smt_notes-html.html

Walsin Technology Corporation

for CHIP-R and MLCC

Soldering Joint Criteria

Page 2

Screen or Stencil Printing of Solder Paste

Placement Accuracy

Visual inspection of soldered joints

Guidelines for Footprint Design

Page 7

Footprint design for Discrete CHIP-R / 8P4R Array Resistor

Footprint design for 10P8R network Resistor

Footprint design for Discrete MLCC / 8P4CArray Capacitor

Stencil thickness

Reflow soldering

Page 11

Wave soldering

Page 14

Adhesive and Adhesive Application

Page 16

PCB design guidelines

Page 18

Precaution of handling

Page 21

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

Soldering Joint Criteria

The components and the printed boards must fulfil the requirements for good solderability and wettability even

after an (accelerated) ageing treatment. The components are tested according to IEC 68-2-58 under the

following conditions :

Property to be tested

Temperature of the solder (

Dwell time (second)

Wettability

235

0.2

Permanent wetting

260

0.5

Speed of immersion

: 25

2.5 mm/sec

Speed of withdrawal

: 25

2.5 mm/sec

Depth of immersion

: Such that the surfaces to be examined are fully immersed.

Most reflow soldering techniques utilize a solder paste that can be applied by screen-printing, stencil-printing or

dispensing. Solder paste is a suspension of pre-alloyed solder powder particles in a flux vehicle, to which

special agents have been added.

Solder balling is a common problem in reflow soldering using solder paste, because during the melting process

the solder powder particles do not completely coalesce, so that isolated spheres of solder are left on the non-

wettable parts of the substrate.

The solder balling test determines the reflow properties of the pre-alloyed solder powder particles in the paste.

Preferred

The molten solder from the paste has
melted into one solder ball.

Acceptable

The molten solder from the paste has
melted into one large solder ball
surrounded by a small number of very
small solder particles (

≤

50um)

Rejected

The molten solder from the paste has
melted into one or more solder balls
surrounded by a lot of small solder
particles

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

Screen or Stencil Printing of Solder Paste

In reflow soldering the quantity of solder at each soldered joint depends on the amount of solder paste

deposited on each solder land. In standard cases, the amount of solder paste should be 0.8 mg/mm

surface area of the solder lands. For fine pitch (

≤

0.8mm) multi-lead components, the amount of solder

paste should be lower, in the range of 0.5 mg/mm

Solder paste printing shifted in x/y direction

Criteria

Preferred

Acceptable

For Components like 0805, 1206 :

Displacement

≤

0.2 mm.

For multi-lead components like array resistor with a pitch

≤

0.8 mm :

Displacement

≤

0.1 mm.

Rejected

For Components like 0805, 1206 :

Displacement > 0.2 mm.

For multi-lead components like array resistor with a pitch

≤

0.8 mm :

Displacement >0.1 mm

Rejected

Contamination of paste, too little paste and no paste

Rejected

Paste outflow more than 0.2mm outside the solder land
caused by too much solder paste or too great a placing
force, (for multi-lead components with pitch

≤

0.8 mm :

outflow more than 0.1mm)

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

Placement Accuracy

In principle all components must be positioned exactly onto the solder lands. As a minimum condition, half

the width of the component termination has to be placed on the solder land. Displacement in the y

direction and/or displacement caused by rotation cause the total displacement. This demand is a practical

value, which guarantees a reliable joint and can be visually inspected in an unambiguous way.

Components shifted in x/y direction

Criteria

Preferred

Acceptable

Half or more of the width of the component is situated on the
solder land. Only acceptable when the conductor is covered
with insulating lacquer.

Rework

Less than half the width of the component is situated on the
solder land.

Rework

The metallisation must at least be partly positioned on the
solder land.

