ep c3 assembly technologies

8/2/2019 EP C3 Assembly Technologies

1/44

Marius RANGU, PhD

ELECTRONIC PACKAGING- 3 -


2/44

ASSEMBLY TECHNOLOGIES

Components placement

Physics of soldering

Manual / semiautomatic soldering

Wave soldering

Reflow soldering

The SMT assembly line


3/44

3

The PCB Assembly ProcessAssembly = Placement + Soldering + Inspection

Manual or automatic ?


4/44

4

Component PlacementComponent carriers Paper tape

THD axial and radial components Axial components needs terminal

forming and cutting

Tape and reel

Small SMD components

Most widely used component carrier

Plastic

carrier

Top Ta

pe

Guiding holesReel


5/44

5

Linear Magazine (Stick)

Medium and large SMD components

Low volume carrier

Waffle tray

Large SMD components

Manual / Semiautomatic Placement Low productivity: takes long time and is prone to human error

Used exclusively for prototyping and low-volume production

Might complement automatic assembly for exotic components: those that are too large

or have to complicated geometries to be handled by automated placement machines Most SMD components are difficult to handle manually


6/44

6

Manual placement productivity tools

Vision system

Tray Feeder

Tape

Feeder

Stick

Feeder

Carousel

Feeder

Manipulator


7/447

Automated pick & place machines

Placement is done automatic, under computer control

Components are picked from tapes or trays

Machine vision ensures precise alignment of the components

May operate with several placement heads, in tandem (on several boards at the same time)

The components list, their positions and orientations are known from CAD-generated pick

and place files


8/448

Pick & place operations

Pick Check Rotate Place Inspect

PICK

A vacuum nozzle extract the component from the feeder, using suction After picking, tape feeders spin the reel to move the next component on the picking position

CHECK

Optical inspection checks the component presence, orientation and marking

Electrical check on the fly is possible for some components

ROTATE

The orientation can be adjusted on the fly

PLACE

The component is placed on the PCB pads with controlled pressure

INSPECT Optical inspection check again the component presence and orientation


9/449

Turret pick & place machines

Multiple nozzles are mounted on a revolving turret

The turret is stationary

The board moves in X-Y directions

The feeder carriage moves on X direction, to selectany component

Several operations are done simultaneously: pick

component, inspect, dispose if bad, rotate, place

Feeder

carriage

Turret

head

PCB

Vacuum

nozzle

Very fast: tens of thousands components / hour

Moderate accuracy: cannot handle fine pitch components


10/4410

Turret pick & place


11/44


12/4412

Gantry pick & place


13/4413

The physics of solderingSoldering = joining two or more metals through metallurgical

bonds with a filler material

The filler material is called soldering alloy (or just solder)

The filler material must be added to the process.

If the melting temperature of the filler is much smaller that the melting point of the

joined metal, the process is called soft soldering

If the melting temperature of the filler is comparable with the melting point of the joinedmetal, the process is called hard soldering

In electronics, soft soldering joins the component terminals to the PCB pads

Solder melting Solder Wetting Base metaldissolution

Formation of theintermetallic layer

The soldering process


14/4414

Heat must be applied, at a high enough temperature and for a sufficient time, in order to

melt the solder

Soldering temperature must be higher than the solder melting temperature !

One way to apply the heat is by direct contact, using a soldering iron.

(There are many others)

The base material is usually covered by oxides and impurities. This is a big problem.

Flux has a crucial role in cleaning the base material

When the solder melts, the surrounding air is heated and filled with flux and impurities

vapors. It is not air at ambient temperature !

Soldering iron

Solder

Flux

Impurities

Base metal

Heated air

+ vapors

1. Solder melting


15/44

15

( a reminder on the surface

tension )

Molecules in a liquid experience intermolecular attractiveforces (residual electrostatic forces)

Intermolecular forces between alike molecules are called cohesion forces.

Intermolecular forces between unlike molecules are called adhesive forces.

