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INTRODUCTION
The electronics industry is continually challenged to
meet customers’ growing demand for smaller, high-
performance semiconductor devices. The market
demand for portable electronic products, in particular,
is driving manufacturers toward increased miniatur-
ization of components. Accommodating these smaller
packages has necessitated improvements and changes
to the methods by which components are assembled
onto a Printed Circuit Board (PCB). This, along with
increased efforts to eliminate ozone-depleting sub-
stances in all phas es of manufacturi ng, have led to the
following changes:
• Convent io nal vapor ph as e re fl ow assembly i s be ing phased out because it re quires t he use of fl uor inated
solvents.
• Reflow sys tems are being de signed to h ave for ced conve ction , whic h improve s the effi ciency and u nifor -
mity of the heat transfer process.
AMD recognizes the impact t h at t he b oard asse mb ly pro cess can h ave on the reliabi l it y of t he component s,
especia ll y the ref l ow temperat ure pr ofi le that is used (e.g ., the peak te mpe ra tur e and tempera tur e g ra dient).
The qualification and reliability testing AMD conducts on its plastic Surface-Mount Devices (SMDs)
includes simulating common board assembly techniques. This chapter provides an overview of the board
assembly methods commonly used by AMD.
PCB DESIGNS
The primary functi ons of the PCB are to pr ovi de mechanical supp ort for the co mp onen ts , provide the elec -
trical interconnection paths, and help dissipate heat. The most common substrate design is a multilayer
laminat e, with f our to six la yers be ing preval ent f or desi gns tha t acco mmodate SMDs. Design ing mo re lay -
ers in the substrate reduces the circuit density, but it increases the thickness and cost of the PCB.
PCB Materials. In choosing the substrate material, designers must consider cost, the material’s Coeffi-
cient of Thermal Expansion (CTE), the glass transition temperature, and the end-use application. (The
glass transition (Tg) temperature is the temperature above which the physical structure of the laminate
changes to a soft form and loses its mechanical strength. Tg is characteristic of polymer.) Common sub-
strate materials are shown i n the table below.
BOARD ASSEMBLY METHODS
PCB assembly methods vary depending upon the type of components being mounted. For through-hole
components, wave soldering is the popular method. For complex surface-mount devices, such as plastic
leaded chip carriers and plastic quad flat packages, reflow soldering methods are used.
Substrate Materi als
Substrate
Resin Coefficient of
Thermal Expansion (ppm/°C ) Glass Transition
Temperature (°C)
FR-4 80 to 90 130
Bismaleimide Triazine (BT) 41 180
Polyimide 50 270
Figure 3.1 Packages are typically surface-mount ed onto a prin te d
circuit board using a solder reflow assembl y method.
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Wave Soldering. The wave soldering process begins with the component leads being inserted into the
plated throu gh h oles on the board. As the as sembly passes thro ugh the wav e sol dering sy stem, all oxida tion
and contamination is cleaned from each hole barrel and component lead, and the underside of the board is
exposed to a wave of molten solder (having a maximum temperature of 240°C to 260°C). The solder is
flowed through each through hole by capillary action. As the lead tip touches the solder wave, the wetting
force causes the solder to climb up the lead through the hole, spreading the solder to form the required sol-
der fillet.
Reflow Soldering. T he r ef low me tho ds c urrentl y i n use are in frared a nd f o rc ed convectio n. The se meth -
ods rely on the bo ar d assembly be ing heated to mel t the solder pas te on the l and pat te rn so that it wets with
the solder coating on the component leads. This allows the required solder joint to form, electrically and
mechanically bonding the component to the surface of the PCB. The primary differences between these
reflow methods are the source of the heat and the means by which the heat is transferred.
•Infrared - The InfraRed (IR) reflow method employs the thermal energy of halogen lamps radiating at a
given wavelength, usually 1 to 5 mm. The light is condensed by reflecting mirrors which raise the tem-
perature enough to reflow the solder paste. The board assembly passes through 4 to 20 zones, each of
which are independently controlled for temperature. The zones are classified as preheat, preflow, and
reflo w. The IR m ethod is att ractive for hi gh-volu me, large boa rd assemb ly operations becaus e it allows
the use of line-light and point-light sources.
• Convection Reflow - The forced convection reflow system is basically an improved modification of the
infrared technique. Within the forced convection system, fans circulate hot air or inset gas. The heat
transfer coefficient of forced convection is higher when compared to IR. Based on the uniformity and
efficiency of heat transfer, convection heating is the optimum source for reflow when doing a mass
reflow of the entire PCB assembly.
Reflow Profile Recommendation. Figure 3.2 on page 3-4 shows a typical reflow profile of an IR/
convection oven for tin/lead solder. The band has a widt h of approximately 25°C over the entire range of
heating and its u pper a nd lower limit s defi ne the proce ss windo w. Eac h temper ature zone of the r eflo w pro -
file is described below.
•Preheat - During preheat, the solvent will begin to evaporate and a 1° to 3°C/second rise rate should be
maintained. If the rate exceeds this slope, thermal shock or cracking of component is risked.
•Thermal Soak - D uring therm al soak , the re is typ icall y a 60 to 120 seco nd exp osure to allo w the p aste
to dry and the flux to activate. Too high or too low a temperature can lead to solder spattering or balling,
as well as oxidation of the paste, pads, and the component terminations.
•Reflow - A critical parameter in the reflow temperature zone is the wetting time, the amount of time each
solder joint is molten. The w etting time should be no more than 120 seconds, and the peak temperature
of the SMD lead should be in the range of 220°C to 225°C. For information on specific packages, see
your AMD sales representative.
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•Cooling - A cooling rate less than
4°C/second is recommended to help
achieve bright solder fillets with a
good shape and low contact angle.
The total profile time varies by SMD
package, board size, board density,
throughput requirements, type of
reflow equipment, and solder paste.
Profiling Method. Attaching a ther-
mocouple to the assembly, using adhe-
sives or Kapton tape, is the most
common profiling method. When pro-
filing for a small component, the ther-
mocouple should be placed toward an
edge or corner of the assembly. For a
component of higher mass, however,
the thermocouple should be placed
towa rd t he ce nter of th e bo ard. (Dur ing
the soldering process, the outside edges
and corners of an assembly will heat up
faster than the center. Also components
of greater thermal mass will heat more
slowly than components of lesser ther-
mal mass.) Additional thermocouples
should also be placed on other parts of
the board or on components that are
most subject to thermal shock or high
temperature damage.
For Ball Grid Array (BGA) packages,
AMD recommends t hat a thermocouple
bead be soldered to a ball under the
package and then placed in the center
of the package. It is here where the package is likely to exhibit the minimum peak temperature and, there-
fore, is the worst case position for a cold so lder joint.
For Ceramic Column Grid Array (CCGA) packages, AMD recommends special care be taken with han-
dling and control measures. See your AMD sales representative for further details.
LAND PATTERNS
The solder land pattern is a critical design feature for a surface-mount component because it affects the
strengt h of the sol der joints, the ease with which the compone nt can be te ste d and re paired, the type of sol-
der defects that can occur (e.g., solder bridging), and the thoroughness with which the board as sembly can
be cleaned. Refer to ANSI/IPC-SM-782 specification for land pattern recommendations.
Figure 3.2 Solder Reflow Temperature Profile