Surface Mounted Technology Notes

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2002a

Walsin Technology Corporation

Components with short leads (SO,SSOP, QFP) and

Array and Network of Resistor / Capacitor

Criteria

Preferred

Lan

Termination

Distance A

≥

3/4 the width of the termination

Acceptable

Lan

Termination

Distance A

≤

half the width of the termination

Rework

Lan

Termination

Distance A > half the width of the termination

Rework

Lan

Termination

Distance A and B > half the width of the
termination

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

Visual inspection of soldered joints

Criteria

Preferred

The height of the meniscus is equal to the
component height. The solder fillet is
concave.

Acceptable

For Components with a termination height H
< 1.2mm : The height of the meniscus must
be at least 1/3 of the height H

For components with a termination height H

≥

1.2mm : The height of the meniscus must

be at least 0.4mm.

Rework

For components with a height H of less than
1.2 mm, the height of the meniscus is less
than 1/3 of the height H.

For components with a height H over 1.2mm,
the height of the meniscus is less than
0.4mm

Rework

The height of the meniscus is less than
0.4mm or the solder fillet is convex.

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

Guidelines for Footprint Design

The first step in circuit board design is to consider

how the surface mounted board will be manufactured

This is because the manufacturing process determines the necessary dimensions of the solder lands, the

minimum spacing between components, the area underneath the SMD where tracks may be laid down, and

the required component orientation during wave soldering. Therefore a footprint related to the manufacturing

process with all this information is an essential tool for SMD circuit board design.

A typical SMD footprint, as shown in following figure is composed of :

Occupied area

Solder resist pattern

Solder land /

Solder paste pattern

Tracks or Dummy tracks

(for wave soldering only)

These footprint details depend on the following parameters :

Component dimensions and tolerances as given in the component data ;

Board dimensional accuracy ;

Placement accuracy of the component with respect to the solder lands on the board ;

Solder paste position tolerances with respect to the solder lands (for reflow soldering only) ;

The soldering process parameters ;

Solder resist position tolerances with respect to the solder lands ;

Solder joint parameters for reliable joints.

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

Footprint design for discrete CHIP-R

Reflow Soldering

Footprint dimensions in mm

SIZE

Processing remarks

Placement

Accuracy

0201

0.75 0.30

0.30 0.30 0.20

1.10

0.50

reflow

soldering

0.05

0402

1.50 0.50

0.50 0.60 0.10

1.90

1.00

0.15

0603

2.10 0.90

0.60 0.90 0.50

2.35

1.45

0.25

0805

2.60 1.20

0.70 1.30 0.75

2.85

1.90

0.25

1206

3.80 2.00

0.90 1.60 1.60

4.05

2.25

0.25

1218

3.80 2.00

0.90 4.80 1.40

4.20

5.50

0.25

2010

5.60 3.80

0.90 2.80 3.40

5.85

3.15

0.25

2512

7.00 3.80

1.60 3.50 3.40

7.25

3.85

IR or hot plate soldering

0.25

Wave Soldering

Footprint dimensions in mm

SIZE

Proposed number & Dimensions

of dummy tracks

Placement

Accuracy

0603

2.70 0.90

0.90 0.80 0.15

3.40

1.90

(0.15x0.80)

0.25

0805

3.40 1.30

1.05 1.30 0.20

4.30

2.70

(0.20x1.30)

0.25

1206

4.80 2.30

1.25 1.70 1.25

5.90

3.20

(0.25x1.70)

0.25

1218

4.80 2.30

1.25 4.80 1.30

5.90

5.60

(0.25x4.80)

0.25

2010

6.30 3.50

1.40 2.50 3.00

7.00

3.60

(0.75x2.50)

0.25

2512

8.50 4.50

2.00 3.20 3.00

9.00

4.30

(1.00x3.20)

0.25

Footprint design for Array Resistor :