Molecules on the separation layer of a liquid experience more cohesion than

adhesion, so they exhibit inwards attractive forces. The inward intermolecular attraction forces the liquid into the shape with minimal

surface area (in isolation: spherical)

The surface tension is defined as the force acting on a line of unit length:

F is parallel to the surface

Fis perpendicular to

l Nm

F

l

=

l

2. Wetting


16/44

16

Solder wetting of padsMolten solder

Base metal

SE

BE

SB

The molten solder spreads on the metal base

The shape of the solder is defined by the equilibrium of the tensions at the

separation between: - Base metal (B)

- Solder (S)

- Environment (E) = air + vapors

The equilibrium condition is known as Youngs Equation

cos BE SB SE

= +

is called the wetting angle. A good solder joint must have a small wetting angle (good spreading on the pads)

Small can be achieved by: - high surface tension substrate- low surface tension solder

- low surface tension flux


17/44

17

Solder wetting of SMD terminalsT

T

The surface tension of the lateral side of the

terminal forces the solder to rise on it

The surface tension on the bottom side of the

terminal and the surface tension of the pad forces

the solder to spread underneath the terminal

(capillarity flow)

P

Solder wetting of THD terminals

P

P

T

T

Capillarity flow forces the solder to

rise in inside the hole

For a good quality joint, the solder

should also wet the top side of the

terminal

Pad finishes plays a crucial role in

wetting


18/44

18

Copper

Solder

Molten

Solder

Base metal

The solder and base metal mix at the

atomic level.

Base dissolution in solder is necessary

for metallurgical bonding (for instance,lack of dissolution prevents soldering

on glass)

The rate of dissolution depends on basemetal, solder, temperature and time

Too much dissolution might reduce the

copper thickness to unacceptable values

3. Base metal dissolution


19/44

19

Chemical reactions between base

metal and solder will create

intermetallic compounds (IMC) at the

interface layer.

Tipical thickness of an intermetalliclayer is a few micrometers

The intermetallic layer has some

desirable and some undesirable effects

Desirable:

Enhances wetting due to increasedsurface energy of the IMC

Slows the base metal dissolution

due to higher melting point of the IMC

Undesirable:

IMC are crystals with low symmetry,

so they are brittle. Thin intermetallic

layers reduce the reliability of the

solder joint.

IMC increase the contact resistance

If exposed to air, IMC reduces

wettability through oxidation.

Molten

Solder

Base metal

Intermetallic

layer

4. The intermetallic layer


20/44

20

Soldering alloysA GOOD SOLDER SHOULD: Have a melting point lower than the melting point of the base metal, and low

enough to be easily achievable using common heating technologies

Have a melting point higher than the maximum expected operating temperature Have a low surface tension

Have high electrical and thermal conductivity

Have good mechanical strength (ability to withstand stresses)

Have a CTE similar to the CTE of the base material

Have a low price

Does such a soldering alloy exists ?In the past century, Tin-Lead (SnPb) was the preferred solder

but

The ROHS Directive (Restriction of Hazardous Materials) prohibits the

usage of Lead in electronic assemblies (except for killing or curing

people). . .

So today, SAC (SnAgCu) is the most widely used alloy(polar bears still die though)

E i S Pb S ld


21/44

21

Eutectic SnPb Solder

(Phase diagram of the SnPb alloy)

E

An eutectic alloy has a lower melting point than any other composition of the


22/44

22

Property Value

Composition 63% Sn + 37% Pb

Melting point 183 []Electrical conductivity 6,8510

6

[1/ m] ( 11.5% Cu)Thermal conductivity 47,7 [W/m] ( 11.9% Cu)

Coefficient of thermal expansion (CTE) 25 [ppm/] ( 147% Cu)

An eutectic alloy has a lower melting point than any other composition of the

same metals

The eutectic alloy doesnt have a pasty transition between solid and liquid. A

pasty phase prevents good wetting.

Properties of eutectic SnPb solder

SnPb solder comes as:(so do lead-free solders)

Solder wire, for manual soldering

Solder bar, for wave soldering

Solder paste, for reflow soldering

L d f S ld


23/44

23

Property ValueComposition 96.5% Sn + 3% Ag + 0.5% Cu

Melting point 218 [] nearly eutecticPasty phase 1.3 [] (216.7 218)Electrical conductivity 6.6106 [1/ m] ( 11 % Cu)

Thermal conductivity 58.7 [W/m] ( 14.6 % Cu)

Coefficient of thermal expansion (CTE) 21.6 [ppm/] ( 127% Cu)

Properties of SAC305 solder

Soldering with lead-free is more difficult than soldering with SnPb,

because:

SAC melts at a higher temperature than SnPb. Equipments, components and PCBmust withstand this higher temperature

