Solder paste resistance is an extreme test for many novice, why do you say so? Because in the common PCB board, welding patch resistance, due to the welding plate grounding heat fast, resulting in welding difficult, for manual welding, small make up this problem has two methods to solve this problem:
1, first in the ground and then use a little inventory of welding solder, soldering iron melting solder, but the circuit board is fully preheated, on chip resistors, welding wire and welding end, the other end. Disadvantages: time and time.
2, the iron temperature is high, the usual welding, pay attention to grasp the good time. The main techniques of circuit board welding are as follows:
A, welding end all pins, with all pins to clean solder solder wetting. Suck off excess solder where needed to eliminate any short circuit and lap. Finally, check whether the tweezers weld, after the completion of the examination, removal of flux from the circuit board, the hard brush dipped in alcohol along the direction of pin carefully wipe, until the flux disappeared.
B, the circuit board welding engineer to remind you of patch resistor is relatively easy to weld, can be the first in a little tin solder, then put a head element, with tweezers element, after welding on the head, and then see if put; if it is placed, and then welded on another head. A lot of practice is needed to really master welding techniques.
C, circuit board welding engineers remind readers in welding before the pads coated with flux, again with iron, so as to avoid the adverse pad tin or oxidized, resulting in bad welding, chips generally do not need treatment.
D, carefully into the PCB PQFP chip board with forceps, be careful not to damage the pin. So that it is aligned with the pad, to ensure that the chip is placed in the right direction. The iron temperature to more than and 300 degrees Celsius, the iron tip with a small amount of solder, use tools have been holding down the alignment position of the chip, plus a small amount of flux in the two diagonal position of the pin, still holding down the chip, welding the two diagonal position of the pin, the chip is fixed and can not move. Re check the position of the chip after the welding is finished to check the position of the chip. If necessary, adjust or remove and re align the position on the PCB board.
E, start welding all pins, should add solder iron tip, all the pins apply flux to the pins to keep moist. By the end of each pin of the soldering iron tip contact chip, until you see the inflow of solder pin. To keep the iron tip and the welding pin is parallel in welding, to avoid excessive occurrence of lap solder.
This is to rely on the experience of SMT, general maintenance personnel love with a knife head of iron, tin content increase put the parts together onto the iron head, in a pad position to move another step pad. Practice more will be. One can not start to take tweezers, a head of a welding head.
If the first solder coated with a point on the circuit board and then use the tweezers chip resistors, resistance is difficult to close to the circuit board welding lead when the solder ran down the first pin, right between parts and pad position, tweezers parts can also withstand ah, flexible change, not difficult, more practice operation! What if the resistance welded on the PCB board is in the wrong position?
If the PCB board welding resistance in the wrong position, the preferred resistance to the demolition, to correct a resistance welding resistance.
Disassembly resistance method:
Method for removing chip resistor element
Tin to two pads on both ends of the resistor, and the electric iron with two heating pads, the iron head opened resistance.
Two, split vertical resistance element method
The resistance of the two pin with tin, pay close attention to the resistance with tweezers and iron while two pins and heated, melted tin with tweezers will drag out resistance. Can also be heating a foot, and then heating the other foot.
Three, open and exclusion method
Add tin on all pins, tweezers slowly pry.
A new welding resistance, first with a multimeter measuring the resistance welding, then go.