IPC-SM-782A 表面安装设计和焊盘设计标准(带BGA).pdf - 第64页

7.4 Soldering Like the selection of autoplacement machines, the soldering process selection depends upon the type of components to be soldered and whether or not they will be used in combination with leaded parts. For ex…

Sequential placement equipment (Figure 7–6) typically uti-

lizes a software controlled X-Y moving table system. Com-

ponents are individually placed on the printed board in

succession. Typical cycle times vary from 0.3 to 1.8 sec-

onds per component.

Sequential/simultaneous placement equipment (Figure 7–7)

features a software controlled X-Y moving table system.

Components are individually placed on the printed board

from multiple heads in succession. Simultaneous ﬁring of

heads is possible. Typical cycle times vary around 0.2 sec-

onds per component.

There are many autoplacement machines available in each

of the four categories. One must establish guidelines for

selection of a machine. For example, what kind of parts are

to be handled? Will they come in bulk, magazine, or on a

tape? Can the machine accommodate future changes in

tape sizes?

Selection and evaluation of tapes from various vendors for

compatibility with the selected machine is very important.

The off-line programming, teach mode, and edit capability

along with computer aided design/computer aided manu-

facture (CAD/CAM) compatibility may be very desirable,

especially if a company has already developed a CAD/

CAM data base. Special features such as adhesive applica-

tion, component testing, board handling, and reserve capa-

bility for further expansion in a machine may be of special

interest for many applications. Reliability, accuracy of

placement, and easy maintenance are important to all users.

IPC-782-7-3

Figure 7–3 Typical process ﬂow for mixed technology type 2c (complex) surface mount technology

''' '

''''

'''' '

'''

Solder Paste

Dry

Paste**

Reflow

Solder

Place

Surface Mt.

Components

Clean*

Clinch Leaded

Thru-Hole

Components

Insert

Thru-Hole

Components

Invert

Board

Invert

Board

Wave

Solder

Apply

Adhesive

Cure

Adhesive

Place

Surface Mt.

Components

Clean* Test

*Optional depending on flux and cleanliness requirements. If no flux is used for solder paste

and/or wave soldering, cleaning and cleanliness test may be omitted.

**Typically used for vapor phase soldering.

IPC-782-7-4

Figure 7–4 In-line placement equipment

•

Moving board/fixed head

•

Each head places one component

•

1.8 to 4.5 seconds/board

IPC-782-7-5

Figure 7–5 Simultaneous placement equipment

•

Fixed table/head

•

All components placed simultaneously

•

Seven to ten seconds/board

▼

December 1999 IPC-SM-782A

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7.4 Soldering

Like the selection of autoplacement

machines, the soldering process selection depends upon the

type of components to be soldered and whether or not they

will be used in combination with leaded parts. For

example, if all components are surface mount types, reﬂow

method (vapor phase, hot air convection or infrared) may

be desirable. However, for through-hole and surface mount

combinations, in mixed technology, a combination of wave

soldering and reﬂow soldering may be used. No process is

best for all soldering tasks. In addition, the number of sol-

dering processes discussed in the following text are by no

means complete.

7.4.1 Wave Soldering Wave soldering is an economical

method of soldering mass terminations. There are four

main process variables that must be controlled in the wave

soldering process: preheat, ﬂuxing, speed, and solder wave.

In preheat, allowance in the conveyer system must be made

for the thermal expansion of the board during preheating

and soldering to prevent board warpage.

In ﬂuxing, ﬂux density, activity and ﬂux foam/wave height

must be closely monitored. A system must be in place to

determine when the ﬂux activity has deteriorated and when

the old ﬂux must be replaced and the new ﬂux added.

Speed is the time sequence and duration of all of the steps

in soldering. By controlling the speed, more uniform and

better joints result. In controlling the conveyer speed, pre-

heating a packaging and interconnecting assembly in two

or three stages minimizes the thermal shock damage to the

assembly and improves its service life. Uniform preheating

is achieved by developing a solder schedule that speciﬁes

preheat settings and conveyer speed for each type of board.

The solder wave is an important variable. Wave geometry

is especially important for preventing icicles and bridges

and for the proper soldering of surface mounted compo-

nents. Wave geometries include uni- and bidirectional;

single and double; rough, smooth and dead zone; oil inter-

mix, dry, and bubbled, and with or without a hot air knife.

Special solder waves just for surface mounted components

are also available.

The concern generally expressed in wave soldering of sur-

face mount devices is damage to the components when

they go through the soldering wave at 260°C [500°F]. The

maximum shift in tolerance of resistors and capacitors is

generally found to be 0.2%. This is a negligible amount

considering the part tolerance of commonly used compo-

nents is +5 to 20%. The components generally spend about

three seconds in the wave but they are designed to with-

stand soldering temperatures of 260°C [500°F] for up to 10

seconds.

In wave soldering, outgassing and solder skips are two

other main concerns. The outgassing or gas evolution

occurs on the trailing terminations of chip resistors and

capacitors. It is believed to be caused by insufficient drying

of ﬂux and can be corrected by raising the packaging and

interconnecting assembly preheat temperature or time. The

other concern, solder skips, is caused by the shadow effect

of the part body on the trailing terminations. Orienting the

part in such a way that both terminations are soldered

simultaneously solves most shadow effect problems. Some

manufacturers use an extra land to serve as a ‘‘solder

thief’’ for active components.