SAC oxidizes more quickly than leaded solder, which affects the wetting process

The higher tin content of the solder makes it more corrosive for wave soldering

pots

Lead-free Solder The most widely used Lead-Free Solder is SnAgCu (SAC)

The eutectic SAC has 3.7% Ag and 1% Cu, and melts at 216.7 []

Fl


24/44

24

Fluxes Most metals (and especially copper) oxidizes very quickly in air

Oxygen satisfies intermolecular bonds:

On the pad: it reduces the adhesion forces

On the molten solder: creates a high surface tension skinthat prevents free movement of the liquid solder

Solder Oxide

Diffusion layer

Flux

Copper

Oxide

Insufficient or inefficient flux is the primary cause of poor wetting !!! The flux has the role to clean the oxide and impurities from the base metal and

solder surface, allowing wetting

Fluxes are mild acids that reduce the oxide to soluble metal salts

To clean the fats and dirt on the soldering surfaces, the acid is mixed with

solvents

Prevents

wetting

A GOOD FLUX SHOULD:


25/44

25

Flux types

A GOOD FLUX SHOULD: Have a low viscosity to penetrate all the narrow gaps of the board

Retain its oxide-reducing capability at soldering temperature

Not carbonize, cake up or become sticky after soldering (easy to clean)

Rosin-based fluxes Rosin is a pine resin soluble in alcohol

It becomes chemically active above 70 []

It leaves a hard coating, difficult to remove Its an insulator, so rosin residue must be cleaned before

testing

Its not very effective with Tin oxides

RMA (Rosin Mildly Activated) Rosin activated with an acid or alcohol

More effective than pure rosin

RMA residues might be electrically conductive (!!!)

RMA residues might be corrosive

Most manual soldering fluxes are RMA !

Boards must be cleaned if soldered with RMA flux !

No clean fluxes


26/44

26

No-clean fluxes Still rosin with activator, but with a very low

solid content (


27/44

27

Manual / semiautomatic solderingManual soldering tools

Tip

Heater

Magneticsensor Switch

Spring

Wiring

Handle

Connector

The most widely used manual soldering tool is

the soldering iron (or soldering station)

It provides controlled heat to a soldering tip

Magneto-regulated soldering iron

The heat regulation is done by a ferromagnetic material

Below the Curie temperature, the material is magnetic and it keeps a switch closed,

closing the electric circuit of the heater

Above Curie temperature, the material becomes paramagnetic and the spring

opens the switch. The soldering temperature will be (aprox.) the Curie temperature

of the sensor

Electronically regulated soldering iron


28/44

28

Electronically regulated soldering iron

The heat regulation is done by an electronic

controller

A sensor measures the tip temperature and

sends the information to the controller The controller regulates the power delivered

to the heating element

Temperature control is more precise than for

magneto-regulated irons

Soldering Tips

Iron

Nickel

Chrome

Copper core

Nickel Soldering tips are made of copper

coated with various metals to protect it

from the corrosive action of the flux

Various tips shapes are available

S i t ti ld i t ti


29/44

29

Semi-automatic soldering stations(rework stations)

VisionSystem

Soldering

Iron

Component

holder

Board holder

Pre-heater

IR heater

Fluxes for manual soldering


30/44

30

Bottle with

brush

Bottle with

needle

Pen

Syringe (paste flux)

Liquid and paste fluxes

Soldering wire with flux

Solder Flux

Fluxes for manual soldering

Manual soldering process


31/44

31

Manual soldering process THD terminals

1. Touch the pad and terminal with the soldering tip, to

preheat them

2. Apply solder while keeping the tip in contact with thepad and terminal.

The solder should melt only at the joint location, not

earlier !

A good THD solder joint should:

Wet at least 75% of the pad on the soldering side

Wet at least 50% of the pad on the opposite side

Fill at least 75% of the hole


32/44

32

THD manual soldering

SMD terminals


33/44

33

SMD terminals1. Apply flux on the pads

2. Place the component and align it with the pads

3. Solder one or two supporting terminas, applying

solder and heat at the same time

4. Apply flux on the pads AND terminals

5. Deposit some solder on the soldering tip

6. Transfer the solder on the terminals

7. Repeat 5 and 6 until all terminals are soldered. Add

more flux if necessary

A good SMD solder joint should:

Cover with solder (C) at

least 50% of the terminal

width (W) and at least 50%

of the pad width (P):

Cover with solder (F)

at least 25% of the

terminal height (H)