Type

0603*4

0402*4

0402*2

Symbol / Item WA06X / WA06T

WA04X

WA04Y, WA04P

2.85 +0.10/-0.05 1.80 +0.15/-0.05

1.20 ± 0.05

0.45 ± 0.05

0.30 ± 0.05

0.40 +0/-0.05

0.80 ± 0.10

0.50 ± 0.1

0.50 ± 0.05

0.80 0.50 0.65

3.10 ± 0.30

2.00 +0.40/-0.20

1.50 +0.20/-0.10

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

Footprint design for 10P8R Network Resistor :

Symbol

WT04X

0.35 ± 0.05

0.50 ± 0.05

0.80 ± 0.10

0.70 ± 0.05

0.65 ± 0.05

3.20 ± 0.10

2.80 +0.40/-0.20

Footprint design for discrete MLCC :

Reflow Soldering

Footprint dimensions in mm

SIZE

Processing remarks

Placement

Accuracy

0402

1.50 0.50

0.50 0.50 0.10

1.75

0.95

0.15

0508

2.50 0.50

1.00 2.00 0.15

2.90

2.40

0.20

0603

2.30 0.70

0.80 0.80 0.20

2.55

1.40

0.25

0612

2.80 0.80

1.00 3.20 0.20

3.08

3.85

0.25

0805

2.80 1.00

0.90 1.30 0.40

3.08

1.85

0.25

1206

4.00 2.20

0.90 1.60 1.60

4.25

2.25

0.25

1210

4.00 2.20

0.90 2.50 1.60

4.25

3.15

0.25

1808

5.40 3.30

1.05 2.30 2.70

5.80

2.90

0.25

1812

5.30 3.50

0.90 3.80 3.00

5.55

4.05

0.25

2220

6.50 4.70

0.90 5.60 4.20

6.75

5.85

IR or hot plate soldering

0.25

Wave Soldering

Footprint dimensions in mm

SIZE

Proposed number & Dimensions

of dummy tracks

Placement

Accuracy

0603

2.40 1.00

0.70 0.80 0.20

3.10

1.90

(0.20x0.80)

0.10

0805

3.20 1.40

0.90 1.30 0.36

4.10

2.50

(0.30x1.30)

0.15

1206

4.80 2.30

1.25 1.70 1.25

5.90

3.20

(0.25x1.70)

0.25

1210

5.30 2.30

1.50 2.60 1.25

6.30

4.20

(0.25x2.60)

0.25

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

Stencil thickness

The following relation between the solder land dimensions (C x D) and the solder paste apertures

dimensions must be taken into account of solder for each solder joint :

Solder land dimensions

Solder paste apertures

Max. Stencil thickness

≥

0.6

≥

0.6

0.4

≤

C < 0.6

D > 0.9

C > 0.9

0.4

≤

D < 0.6

(C – 0.1) x (D – 0.1)

0.20 mm

0.4

≤

C < 0.6

0.6

≤

0.9

C x (D – 0.1)

0.20 mm

0.6

≤

0.9

0.4

≤

D < 0.6

(C – 0.1) x D

0.20 mm

0.5

≤

D < 0.6

0.5

≤

D < 0.6

C x D

0.20 mm

C < 0.4

D > 0.6

(D – 0.03) x (C – 0.1)

0.15 mm

C < 0.4

≤

0.6

(D – 0.03) x (C – 0.05)

0.12 mm

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

Reflow Soldering

The most common method used for reflow soldering in mass production is an infrared or convection oven.

In special cases vapour phase reflow is more suitable. In some situations it is required to assemble only

one or two SMDs (e.g. a very fine pitch QFP after the assembly of more standard components, or for repair

purposes). For this application reflow methods like resistance soldering, hot gas soldering, or laser

soldering can be used. The following figure show the process of reflow soldering :

Solder paste printing

Component Mounting

Reflow soldering

Cleaning (by case)

Temperature profile

In cases where a combination of small and large SMDs must be reflowed, the situation is complex. When the

solder paste for small components (e.g. MLCC, Chip-R) start to melt, the temperature near the large

components (e.g. IC) is far below the melting point. As soon as the paste for the large components start

melting, the temperature of the

small components can have

reached damaging levels.

