2 seconds, IMC is thin, and the shelf life is longer;
2 Tinning time is short wettingtime, about 1 second;
3 The board is heated evenly, mechanical properties are kept well, and the board is less warped;
Horizontal spray tin process flow:
Pre-cleaning process---Preheating---Flux application---Horizontal spray tin—Heat scraper scraping tin—Cooling—Post-cleaning treatment
1. Pre-cleaning treatment:
Mainly micro-etching copper surface cleaning, micro-etching depth is generally 0.75-1.0 microns, while removing the attached organic contaminants, so that the copper surface is truly clean, and effective contact with the molten tin, and the rapid generation of IMC; uniform micro-etching Will make the copper surface with good solderability; hot air blows dry after washing;
2. Preheat and flux coating
Pre-tropical 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; '60mil (1.5mm) board speed is generally 4.6-9.0m/min Between the surface temperature of 130-160 degrees for flux coating, double-sided coating, hydrochloric acid can be used as an activating flux; preheating can effectively prevent the preheating section of the metal before flux coating Part of it will not rust or burn out due to dripping flux;
3. Tin soldering: tin bath containing about 430 kilograms of tin, 63/37 eutectic solder alloy composed of eutectic temperature is maintained at around 260 degrees;
In order to avoid the contact of solder with air and cause oxidation scum, a layer of glycol oil is intentionally floated on the surface of molten solder in the soldering furnace. The oil should be considered compatible with the flux compatibility; the board passes the transfer wheel. The rolling speed is about 9.1m/min. There are three rows of up and down rollers in the tin furnace area. The dwell 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 plate can be processed. The upper limit is 24 inches; the air knife is blown up and down, the distance between the upper and lower air knifes is 15-30 mil, and the air knife is inclined by 2-5 degrees from the vertical direction in the month, which is beneficial to blow away the tin in the hole and the tin pile on the board surface. ;
4. Factors related to hot air pressure setting: board thickness, pad spacing, pad shape, tin thickness (in order to prevent air knife and deformed board from scratching in vertical spray tin, air knife and board surface The distance between them is quite wide, so it is easy to cause unevenness on the tin surface of the pad.)
5. Cooling and post-cleaning treatment: firstly blow cold air on the air bed of about 1.8 meters from top to bottom, float the board surface, cool the lower surface first, and continue to blow cold air from top to bottom in the load bearing area of ​​about 1.2 meters; The cleaning process removes the flux residue while also not bringing too much thermal shock to the thermalshock
6. The thickness of horizontal spray tin is divided into three types:
2.54mm (100mil), 5.08mm (200mil), 7.62mm (300mil). Tin thickness can be determined by microslicing: after fine polishing, the IMC thickness between the copper-tin alloys is found by microetching, and the simple preparation of the microetching liquid is performed. : The volume ratio of hydrogen peroxide to ammonia 1:3 is 10-15 seconds; the thickness of the interface alloy is usually 6 microinches at one time and about 1.8 microinches at 2 times; the thickness of sprayed tin can be measured with x-ray fluorescence. Thickness measurement; flatness of the board is mainly flat (bow board long direction tilt) and tilt (twist, board diagonal direction tilt); board size changes
7. The relationship between spray thickness and air knife: The tin thickness retained on the pad is affected by two kinds of force factors: a. The surface tension surface tension determines the tin thickness after the final balance, when the area of ​​the pad is large, the cured The thickness of tin is also higher b. The pressure of the air knife; air knife pressure, the final tin thickness will also be reduced, the smaller the surface of the pad is usually relatively large surface tension, can withstand hot air knife pushing, Therefore, thicker solder can be left; a larger-sized pad has less surface tension, and the hot air knife scrapes more tin, leaving only a small tin crown cresb at the end of the pad;
8. Thickness of tin on the wall of the through hole: If the hole wall is led out or extended by the inner flat ring, it will cause a heat sink effect of the heat sink, making it easier for the molten tin to be sprayed to cool and solidify, and the solid tin layer is thicker. Generally, there is no hole inside. There is no apparent correlation between the thickness of tin that can be maintained in the through hole of the plated through hole and the aspect ratio of the through hole; the thickness of the tin at the corner of the hole is about 0.75 micron and 30 micro inches, and the corners of the hole are turned from the two ends of the hole to the center of the hole. The thickness of the tin increases gradually; the reduction of the pore diameter is about 18-30 micrometers, and the center of the pore is reduced most significantly, and the tin layer is the thickest at this point;
9. IMC, Flatness, and board size change: The thickness of the IMC sprayed once is 6 microinches. These three data are the best tools for checking the suitability of the horizontal spray temperature curve. The variation of the three are all related to the temperature. The IMC ie etaphase Cu6Sn5, good solderability, malignant epsolonphase of Cu3Sn, good pretreatment favors the formation of a good alloy layer; the malignant epsolonphase of Cu3Sn, is positively related to the time of tin spraying, and the thickness of spray tin;
The flatness of the board is mainly affected by the following factors: a. The symmetry of the board thickness and the arrangement of the layers, b. The distribution of conductor lines on the board surface is uniform; c. How much moisture is absorbed by the air from the air; Before the spray of tin The board is baked at 100 degrees for 3 hours. The dimensional stability is good. Baking can drive out all kinds of volatiles in half.
