SMT Soldering Handbook surface mount technology 2nd phần 8 pptx

job:LAY07 page:4 colour:1 black–text

Figure 7.1 Angular accuracy of placement

Positional accuracy

With fine-pitch lead spacing, the width of the footprints is half their distance.

Fine-pitch below 0.5 mm/20 mil and ultrafine-pitch with 0.25 mm/10 mil spacing

have become real demands. This means footprints 0.125 mm/5 mil wide, and lateral

placement accuracies of ±0.06 mm/2.5 mil. Pick-and-place equipment with a

placement accuracy of ±0.05 mm/2 mil, and a repeatability of ±0.02 mm/0.8 mil is

commercially available.

Angular accuracy of placement also matters. With a QFP of dimensions 25 mm/

1 in
25 mm/1 in, an angular twist of 1° means a lateral displacement of 0.22 mm/

8 mil at every corner. As a result, with a fine-pitch layout, about half the legs would

sit on the wrong footprint (Figure 7.1). Therefore, with large fine-pitch compo￾nents, angular placement accuracy has to move into the ±0.1° bracket.

Because BGAs and flip-chips are able to correct even massive misplacement by

self-alignment as soon as the solder melts (see section 2.2), the accuracy of their

placement is less critical.

Component identity and functionality checks

The number of placement errors is a measure of the reliability of a placement

system. Until not so long ago, wrong-value chips and melfs in blistertape or bulk

packages contributed significantly to manufacturing reject rates. Smart placement

systems, which detect and correct such errors during placement ‘on-the-fly’

brought a drastic improvement and have become a common feature with most

automatic placement machines.

252 Component placement

job:LAY07 page:5 colour:1 black–text

7.3 Placement options

A user must decide between two strategies of component placement: on the one

side are the manual and semi-automatic manual methods, on the other the fully

automatic ones, which also fall into two categories, the sequential and the simulta￾neous systems. The choice between them depends on several factors:

1. A newcomer to SMD technology who operates on a small-to-medium scale

will tend to opt for a manual or semi-automatic system, unless he is part of an

organization where in-house know-how and technical assistance with fully

automatic systems are available.

2. The type of product and the volume of production are crucial factors. If there

are no more than about 50, at most 100 components, on a board, however

complex their function and the layout, and if the number of boards does not

exceed a few hundred per working day, a manual, probably semi-automatic

placement system may well be the best choice.

As the number of boards to be processed per day rises, the cost effectiveness of

purely manual placement soon drops. Semi-automatic placement reaches its maxi￾mum cost efficiency in the middle range of production volume, particularly where

full-time working is not always guaranteed.

A further factor which affects the choice of system is the product mix: if the

boards are all customer-specific boards, each with a short or unpredictable length of

run, and if production must be flexible and capable of coping with frequent

changes, semi-automatic manual placement may be best. It is worth noting, though,

that recent years have seen the arrival of several fully automatic pick-and-place

machines of great flexibility, with facilities for a rapid change-over from one

working program to another.

In the last resort, the size of the necessary investment must be decisive. Naturally,

manual and semi-automatic equipment is cheaper, and writing it off is less of a

problem when faced with a fluctuating and highly differentiated demand. On the

other hand, where one or a number of soldering lines must be fed with assembled

boards, without the risk of costly interruptions, one or more fully automatic

pick-and-place installations are the best, if not the only choice.

7.3.1 Fully manual placement

Like every placement system, placing SMDs by hand involves two steps: finding and

fetching the component, and then putting it down on its footprints, having rotated

it into its correct orientation (Figure 7.2). With boards to be wavesoldered,

placement is preceded by putting down the spots of adhesive with a handheld

syringe or by hand stencilling. The footprints of boards to be reflowsoldered are

provided with solder paste, again from a syringe, a metering dispenser, or by manual

stencilling.

As with all manually operated processes, good ergonomic design of the work

place is the precondition to get the best possible results with a minimum of errors.

Even so, manual placement without any additional aids like assigning to each type of

Component placement 253

job:LAY07 page:6 colour:1 black–text

Figure 7.2 The basic tasks of manual placement

component its proper set of footprints, needs good housekeeping and unremitting

concentration. It should be practised only for assembling small numbers of proto￾type boards, or very simple assemblies with few types of components.