To prevent this large

temperature difference it is

advisable to use a temperature

profile as that described in

following figure. It is important to

know that the temperature

differences are the only reason

for the typical shape of the

temperature profile.

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

In many cases the following figures can be used :

The temperature rise in the first zone of the oven (

) should be

≤

C because of possible solder

paste spattering.

The temperature in the equalise zone (Te) should be

≤

160

C and the time in this zone (Te)

≤

minutes (and < 1minute if possible) because of possible solder paste degradation.

The peak temperature (Tp) in the last heating zone should be at least 213

C with a max. of 280

C (for

double-sided reflow the max. temperature is 265

C).

Tp : 213

C is chosen to guarantee good soldering for 80Sn/20Pb metallisations, and 280

C is the max.

temperature for the glass epoxy board.

The time that the solder temperature is above the melting point (Tr) should not be longer than 1 minute

because of possible decreasing joint strength by intermetallic layers.

Recommendation of soldering profile

The robust construction of chip resistors allows them to be completely immersed in a solder bath of 260

C for

one minute. Therefore, it is possible to mount Surface Mount Resistors on one side of a PCB and other
discrete components on the reverse (mixed PCBs).

Surface Mount Resistors are tested for solderability at 235

C during 2 seconds. The test condition for no

leaching is 260

C for 60 seconds. Typical examples of soldering processes that provide reliable joints without

any damage are given in below.

Figure. Infrared soldering profile for Lead-Free Chip Resistors

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

Wave Soldering

Wave soldering was introduced to the electronic industry in the mid-fifties. Since then wave soldering

machine have evolved from simple soldering units to fully automated systems.

For mass production

purposes, wave soldering

is very often preferred to

reflow soldering. For mixed

print assemblies (leaded

components on the topside,

SMDs underneath) on

single-sided boards, wave

soldering is the only

reliable soldering

technology. The typical

wave soldering process is

shown as following :

Gluing

Component pick & place

Drying

Leaded component placing

Fluxing

Drying

Wave

Cleaning

Wetting behaviour of SMDs

SMD wave soldering places two opposing demands on the

solder wave :

a. The wave must reach all components terminations ;

b. Bridges between adjacent narrowly placed leads must be

avoided.

During soldering, the component bodies are immersed in the

solder. “Skips” are mainly caused because plastic bodies are

not wetted by soldering, creating a depression in the solder

wave, which is enhanced by surface tension. This can cause a “shadow” behind the component and prevent

solder from reaching the solder lands at trailing side of the SMD. The higher the body, the shorter the leads,

and the smaller the footprint, the more severe the shadowing effect. The design of the circuit on the board

plays a major part in the soldering result. The conditions for wave soldering are rather critical, especially for

multi-lead components with closely spaced leads.

Surface Mounted Technology Notes

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2002a

Walsin Technology Corporation

The shadow effect in wave soldering : Surface tension
of the molten solder prevents wetting of the
downstream solder land.

Extending the solder land to overcome the shadow
effect.

Recommendation of soldering profile

Surface Mountable Resistors are

tested for solderability at a

temperature of 235

C during 2

seconds. The test condition for no-

leaching is 260

C for 60 seconds.

Typical examples of soldering

process that provide reliable joints

without any damage, are given in

following figure for wave soldering.

Wave soldering of multi-lead components

The prevention of bridging between the leads of integrated circuit packages is a particular problem.

Sometimes it is necessary to shorten the protruding solder

lands, and reduce the conveyor speed, or to use

solder

thieves

Wave soldering is a sell-established process, that has some
limitations in SMD applications. For example, an adhesive is
required, and care must be taken to ensure that this does not
contaminate solder lands, leads or terminations, which would
affect solderability and thus quality of the joints. There are some
limitations regarding component orientation and conductor
spacing. On the other hand, wave soldering is useful for mass
production.