To bake before spraying, the baking board must be baked before the aging board is assembled, so as to reduce the blowhole blowhole generated by the outlet of the through hole;
10. Comparison of horizontal and vertical IMC thickness
Project Horizontal Spray Tin Vertical Spray Tin
Once 6---8 microinches 12.6—17.7 microinches
Micron 0.32---0.4470.152—0.27
There are currently two kinds of spray tin: vertical spray tin and horizontal tin spray;
The main role of spray tin:
1 Prevention of bare copper surface oxidation;
2 maintain solderability;
Other methods of surface treatment include: hot melt, organic protective film OSP, chemical tin, chemical silver, chemical nickel gold, nickel plating gold, etc.;
Vertical spray mainly has the following disadvantages:
1 The board is unevenly heated from top to bottom, and the board is bent in and out first.
2 The tin thickness on the pad is uneven. Due to the blowing force of the hot air and the force of gravity, solder sinkers are generated at the lower edge of the pad, which makes the soldering of the SMT surface mount component less stable and tends to result in partial deviation of the soldered component. Move or erect phenomenon tombstoning
3 The contact time between the pad on the bare copper board and the hole wall and the solder is longer, generally more than 6 seconds. The amount of copper dissolved increases rapidly in the solder furnace. The increase of the copper content directly affects the solderability of the pad because of the The thickness of the IMC alloy layer is too thick, so that the shelf life of the board is greatly shortened;
Horizontal spraying of tin greatly overcomes the above drawbacks. Compared with vertical spraying of tin, it has the following advantages:
1 The contact time between tin and bare copper is short, about 2 seconds, IMC is thin, and the shelf life is longer;
2 Tinning time is short wettingtime, about 1 second;
3 The board is heated evenly, mechanical properties are kept well, and the board is less warped;
Horizontal spray tin process flow:
Pre-cleaning process---Preheating---Flux application---Horizontal spray tin—Heat scraper scraping tin—Cooling—Post-cleaning treatment
1. Pre-cleaning treatment:
Mainly micro-etching copper surface cleaning, micro-etching depth is generally 0.75-1.0 microns, while removing the attached organic contaminants, so that the copper surface is truly clean, and effective contact with the molten tin, and the rapid generation of IMC; uniform micro-etching Will make the copper surface with good solderability; hot air blows dry after washing;
2. Preheat and flux coating
Pre-tropical 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; '60mil (1.5mm) board speed is generally 4.6-9.0m/min Between the surface temperature of 130-160 degrees for flux coating, double-sided coating, hydrochloric acid can be used as an activating flux; preheating can effectively prevent the preheating section of the metal before flux coating Part of it will not rust or burn out due to dripping flux;
3. Tin soldering: tin bath containing about 430 kilograms of tin, 63/37 eutectic solder alloy composed of eutectic temperature is maintained at around 260 degrees;
In order to avoid the contact of solder with air and cause oxidation scum, a layer of glycol oil is intentionally floated on the surface of molten solder in the soldering furnace. The oil should be considered compatible with the flux compatibility; the board passes the transfer wheel. The rolling speed is about 9.1m/min. There are three rows of up and down rollers in the tin furnace area. The dwell 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 plate can be processed. The upper limit is 24 inches; the air knife is blown up and down, the distance between the upper and lower air knifes is 15-30 mil, and the air knife is inclined by 2-5 degrees from the vertical direction in the month, which is beneficial to blow away the tin in the hole and the tin pile on the board surface. ;
4. Factors related to hot air pressure setting: board thickness, pad spacing, pad shape, tin thickness (in order to prevent air knife and deformed board from scratching in vertical spray tin, air knife and board surface The distance between them is quite wide, so it is easy to cause unevenness on the tin surface of the pad.)
