IPC-SM-782A 表面安装设计和焊盘设计标准(带BGA).pdf - 第66页

There are various tools available for removing components. Resistance heating tweezers are also used for removing surface mounted components. V arious types of hot air/gas and IR systems are also used for removing surfac…

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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There are various tools available for removing components.

Resistance heating tweezers are also used for removing

surface mounted components. Various types of hot air/gas

and IR systems are also used for removing surface

mounted components. One of the main issues when using

hot air/gas devices is preventing damage to adjacent com-

ponents.

No matter which system tool is used, all the controlling

desoldering/soldering variables, such as number of times a

component can be removed and replaced, desoldering tem-

perature and time, and damage to the packaging/

interconnection assembly, need to be addressed.

IPC-SM-782A December 1999

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Land Pattern Details

The information in the following sections is intended to represent requirements for families of surface-mountable compo-

nents that have been standardized by standard developers throughout the world (e.g., EIA, JEDEC, and EIAJ). The prin-

ciples used in the data embody the concepts of ‘‘design for manufacturability,’’ and specify criteria for analysis or modiﬁ-

cation in order to make the manufacturing process as robust as possible.

Each component family type has been organized into a four-page set that has its own number identiﬁcation (a paragraph

number of the mother document—IPC-SM-782) and its own revision status. As changes are necessary (the addition or dele-

tion of information, or corrections to modify the text or numerical data), a four-page signature(s) will be reissued as a set

for replacement in this handbook.

A summary of the contents with revision status is provided at the conclusion of this explanation.

Introduction

Page 1 of each data set (four-page signature) provides general information about the part, its usage or performance require-

ments, and techniques for handling the attachment of the component. Any useful information about the particular compo-

nent type is detailed on this introductory page.

Component Dimensions

Page 2 of each data set describes the critical component dimensions necessary to make judgements for reliable mounting

recommendations. The standards organizations provide many more dimensions to deﬁne the requirements for manufacturing

the speciﬁc components in a family class; only those dimension that are necessary for land pattern development are repeated

on page 2 of each data set.

Every attempt has been made to check and correlate the dimensions used against the published standard. Unfortunately, the

standards do not always provide adequate information, or full disclosures of maximum and least material conditions of the

component dimensions used in land pattern development. At times, the numbers shown in the data set have been enhanced

(nominal dimensions converted to maximum/minimum dimensions), at times the numbers shown have been derived (termi-

nal dimensions subtracted from an overall dimension with their inherent tolerances considered in an RMS condition), and

at other times the dimensions have been tailored (total tolerances spread between maximum/minimum limits reduced to a

reasonable amount) in order to facilitate a producible land pattern and assembly.

Users are encouraged to check with their component suppliers in order to ascertain that the suppliers certify to the numbers

used on page 2 in order to use the registered land patterns on page 3. In the event of disparities, if they are component-

supplier speciﬁc, users are encouraged to modify their use of the land patterns shown on page 3 in accordance with the prin-

ciples outlined in Section 3.3 on the registered land patterns dimensioning system. If the disparities are industry speciﬁc,

please inform the IPC using the form at the end of this section to initiate a data set change.

Land Pattern Dimensions

Page 3 of the data set provides the details for the land pattern. Since the design concepts are dedicated to establishing the

most favorable solder joint conditions, all land pattern dimensions are shown at maximum material condition (MMC). This

is the ‘‘target value’’ for the board manufacturer; moving away from MMC to the least material condition (LMC) reduces

the opportunity for formation of the optimum solder ﬁllet. The LMC dimensions should not exceed the fabrication (F)

allowance shown on page 4. The LMC and the MMC provide the limits for each dimension.

In addition to specifying maximum and minimum limits, the resultant land width is provided as a reference (dimension

‘‘Y’’). Designers are encouraged to incorporate the MMC conditions into their library symbols, and when sending electronic

data to their board manufacturer they should identify that the data is at MMC. Specifying minimum conductor width as a

manufacturing goal should be avoided, since the manufacturer is compensating and scaling data to accommodate process

allowances in the phototool, and is concentrating on meeting minimum requirements. This is opposite to the desire to have

robust land patterns which should be at maximum material conditions.

Page 3 also provides an area around the land pattern known as the ‘‘grid courtyard.’’ The description is in the 0.05 mm ele-

ments of the international grid. An area described as 4x6 is equal to 2.00 mm by 3.00 mm. When placing parts on a printed

board, the highest density is when one courtyard touches another. Courtyards are intended to encompass the land pattern

and a component body that is centered on the land pattern.

December 1999 IPC-SM-782A

电子技术应用　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