BGA welding process requirements

BGA welding process requirements

In the assembly process of BGA, every step, every tool will have an impact on the welding of BGA.

1.Solder paste printing

The solder paste is a important part of SMT production. Selection of solder paste will normally consider the

following aspects: good printability good weldability, low residue can be good weldability. Generally speaking,

we use the alloy composition for low residue paste type paste containing 63% tin and lead 37%.

Table 3 shows how to select the appropriate components of the pin spacing of solder paste. From the table we

can see that the components of the lead tin solder paste show ingenuity, a smaller, relatively good hair is printed.

But is not to say that the choice of solder paste, tin star, the smaller the better, because from welding effect, tin

powder particle paste soldering effect to smaller than the particles of tin solder paste is good. Therefore, we

should consider the factors synthetically in the selection. Due to the smaller pin BGA, the template mesh to open

small hole, so we use the diameter below the 45m solder paste, in order to guarantee good printing effect.

Table 3 solder tin powder shape and particle diameter

Printed wire mesh template is generally used in stainless steel material. Because of the smaller pin spacing of

BGA components, the thickness of the steel plate is thin. General steel plate thickness is 0.12MM-0.15MM.

The opening parts of the steel plate are determined, and the opening of the steel plate is usually less than

that of the pad.

For example: size 35MM, 1.0MM PBGA between pin show ingenuity, welding rib diameter is 23MIL.

We generally control the opening of the steel plate in the size of the 21MIL.

In printing, the 60 degree metal scraper is usually made of stainless steel. Print pressure control has a range of

3.5KG-10KG. The pressure is too large and too small to be printed. Printing speed control in the

10MM/SEC-25MM/SEC, the smaller the pin spacing between components, the slower the printing speed.

Print out the speed of the general set to 1MM/SEC, if it is BGA u or CSP device release rate should be more

slowly about 0.5MM/SEC. In addition, in the printing press to pay attention to control the operation of the

environment. The temperature control of the working field is about 250C, and the temperature is controlled

at 55%RH. After printing the PCB as far as possible in less than half an hour into the reflow solder paste,

to prevent exposure too long to affect the air quality.

2.Device placement

Accurate placement of BGA depends on the accuracy of the placement machine, as well as the recognition ability

of the image recognition system. On the current market, a variety of brands of multi function chip, can be placed

in the BGA of the placement of the accuracy of its chip to reach about 0.001MM, so there is no problem in the

placement accuracy. As long as the BGA device through the mirror image recognition, it can be accurately placed

on the printed circuit board.

However, sometimes the BGA is not 100% of the Z of the welding of the device, it is possible that the direction of

the ball is slightly smaller than the other. In order to ensure the good weldability, we usually can the BGA device

thickness minus 1-2MM, also with delay in closed vacuum system about 400 msec, the BGA device placed in the

solder ball can fully contact with the solder paste. This way can reduce the phenomenon of the BGA of a pin empty.

However, we do not recommend the use of the BGA u and CSP devices to prevent the occurrence of welding defects.

3.Reflow soldering

Reflow soldering is the most difficult step in the process of BGA assembly. Therefore, the key to get good welding of

BGA is to get a better return air line.

A.Preheating stage

During this time the PCB uniform heating temperature, and stimulate the active flux. General warming of the speed is

not too fast, to prevent the line arc heated and produce a large deformation. We try to temperature control in the

following 30C/SEC, the ideal temperature for 20C/SEC. Time control in 60-90 seconds.

B.Infiltration stage

This stage started volatile flux. The temperature should be 60-120 seconds in between 1500C-1800C, so that the flux

can play its role fully. The heating rate is generally 0.3-0.50C/SEC.

C.Return phase

This phase of the temperature has exceeded the melting point of the solder paste, solder paste is melted into liquid,

component lead tin. In this stage, the temperature in the 1830C should be controlled between 60-90 seconds. If the

time is too small or too long will cause welding quality problems. The temperature in the 210-2200C range of time

control is the key, the general control in 10-20 seconds for the best.

D.Cooling stage

This phase solder solidifies, the components are fixed on a circuit board. The same is the speed of cooling can not be

too fast, the general control in the 40C/SEC, the lower the temperature is 30C/SEC. Due to the fast cooling rate will

cause the circuit board to produce cold deformation, it will BGA weld quality problems caused by, especially the pins

of the outer ring of the BGA weld.

In the measurement of the temperature curve of reflow soldering, the measuring point for the BGA element should be

between the BGA pin and the circuit board. BGA try not to use high temperature adhesive tape, and the use of high

temperature solder welding and thermocouple fixed, in order to ensure access to more accurate curve data.

In short, BGA welding is a very complicated process, it is also subject to the circuit board design, equipment capacity

and other aspects of the impact, if only to take into account a certain aspect is not enough. We also need to study and

explore in the actual production process, and strive to control the factors that affect the BGA welding, so that the welding

can achieve the best results.