IPC-D-279 EN.pdf - 第82页

IPC-279-B3 Figure D−3 Use of Heat Slug July 1996 IPC-D-279 71

convection. For most products using low power devices,

strategic placement of a few ICs and careful venting in a

housing or enclosure is adequate. For the products used in

a friendly environment, such as the typical air conditioned

office, one board assembly will survive indeﬁnitely. But as

the complexity and component density increases, creative

thermal management techniques must be adopted.

Transferring heat from the component body of ICs and

power regulators usually requires some form of physical

contact to a mass of material attached to the device. Using

hardware or thermally conductive adhesives, as shown in

Figure D-1 is a common practice for thermal management.

The thermal rise can be distributed to a larger surface area,

thereby keeping the component within a recommended

operating temperature range. The efficiency of this dissipa-

tion and distribution is reliant on the area of the heatsink

mass and the loss expected through the interface media.

When attachment hardware is used, a thermal compound is

applied to prevent air, moisture and contaminants from

forming between surfaces. When epoxy attachment is nec-

essary, a thermally conductive material is chosen. The most

efficient thermal conductive epoxy usually contains an

electrically conductive ﬁller. When maximum insulation is

also critical, the heat transfer efficiency from the compo-

nent to the heatsink will be reduced. Researchers are work-

ing on new compounds each year to further improve these

thermal transfer characteristics, and heat transfer tech-

niques for through hole devices are as varied as the engi-

neer’s imagination.

Care must be taken that the thermal planes do not cause

tensile loading of the solder joints.

Typically signal layers on a printed board board are etched

⁄

ounce or 1 ounce copper clad dielectric. But 2 and 3

ounce copper is also available for use on power and ground

layers. Of course, if the heat transfer is a mechanical

attachment to the chassis, the ground plane or planes would

provide a thermal conduit to transfer the heat from the

component’s body into the internal copper layer of this

substrate. When specifying the copper thickness on each

side of one dielectric layer, the layers and copper thickness

should be distributed evenly as the example in Figure D-1.

A non-symmetrical lamination may warp excessively dur-

ing reﬂow solder process.

The thermal rise expected from the components must be

determined, and if this collective temperature rise is

beyond the recommended operating temperature limits of

the device; a careful evaluation of the component grouping

is in order. Distribution of higher power devices on the

outer edge of the substrate panels will provide the neces-

sary correction to control and distribute thermal rise away

from the components. The more direct the thermal path is

away from these components the better; don’t locate high

power devices in the center of the substrate. Rather, place

them closer to the heat transfer edge leaving low power

devices in the center area. See ﬁgure D-2.

Figure D-3 describes techniques used to transfer thermal

rise of a SMT device through the outer dielectric layer of

the printed board to an internal ground plane of metal core.

Multilayer printed boards may have power and ground

planes internally layered and intermixed with signal layers.

This mass of copper planes can be an efficient thermal

conductor to transfer heat to the outer edge of the substrate

or the chassis.

IPC-279-D1

Figure D−1 Thermal Vias and Planes

IPC-279-B2

Figure D−2 Other Methods of Conductive Heat Transfer

IPC-D-279 July 1996

IPC-279-B3

Figure D−3 Use of Heat Slug

July 1996 IPC-D-279

Appendix E

Environmental Stresses

The printed board assembly starts its exposure to environ-

mental stresses during bare board fabrication, assembly and

repair/ rework before it even enters the service environ-

ment. These stresses can be thermal, chemical, mechanical

or electrical in nature. It is important to understand the

impact of environmental stresses to assure the reliability of

the assembly during its service life.

E-1.0 THERMAL

E-1.1 Effects of Rework and Repair

‘‘Touch up’’ is the

application of heat and solder to a solder joint which is

deemed cosmetically imperfect. Rework is the correction

of a defect before the SM PWA leaves the plant. Repair is

the correction of a defect found in the ﬁeld. Information on

rework and repair may be found in IPC-R-700. Each cor-

rection requires the heating of one or more solder joints

above the liquidus temperature of lead-tin eutectic solder

(183°C), typically above 210°C, and may involve the

removal and replacement of a component. Note that this

temperature of 183°C is well above several critical tem-

peratures in a SM PWA.

E-1.2 Glass Transition Temperature for Printed Boards

The glass transition temperature (T

) of most epoxy-glass

boards is ~125°C. For multi-functional epoxy glass boards,

is ~160°C. T

is greater for polyimide glass printed

boards which have a T

of 250°C. There are many other

printed board materials that fall between these two

extremes. Exceeding the T

of the printed board results in

signiﬁcant expansion of the printed board in the z axis,

hence in stresses in the barrels of plated through holes and

vias; the result can be intermittent or permanent opens. The

intermittent opens can generally be detected during tem-

perature changes of the PWA (see Appendix B).

Printed boards with a lower T

such as epoxy-glass boards

when heated locally, during component removal or compo-

nent replacement, to temperatures > T

, will tend to bulge

in the heated area. The solder joints of the replacement

component will be under tension and may fail when the

assembly cools down.

When using hot gas or IR systems to remove or replace

components, minimize printed board time at temperature.

During component removal, there should be no component

lifting or twisting during removal until solder is liquid on

all lands. This minimizes stress on the leads where they

enter the component package (with the result of cracked

plastic package body, damaged glass to metal seals), as

well as stress on the lands and their adhesive bond to the

printed board (with the result of lifted lands).

E-1.3 Intermetallic Compound Growth The copper in

the printed board conductors and component leads reacts

more rapidly with the tin in the solder to form a brittle

intermetallic compound (IMC) such as Cu

Sn and Cu

at higher temperatures. The original eutectic solder will be

transformed into a lead-rich layer. In the worst case, the

surface is exposed and oxidized; this lead oxide rendering

that surface unsolderable.

A solder joint kept at temperatures exceeding 150°C will

result in a lead-rich joint which is less ductile than one with

eutectic solder and the fatigue life of the joint will be

degraded. The IMC layer will tend to act as a shear plane

under overload conditions. These comments apply to

neighboring joints during component removal or replace-

ment and to ‘‘touch up’’ of a joint; some data indicate that

solder joints which undergo a ‘‘touch up’’ are less reliable.

If the molten lead is removed from the land by wiping, the

exposed IMC layers can rapidly oxidize and become unsol-

derable.

If the heat source is solder (wave or fountain), the interme-

tallics will be carried away and in the extreme, the copper

land will be dissolved.

When using hot gas or IR systems to remove or replace

components, keep the heat away from neighboring compo-

nents and joints. Hot air deﬂectors and IR shielding panels

may be required for component protection.

E-1.4 Glass Transition Temperature for Plastic Encap-

sulating

The T

range of most molding compounds used

in the plastic encapsulation of semiconductors and passive

networks is in the range of ~150°C to 180°C.

Exceeding the T

of the moulding compound of the com-

ponents during component removal results in internal

stresses to the metallization and passivation of the silicon

chip. This damage can hinder failure analysis of the

removed component and can also be suffered by a neigh-

boring component. Disruption of the metallization can

result in opens and dysfunction of the chip. Damage to the

passivation can result in long term corrosion of and opens

in the metal. Exceeding the T

can also cause delamination

of the molding compound from the surface of the silicon

chip or the surfaces of the leadframe; condensed moisture

can pool in the delaminated sites and lead to dendrites and

corrosion. Where the delamination coincides with bonding

pads and wire bonds, bond shear can occur.

When using hot gas or IR systems to remove or replace

components, if subsequent failure analysis is to be per-

formed on the removed component, dry the component

IPC-D-279 July 1996