Multivacuum and Dynamic Profiling

the new reference for efficient voidfree SMD vacuumsoldering process

Worldwide leading in vapor phase soldering technology

Formation of Voids

What are Voids?

Crystalline residues of fluxes

Gas bubbles from outgassing processes of solvents from solder paste

Outgassing from non-completed polymerisation processes in and on thecircuit board (e.g. solder resist)

Embedded humidity from modules and component materials

Reactive gases arising while removing the oxide layers with fluxes

Trapped air from printing and mounting process (scooping out the solderpaste with a squeegee in case of missing squeegee support)

Restricted heat flow from components to the circuit board (heat sink) Reduced strength of solder joints Reduced vibration stability Restricted performance in the high frequency range Reduced electric capacity of components (e.g. powermodules)

What are the negative influences of Voids

Using optimised solder pastes Applying special stencil geometries in the printing process to prevent the

solder paste from being scooped out during printing- Good: for example star- or cross geometries known as drainage print- Bad: for example flat, grid-shaped or pointed geometries

Exact fitting of the solder paste volume to the pad geometry Drying process for modules and components Avoid tempering PCBs as this increases oxidation Use metallisation with a low level of oxidation

By optimising all above mentioned parameters, void rates between 10% and15% can be achieved in a series production.

Measures to reduce the voiding rate beforesoldering

Applying the vapour phase - vacuum soldering technology / MULTIVACUUM

Ensuring an exact reproducible soldering profile that is optimised to theproduct by DYNAMIC PROFILING

By using the vapour phase - vacuum soldering technology, voids can bereduced to less than 1% in the series. This applies even to large-scalesoldering areas.

Measures to reduce the voiding rate duringsoldering

How the vacuum works

Negative pressure enlarges voids -> Moving towards edge of solder joint

By a further growing the void breaks through the liquid meniscus

The void gets evacuated and becomes smaller, the ventilation-channel closes

When returning to normal atmospheric pressure, the remaining void iscompressed into its final minimised size.

Cooling down the solder joint and “freeze” the void-less condition.

Example: behavior of a balloon in the hermetically sealed glass cylinder at high and low pressure

Trends and Limits of Vacuum Soldering

Increasing solder pads lead to larger air volume that gets trapped in the solderjoint during printing and pick and place process (e.g. scooping out the solderpaste with a squeegee in case of missing squeegee support). If these voidsdon’t get removed this will cause solder splashes besides the alreadymentioned negative influences.

Additionally the increasing solder pads and corresponding higher volume ofsolder paste cause major difficulties in evacuating these voids by only onesingle vacuum step.

The various viscosities depending on different soldering alloys have significantinfluences on the agility of voids during vacuum process.

The Solution: MULTIVACUUM

Several fully adjustable vacuum steps allow an optimisation of the vacuumprocess to the requirements of the specific product.

Step 1: The adjustable vacuum step before solderingEncapsulated air from the printing process gets removed. There is nothermal process during this step. Certain solvents from the solderpaste evaporate already at this point.

Step 2: One or several vacuum steps after solderingFinal pressure and dwell times can be adjusted to the specific productrequirements. A double-vacuum step after soldering is especiallyrecommended for very large-scale soldering areas.

Function Pre-Vacuum

Large voids from printing processget removed before soldering process

Function double vacuum after soldering process

Step 1 brings void to the edge of the solder joint Step 2 evacuates the void completely

Solder joint before vacuum Result after double vacuum process

Supporting the Vacuum Process

The results from the MULTIVACUUM process can be assisted and improved bya product optimized temperature profile.

DYNAMIC PROFILING allows achieving this product optimized temperatureprofile very easily.