5. Cooling and post-cleaning treatment: firstly blow cold air on the air bed of about 1.8 meters from top to bottom, float the board surface, cool the lower surface first, and continue to blow cold air from top to bottom in the load bearing area of ​​about 1.2 meters; The cleaning process removes the flux residue while also not bringing too much thermal shock to the thermalshock
6. The thickness of horizontal spray tin is divided into three types:
2.54mm (100mil), 5.08mm (200mil), 7.62mm (300mil). Tin thickness can be determined by microslicing: after fine polishing, the IMC thickness between the copper-tin alloys is found by microetching, and the simple preparation of the microetching liquid is performed. : The volume ratio of hydrogen peroxide to ammonia 1:3 is 10-15 seconds; the thickness of the interface alloy is usually 6 microinches at one time and about 1.8 microinches at 2 times; the thickness of sprayed tin can be measured with x-ray fluorescence. Thickness measurement; flatness of the board is mainly flat (bow board long direction tilt) and tilt (twist, board diagonal direction tilt); board size changes
7. The relationship between spray thickness and air knife: The tin thickness retained on the pad is affected by two kinds of force factors: a. The surface tension surface tension determines the tin thickness after the final balance, when the area of ​​the pad is large, the cured The thickness of tin is also higher b. The pressure of the air knife; air knife pressure, the final tin thickness will also be reduced, the smaller the surface of the pad is usually relatively large surface tension, can withstand hot air knife pushing, Therefore, thicker solder can be left; a larger-sized pad has less surface tension, and the hot air knife scrapes more tin, leaving only a small tin crown cresb at the end of the pad;
8. Thickness of tin on the wall of the through hole: If the hole wall is led out or extended by the inner flat ring, it will cause a heat sink effect of the heat sink, making it easier for the molten tin to be sprayed to cool and solidify, and the solid tin layer is thicker. Generally, there is no hole inside. There is no apparent correlation between the thickness of tin that can be maintained in the through hole of the plated through hole and the aspect ratio of the through hole; the thickness of the tin at the corner of the hole is about 0.75 micron and 30 micro inches, and the corners of the hole are turned from the two ends of the hole to the center of the hole. The thickness of the tin increases gradually; the reduction of the pore diameter is about 18-30 micrometers, and the center of the pore is reduced most significantly, and the tin layer is the thickest at this point;
9. IMC, Flatness, and board size change: The thickness of the IMC sprayed once is 6 microinches. These three data are the best tools for checking the suitability of the horizontal spray temperature curve. The variation of the three are all related to the temperature. The IMC ie etaphase Cu6Sn5, good solderability, malignant epsolonphase of Cu3Sn, good pretreatment favors the formation of a good alloy layer; the malignant epsolonphase of Cu3Sn, is positively related to the time of tin spraying, and the thickness of spray tin;
The flatness of the board is mainly affected by the following factors: a. The symmetry of the board thickness and the arrangement of the layers, b. The distribution of conductor lines on the board surface is uniform; c. How much moisture is absorbed by the air from the air; Before the spray of tin The board is baked at 100 degrees for 3 hours. The dimensional stability is good. Baking can drive out all kinds of volatiles in half.
To bake before spraying, the baking board must be baked before the aging board is assembled, so as to reduce the blowhole blowhole generated by the outlet of the through hole;
10. Comparison of horizontal and vertical IMC thickness
Project Horizontal Spray Tin Vertical Spray Tin
Once 6---8 microinches 12.6—17.7 microinches
Micron 0.32---0.4470.152—0.27
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