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

Adhesive and Adhesive Application

When an assembly has to be wave soldered, an adhesive is essential to bond the SMDs to the substrate.
Under normal conditions reflow-soldered substrates do not need adhesive to maintain device orientation,
since the solder paste does this. Sometimes and exception occurs in the case of double-sided, reflow-
soldered SMD boards. Both adhesive and solder paste are required to hold devices on the bottom of the
substrate during the reflow process, if the ration between the weight of the component and the component
surface wetted by solder paste is too high.

Application Techniques

There are three different methods of applying adhesive in SMD assembly systems. The choice depends on
the required SMD placement capacity, and the types of components used.

Pin transfer

A pin picks up a droplet of adhesive from a reservoir and transfers it to the surface of the substrate.
Surface tension causes a portion of the droplet on the pin to form an adhesive dot on the substrate.

Screen or stencil printing

A fine mesh screen, coated with emulsion except for the areas
where adhesive is required, is placed over the substrate. A
squeegee passing over the screen forces adhesive through the
uncoated areas of the mesh and onto the substrate. The same
can be done by means of stencil.

Dispensing

The dispenser (either a time-pressure system or a rotary pump) is
positioned over the printed board by means of a computer
controlled X-Y-Z moving system. The adhesive dot size is
determined by the amount of time the dispenser is activated.

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

The next table summarises the principal benefits and disadvantages of the automated adhesive application
systems :

Method

Advantages

Disadvantage

Pin transfer

Compact system

Simple, fast process

Little maintenance

Simultaneous dot placement

Accepts pre-loaded mixed-print board

Controls adhesive quantity

Needs flat board surface

Open system subject to outside
influences

Can not use high yield point adhesive

Screen / Stencil printing

Simultaneous dot placement

Simple process

Uniform dot height

Needs flat board surface

No obtrusions on surface

Screen maintenance required

Open system

Dot height limited by screen emulsion
or stencil thickness

Dispensing

Handles irregular surfaces

Accepts mixed-point boards

Controls adhesive quantity

Closed system not subject to outside
influences

Accepts most adhesives

Requires more maintenance

Equipment is bulky

Much slower system

Surface Mounted Technology Notes

Page

2002a

Walsin Technology Corporation

PCB design guidelines

Mounting density

Chip resistors are designed for handling by automatic chip placement systems.

The temperature rise in a resistor due to power dissipation, is determined by the laws of heat - conduction,

convection, and radiation. The maximum body temperature usually occurs in the middle of the resistor and

is called the hot-spot temperature and depends on the ambient temperature and the dissipated power.

The hot-spot temperature is important for mounting because the connections to the chip resistors will reach

a temperature close to the hot-spot temperature. Heat conducted by the connections must not reach the

melting point of the solder at the joints. Therefore a maximum solder joint temperature of 110

C is advised.

The ambient temperature on large or very dense printed-circuit boards is influenced by the dissipated power.

The ambient temperature will again influence the hot-spot temperature. Therefore, the packing density that

is allowed on the PCB is influenced by the dissipated power.

Example of mounting effects

Assume that maximum temperature of a PCB is 95

°C

and the ambient temperature is 50

°C

. In this case

the maximum temperature rise that may be allowed is 45

°C

. In the graph below (fig.1), this point is found

by drawing the line from point A (PCB=95

°C

) to point B (Tamb = 50

°C

) and from here to the left axis.

To find the maximum

packing density, this

horizontal line is

extended until it

intersects with the

curve, 0.125W (point

C). The maximum

packing, 19

units/50*50mm

(point

D), is found on the

horizontal axis.