The most common method for solving both outgassing and

shadow effect is by switching to the dual wave system

where the ﬁrst wave is turbulent and the second wave is

IPC-782-7-6

Figure 7–6 Sequential placement equipment

•

X-Y Movement table/head

•

Components placed in succession

individually

•

0.3 to 0.8 to 4.5 seconds/board

▼

IPC-782-7-7

Figure 7–7 Sequential/Simultaneous placement equipment

•

X-Y table/fixed head

•

Sequential/simultaneous firing of heads

•

0.2 seconds/component

▼

IPC-SM-782A December 1999

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laminar. The turbulent wave serves to provide an adequate

amount of solder across the surface of the packaging and

interconnecting structure in order to help eliminate outgas-

sing and solder skips. The laminar wave is used to help

eliminate icicles and bridging.

7.4.2 Vapor Phase Soldering

Vapor phase soldering, also

known as condensation soldering, uses the latent heat of

vaporization of an inert liquid for soldering. The latent heat

is released as the vapor condenses on the part to be sol-

dered. The soldering temperature is constant and is con-

trolled by the type of ﬂuid. Thus, unlike wave, IR, convec-

tion and laser soldering, vapor phase soldering does not

require control of the heat input to the solder joints or to

the board. It heats independent of the part geometry, heats

uniformly, and does not exceed the ﬂuid boiling tempera-

ture. This process is also suitable for soldering odd-shaped

parts, ﬂexible circuits, and pins and connectors, as well as

for reﬂow of tin-lead electroplate and surface mount pack-

ages. Since heating is by condensation, the rate of tempera-

ture rise depends on the mass of the part. Therefore, the

leads on the package in contact with the packaging and

interconnecting structure heat up faster than the component

body. This may lead to wicking of the solder up the lead.

All these features make vapor phase soldering an easily

automated process. It does not necessarily require the ﬂux-

ing, preheating and soldering adjustments so critical in

other process, although prebaking and preheating is recom-

mended to remove moisture and reduce thermal shock in

the boards. Vapor phase soldering is amenable to automa-

tion but does have process related problems such as a

higher incidence of solder balls, part movement, which can

be advantageous for alignment, and damage to

temperature-sensitive parts.

Both inline and batch type systems are available. The inline

system is suitable for mass production. For low volume

production or for research and development, a batch pro-

cess is generally used. For both processes, the major disad-

vantage is price of the liquid due to vapor loss. The batch

process minimizes vapor loss by using a less expensive

secondary ﬂuid as a blanket over the primary ﬂuid. Vapor

loss does not change, but a cheap vapor is substituted for

expensive ones. The cooling coils are used to minimize

vapor loss.

7.4.3 IR Reflow

In infrared (IR) reﬂow soldering, the

radiant or convective energy is used to heat the assembly.

There are basically two types of IR reﬂow process—

focused (radiant) and nonfocused (convective). The latter is

proving more desirable for SMT. The focused IR radiates

heat directly on the parts and may unevenly heat assem-

blies. The heat input on the part may also be color depen-

dent. In nonfocused or diffused IR, the heating medium can

be air or an inert gas or simply the convection energy. A

gradual heating of the assembly is necessary to drive off

volatiles from the solder paste. After an appropriate time in

preheat, the assembly is raised to the reﬂow temperature

for soldering and then cooled.

7.4.4 Hot Air

7.4.5 Laser Reflow Soldering

Laser soldering is a rela-

tive newcomer to the soldering technology. It complements

other soldering processes rather than replacing them and,

as with in-line reﬂow soldering, it lends itself well to auto-

mation. It is faster than hand soldering but not as fast as

wave, vapor, IR soldering or hot air convection. Heat-

sensitive components that may be damaged in reﬂow pro-

cesses can be soldered by laser. Process problems include

thermal damage to surrounding areas and solder balls.

7.5 Cleaning

Cleaning of surface mount assemblies, in

general, is harder than that of conventional assemblies

because of smaller gaps between surface mount compo-

nents and the packaging and interconnecting structure and

because of the more complex solder paste residues left on

the assembly and under components. The smaller gap may

entrap ﬂux which may cause potential reliability problems

if the packaging and interconnecting assembly is not prop-

erly cleaned. Hence the cleaning process to be used is

dependent upon the spacing between component leads,

spacing between the components and substrate, the source

of the ﬂux residue and the soldering process.

Flux requiring solvent cleaning—synthetic or rosin based

ﬂuxes are generally known as synthetic activated (SA),

synthetic mildly activated (SMA), rosin activated (RA) or

rosin mildly activated (RMA). Stabilized halogenated

hydrocarbon/alcohol azeotropes are the preferred solvents

for removal of synthetic and rosin based ﬂux residues.

Flux requiring a water clean process—Companies that use

organic acid (OA) ﬂux for reﬂow or wave soldering must

ﬁlter residue from water before disposal. Residues from

OA ﬂuxes are generally removed with water.

Requirements for cleaning are dependent upon the type of

equipment as classiﬁed in Section 1.3. Some individuals do

not clean assemblies used for products; however, assembly

performance is predicated on the type of ﬂux being used to

assist in the soldering operation.

ANSI/J-STD-001 provides characteristics of various ﬂuxes

correlated to cleaning procedures. Users are cautioned to

thoroughly understand the corrosive or conductive proper-

ties of the ﬂux and ﬂux residue, before making a decision

on whether to clean or not to clean based on the end prod-

uct environment of the equipment.

7.6 Repair/Rework The repair/rework of surface mount

assemblies requires special care. Because of the small land

geometries, heat applied to the board should be minimized.

December 1999 IPC-SM-782A

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