Even with entirely manual placement, no component should ever be touched

with bare fingers. As has been said several times already, however clean fingers are,

they will transfer fatty acids and salts to the components and their soldering surfaces.

This affects their solderability, especially if adhesive joints must be cured before

soldering. The curing heat greatly intensifies the damaging effect of any surface

contamination on a soldering surface.

Tweezers can be used for handling the components, but a vacuum pipette with a

finger-actuated rotatable head is much more convenient. The pipette may be

handheld, or mounted on a gantry which is operated by a ‘joystick’. With both, the

accuracy of placement depends on the normally very high degree of coordination

between the human eye and the human hand. Most operators, with some training,

have no problem in putting down components on fine-pitch footprints without

smudging the solder paste. It may be worth remarking here that, so far, no vendor

seems to have found it worth his while to provide manual placement equipment

which can be converted for left-handed operators.

With manual placement, there are three kinds of possible errors: picking the

wrong component, putting it in the wrong place and, with active components,

placing it the wrong way round. Every manual placement system, however well

conceived, which uses a bulk feed system of loose melfs or chips contains one

further, and dangerous, source of placement error: one or more stray components

may have found their way into the wrong compartment of a carousel or feeder box.

If such an error is noticed, or maybe only suspected, it may be simpler and cheaper

to discard the contents of the whole compartment than to try to find the rogue

components amongst several hundred correct ones.

It is important to allow the operator regular intervals of rest, at least five to ten

minutes every hour. It has been found that, depending on the complexity of the

board and the number of component types, the error rate rises rapidly with less than

that amount of rest time. With ten to twenty components per board and not more

254 Component placement

job:LAY07 page:7 colour:1 black–text

than ten different types of components, placement rates of up to 500 or 600

components per hour can be realized with purely manual placing. This can drop to

300/hour with more complex boards.

Components may be picked from a turntable (carousel) which is subdivided into

a number of compartments holding loose components, from a row of horizontal

feeders, which present the components from the open ends of blistertapes, from

stick magazines or waffle trays.

With purely manual placement, the error rate may drop below 0.1%, i.e.

1000 ppm. But errors there will be, and in order to avoid expensive rework it is

strongly advisable to inspect every board for correct placement before soldering it.

Corrections are made by lifting off the offending component. Melfs and chips,

unless valuable, are best discarded. SOs and gullwing-legged components can be

re-used, after the legs have been cleaned with isopropanol. The footprints are wiped

clean of solder paste with a small piece of cotton or linen soaked with isopropanol.

Dots of adhesive are best left alone, lest they be smeared over a footprint, which will

become unsolderable. Adhesive vendors may be able to recommend or supply a

suitable solvent to clear off an adhesive spot. The place having been cleaned up,

fresh solder paste (or adhesive) is put down and the component is replaced.

7.3.2 Semi-automatic placement

Semi-automatic placement machines take over some of the tasks of manual place￾ment. These are principally those where human error could creep in, such as

picking the right component and putting it in its correct place. Moving the

components from their feeders to their footprints, and putting them down with the

necessary precision, is still left to the sensory and muscular feedback system between

the human eye and hand, a system which can be replicated by electromechanical

and opto-electronic means only at great expense.

All semi-automatic manual placement machines are linked with a computer

system, which can be either integral to the machine or operated by a separate PC.

Such a system can be programmed to indicate by an LED or a spot of light the feeder

from which the next component must be fetched. At the same time, the place

where it has to be put down is illuminated by a beam of light, or indicated in some

other way.

These functions can be refined and added to. Some semi-automatic placement

machines make only the correct feeder accessible, while the others are covered.

Feeders are mechanized to automatically present the next component after the

preceding one has been collected.The vacuum pipettewhich handlesthe component

can be mounted on a traversing mechanism which first guides it to the correct feeder,

and then to the correct location above its placement position. The operator then

lowers the pipette and guides it to its exact position. As the component touches

down, the vacuum in the pipette may be released automatically. Finally, the

controlling computer can be programmed to work out the most economical

placement sequenceto savetime-wastingmovements.Depending onthe complexity

of the board and the mix of components, the capacity of semi-automatic placement

systems with full computer support can rise to about 900 components per hour.

Component placement 255