KE-750_MAINTE.pdf - 第95页

- 92 - For large parts, as shown in Figure 3, the pressure in side the nozzle is negative when the part is m ounted on the board, and the part is firmly held. However, for chip parts, the nozzle cannot be held above t he…

Figures 2 and 3 show the vacuum blow timing when mounting on KE-730/740.

For chip parts, the vacuum is set off 70 ms before the nozzle (Z axis) moves

down, and the part is mounted being held by the remaining pressure (negative

pressure) in the pipe. For heavy parts, the vacuum is set off 40 ms (or 20 ms)

before the downward movement is complete, and the nozzle is held for 30 ms

(or 50 ms) above the board. Then, the nozzle is moved up after vacuum

pressure is released.

Square chip parts

Z axis

Vacuum

Blow

Figure 2

For Melf, aluminum electrolytic capacitor, network resistor, trimmer, SOT, SOJ, SOP, and

PLCC

Z axis

Vacuum

Blow

For QFP, TSOP, BGA, connector, and socket

Figure 3

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For large parts, as shown in Figure 3, the pressure in side the nozzle is

negative when the part is mounted on the board, and the part is firmly held.

However, for chip parts, the nozzle cannot be held above the board (otherwise

mount accuracy becomes worse), and the part is held by rather low remaining

pressure when it is immediately before mounting.

3.8.2 Improving point of KE-750/760

For KE-750/760, as described before, the part is held at low negative pressure.

In fact, the part is better to be firmly held immediately before mounting.

However, to set the pressure in the nozzle to normal room atmosphere within a

short time, blow is necessary. When a blow operation is performed on

KE-730/740, the pressure in the nozzle occasionally becomes negative and

positive. There are two reasons as follows:

① Location of air pressure related parts, and air pipe

② Blow operation timing

3.8.2.1 Location of air pressure related parts, and air pipe

Figure 4 shows the location of air pressure related parts, and air pipe on

KE-730/740. The problem is with the relation among the ejector, the blow SV,

and the nozzle. The length of the air tube between the blow SV and the

ejector is 1180 mm but it is 130 mm between the blow SV and the nozzle.

Therefore, when the blow SV is turned on and vacuum destruction is started,

the pressure at the nozzle immediately becomes from negative to normal room

atmosphere but the pressure at the ejector part does not. At this moment, if

the blow SV is turned off, the pressure at the tip of the nozzle is pulled negative

by the remaining negative pressure at the ejector. If blow is continued without

turning off the blow SV, the pressure at the nozzle becomes positive. The

direction of union Y used at the connection part of the ejector, the blow SV, and

the nozzle is also set for the blow air to easily move to the nozzle.

Ejector

Pressure sensor

Filter

Blow SV

Nozzle

Figures indicate the length of the air tube.

Figure 4 KE-730/740 Figure 5 KE-750/760

Figure 5 shows the location of the pressure related parts and the pipe on

KE-750/760. The blow SV is located close to the ejector so that the blow air

directly flows to the ejector. The direction of union Y between the blow SV and

the nozzle is also set so that the blow air hardly flows. In this construction, the

pressure at the ejector side, which is the source of negative pressure, can be

set at normal room atmosphere within the time for the pressure at the nozzle

part becomes normal room atmosphere. The pressure at the nozzle part

therefore is not pulled to negative after turning off blow.

3.8.2.2 Blow operation timing

As shown in Figure 6, vacuum and blow timing when mounting parts is

determined using four mechanism control parameters. Please note the timing

to turn off the blow SV. Condition, W Blow off [n]≦W Place Vac [n] applies to

KE-730/740.

Where:

W Blow off [n]: Time for blow to be turned off before the Z axis moves

upward.

W Place Vac [n]: Time for the Z axis to be held above the board after it

moves downward to the surface of the board.

Therefore, blow continues until the Z axis moves downward completely even if

the blow is turned off quickly. (When W Blow off [n] = W Place Vac [n]) The

blow time cannot be made long, there is almost no effect of blow. (If the blow

time is made long, the pressure at the tip of the nozzle becomes positive.)

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