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The product optimized soldering profile –The Challenge of the Soldering Process

Automatic setup and adjustment of soldering profile Real time measuring of each soldering process on product level Constant profile for each soldering process Traceability in the sense of quality management Applicable from batch size one to serial production Simple adjustment of soldering profile based on nominal values

Parameters of the soldering profile1. Preheating: Power setting (ETR) for adjustment of

t1 t2 preheat gradient

2. Soak zone: Start temperature of soak zone in °C.t2 t3 soaking time in sec.

3. Soldering: Power setting (ETR) ) for adjustment oft3 t4 soldering gradient

4. Time above Liquidus: Soaking time (sec.) after reaching end temperaturt4 t5

t1 t2 t3 t5t4

Tin°C

T1

T2

T3

t in sec.

t6

1

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The Solution: DYNAMIC PROFILING

Using a measuring standard that is heated up together with the PCB in the solderingsystem.- The temperature profiles of measuring standard and PCB are nearly identical- A thermocouple for creating a temperature profile is embedded in the measuringstandard.- Before each soldering process, the measuring standard is actively cooled down close toproduct temperature (room temperature).

The temperature- / time behaviour (warming up) of the measuring standard is used as asetting for regulating the profile via SPC-control.- The temperature profile is permanent controlled during each soldering process.- Influences on the process such as different loading or different temperature of thework piece carrier are compensated.

Full traceability and control of soldering profiles is guaranteed at any time by using thesoftwaretool ASSCON BDE/SQL-Database.

DYNAMIC PROFILING

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Setting up a Soldering Profile in Practice

Procedure for new productswith DYNAMIC PROFILING:

The reference PCB (goldenboard/GB) gets stored.Temperature profiles ofreference PCB and measuringstandards are recorded.

Then, this measuring standard isselected, whose profile matchesthe reference PCB best.

The required soldering programis created and the chosenmeasuring standard is selected.

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GB

Archiving Soldering Profiles(ASSCON-BDE / SQL-Database

All relevant process data,as well as temperatureprofiles can be stored andarchived with the datacollection software.

Possibilities of Application

Traceability

Soldering ProfileOptimisation

Golden BoardMeasuring

Process Visualisation

Examples of Profiles from Practice

Linear- and Ramp Profile Different Gradients Different Soak Times Different Times above Liquidus Various Holding Temperatures Profiles for leaded and lead-free soldering Compensating different Starting Conditions Repeatability

Linear- and Ramp Profiles

Linear- and ramp profiles caneasily be realised depending onthe product requirement

Comparison: Linear- and Ramp Profile

Infinitily variable Gradient

The gradient can be adjustedto the specific productrequirements

Various Gradients

Various temperatures of the soak zone

The starting temperature ofthe soak zone can be adaptedto the product requirements,e.g. for different outgassingtemperatures of solder pastes.

Comparison: High and low starting temperature of soak zone

Different Soak Times

The soak time can be adaptedto the product requirements,e.g. for an extended outgassingof solder paste to archievelower voiding rates.

Comparison: Short and long soak times

Different Times above Liquidus

Time above Liquidus isindividually adjustabledepending on theproduct requirements

23Comparison: different times above Liquidus t1 < t2

Soldering with and without Vacuum

Soldering with vacuum minimises the voiding rate The vacuum treatment starts after the soldering

process has finished It is possible to achieve a voiding rate of below

0,1% during soldering of BGAs, flip-chips, powermodules etc.

Comparison: Identical soldering cycle with and without vacuum

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Leaded and Lead-free Soldering in same machine Max. soldering temperature,

leaded T1 Max. soldering temperature,

lead-free T2 T - leaded is slightly higher depending on theproduct, as the soldering process is aborted beforereaching the evaporating temperature.

Comparison: leaded and lead-free soldering cycle with identical vapour temperature(Galden LS230)

Comparing different Starting Conditionse.g. cold / hot work piece carrier

If the profiles are compared to eachother (superimposing), the consistentbehaviour in the relevant areasbecomes visible.

T = T Cold work piece carrier

= T Hot work piece carrier

Precicion of Repeatability

This picture shows theprecision of repeatability for asequence of seven solderingcycles.

T2

T1

Thank you very much!

This presentation is copyrighted material owned by ASSCON Systemtechnik-Elektronik GmbH,Messerschmittring 35, 86343 Königsbrunn, Germany.