100

150

100

150

mounting density (units/50*50mm )

PCB temperature (normally around 100 C)

100

150

ambient

temperature

( C)

(K)

0.0625W

0.25W 0.125W

Fig.1 PCB temperature as a function of applied power, mounting density and ambient temperature

Surface Mounted Technology Notes

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2002a

Walsin Technology Corporation

PCB design

To take full advantage of design features embodied in the new resistors, equipment manufactures can also

take steps to minimise thermal stress:

1. By using forced cooling with a fan

2. By using a board material with a high thermal conductivity (such as ceramic)

3. By using a heat sink to reduce the temperature coefficient of the PC board

Figures 2, 3 and 4 shows the effect of using the heat

sink. Figure 2 plots the temperature rise as a function

of dissipated power at the hot spot, solder spot and the

reverse side of the PC board without heat sink. Figure

3 shows same measurements with the board mounted

on a 400mm

Heat sink, and figure 4 shows an

example of the temperature rise reductions that can be

expected when a heatsink is added on the underside of

an FR4 PC board.

Fig.4 Temperature rise reduction of solder spot as a

function of heatsink area

Fig.2 Temperature rise at hot and solder spot as a

function of dissipated, mounted on FR4 without

use of a heatsink.

0.4

0.8

1.2

1.6

2.0

120

240

solder spot

(K)

P(W)

160

200

reverse side

100

200

300

400

heat sink area (mm )

temperature rise

reduction ratio

(%)

0.4

0.8

1.2

1.6

2.0

120

160

(K)

P(W)

solder spot

hot spot

200

reverse side

Fig.3 Temperature rise as a function of dissipated

power with Philips test substrate mounted on

a 400 mm heatsink

156

Surface Mounted Technology Notes

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2002a

Walsin Technology Corporation

Precaution of handling

a. Precaution for soldering

1. Note that rapid heating or rapid heating or rapid cooling or local heating will easily damage

CHIP-R/MLCC.

2. DO NOT give the heat shock over 100

C in the process of soldering. We recommend taking

preheating and gradual cooling.

b. Precaution for handling of substrate

1. DO NOT exceed to bend the PCB

after soldering this product extremely.

2. Mounting place must be as far as

possible from the position that is
closed to the break line of PCB, or
on the line of large holes of PCB.

3. DO NOT bend extremely the PCB in

mounting another components. IF
necessary, use Back-up pin (Support
pin) to prevent from bending
extremely.

4. DO NOT break the PCB by hand.

We recommend to use the machine or the jig to break it.

c. Caution of wave soldering

1. We do not recommend the wave soldering to this product, because of solder bridging

happens owing to narrow 0.8mm pitch of this product.

d. Soldering gun procedure

Note the follows, in case of using soldering gun for replacement :

1. The tip temperature must be less than 280

C for the period within 3 seconds by using

soldering gun under 30W.

2. The soldering gun tip shall not touch this product directly.

The tip of soldering iron
should not directly touch
the chip component to
avoid thermal shock on the
interface between
termination and body
during mounting, repairing
or de-mounting process
(shall ensure both
termination had been
molten still before chip
component removed).

Ha n d s old erin g m eth od

Bod y (cera m ic)

Termination (metal)

Solder pad

Surface Mounted Technology Notes

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2002a

Walsin Technology Corporation

e. Storage conditions

Note the following, in case of storing this product :

1. Avoid the atmospheres which are high temperature, high humidity, dusty and having

corrosive gas (Hydrogen chrolide, Sulfurous acid gas, Hydrogen sulfide etc.) to prevent
terminal solderability fromdeclining. Keep the storage conditions less than 40

C and 70%

relative humidity, and use up this product in 6 months as far as you can.

2. Avoid direct heat and sunshine to prevent the tape of package from transforming and

sticking to this product.

3. Capacitance value occasionally will descend in several percent at the characteristic of high K

capacitor material.

Contact Information

For more information, please contact with:

Walsin Technology Corporation

Tel

886-3-475-8711

Fax No.

886-3-475-7130

Web Site

http://www.passivecomponent.com

e-Mail :

info@passivecomponent.com

Address :

566-1, Kao-Shi Road, Yang-Mei, Tao Yuan, Taiwan, R.O.C.

or your local

Walsin Technology Corp. Sales Representatives.

Surface Mounted Technology Notes

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2002a