Provide SOME padsolder terminaloverlap


34/44

34

SMD Chip manual soldering


35/44

35

SMD IC manual soldering

W S ld i


36/44

36

Wave Soldering The backside of the board travels through the crest of a wave of molten solder

If good wetting can be provided, solder will adhere to the terminals and pads,forming the joints

Initially developed for THD soldering, it can also be used to solder SOME types of

SMDs (not fine pitch)

PCB

THDs on topSMDs on bottom

Molten solder

Solder wave

Wave soldering process


37/44

37

Flux Preheat Solder Wave Cooling

Wave soldering process

PCBs are mounted on frames and

transported by a conveyor system

Liquid or foamed flux is first applied, to

provide wettable surfaces for soldering

The board is preheated to avoid thermalshocks

The backside of the board travels through

the solder wave. Heat and solder are

simultaneously applied

Solder is cooled to room temperature and

solidifies

1 Applying the flux


38/44

38

1. Applying the flux

Compressed

Air

Liquid flux

Foam fluxCompressed

air

Metering

pump

Liquid flux

Flux must cover the backside of the board with a thin, uniform film

It must also penetrate on the inside of the insertion holes

FLUXING IS CRITICAL FOR WAVE SOLDERING !!!

FOAM FLUXING SPRAY FLUXING

Compressed air is forced on the middle

of a cylindrical porous body

(foaming stone)

Foam is guided upwards by a nozzle

Some of the flux is recircullated

Compressed air extracts the liquid flux from

a bath (through suction) and sprays it on thebackside of the board

A metering pump controls the mixture

Flux can be propelled into narrow gaps

Flux is always fresh.

Most suitable for SMD

2 Preheating Reflector


39/44

39

2. Preheating

The board and components are

heated to 80 100, to avoid thermal

shock (due to CTE mismatches) and

reduce the temperature step at

contact with the wave

During preheating, the solvents from

the flux evaporates

IR heaters

Hot air blower

Reflector

1st preheater 2nd preheater

Wave contact

Wave exit

Solder melting

temperature

Preheating is done by infrared

heaters

A reflector is placed above the

conveyor, for thermal efficiency anduniform distribution of temperature

Forced convection is sometimes

used as a supplement, to ensure

ventilation of the (potentially

flamable) solvent vapors

2 The solder wave


40/44

40

2. The solder wave

Solder is kept molten in a pressure chamber

An axial impeller pump forces the solder upwards through one or more nozzles, creating

stationary waves Most wave soldering machines use a double-wave configuration to avoid solder bridging

The board travels through the crest of the waves on an inclined path (7)

1st wave 2nd wavepump

Heaters

Pressure

chamber

1ST WAVE( )


41/44

41

2ND WAVE

Before SMDs wave soldering machines used only

one wave, with the characteristics of what is nowknown as the second wave

An asymmetrical wave: it provides a turbulent

entry and a smooth exit

On the entry part, the solder flows on the

opposite direction of board traveling. It has the

same effect on the board as the first wave.

1 WAVE High kinetic energy wave (turbulent)

Fills every hole and narrow gaps

At the exit point, adhesion to the joints keeps the solder

attached to the board until its own weight forces it back to

the bath (peelback)

The peelback causes bridges and excessive solder deposits.

Doe to peelback, a turbulent wave cannot by used by itself

The exit part is flat and the solder flows in the same direction and with the same speed

as the board. The solder is stationary with respect to the board, on the horizontal axis.

Since on the exit path the board only moves on vertical direction with respect to the

solder (due to the inclined path), it has the same effect as lifting the board from a

stationary bath: it forms conical joints with minimum excess solder.

The second wave smoothes the solder joints and removes the excess solder

WAVE S ld i


42/44

42http://www.youtube.com/watch?v=WXLv7MNjGL0

WAVE Soldering

Wave soldering of SMDs


43/44

43

Apply adhesive

Wave soldering of SMDs

Place SMD

Cure adhesive

Flip board & place THD

Wave solder

The component body can restrict solder access to pads situated on the rear end of the


44/44

component (with respect to traveling direction)

The first turbulent wave can diminish this shadowing effect, but not eliminate it entirely

In order to wave solder SMDs, the board must be designed with:1. THDs on top and SMDs on bottom

2. SMDs with specific orientation relative to the board traveling direction during

wave soldering:

Board traveling

direction

ep c3 assembly technologies

Documents