Get There Without Hassle – Professional Lead Free Rework Equipment

The use of Lead Free materials in Rework leads to more stringent demand for technology and users.  Higher temperatures, tighter process windows and noticeably reduced wetting by the solder materials are just some of the parameters.  Especially in rework, all the new requirements are concentrated, as a rework station has to deal with all aspects of placement and soldering. Here we have a camel to go through the eye of a needle.  To have any chance of success the equipment must provided adequate certainty that it can cope with the new demands.

By means of example two typical SMDs show how tight the limits are for Lead Free rework.  These limits have to be adhered to by professional rework stations to ensure the quality of solder joints.  The aim of damage free rework of electronic components is to re-establish the original service life.

Table of Contents:

1. Theory and Practise of Lead Free Rework
2. Selected Standards
3. Lead Free Soldering with Precision
4. Set Up and Procedure
5. Results
6. Literature
   
   
   

All in One Rework Solution

All in one place
The types and sequence of the steps in rework technology do not change with Lead Free.

Changed Wetting Behaviour of Solder Materials 
A new property of Lead Free solders is their changed wetting behaviour.  We now know how leaded solders have spoilt us with their good-natured flow characteristics, whereas Lead Free solders hardly flow at all.  It is longer sufficient to print new solder onto just sections of the contact areas.  Instead, when selectively printing Lead Free solder paste, very precise alignment of the print head is required.  In addition, the design of the stencil apertures, the stencil thickness and its shape have to be re-assessed in the light of the available space on the printed circuit board.

Reduced Self-Alignment Effect

In the past there was the assumption that a component misplaced by up to 25µm would be pulled back to its correct position during reflow. This luxury with leaded solder is virtually gone with Lead Free solder. In view of increasing PCB packing density and the continued process of shrinking component sizes (eg 01005s) placement accuracy of 10µm appears to be reasonable.

The standard, recognised in the USA and Europe, for establishing the temperature sensitivities of semi conductor elements and the conditions for storage and processing derived from them is the J-STD-020 Standard, jointly published by IPC (Institute for Printed Circuits) and JEDEC (Joint Electron Device Engineering Council).  It concerns the processing of moisture sensitive SMDs and maintaining their body integrity during soldering.  The temperature critical for component reliability is measured at the body surface and is known as the Package Peak Temperature (PPT).

Apart from the JEDEC requirements, which mainly concern the components, the specifications issued by the solder manufacturers and the temperature limits printed circuit boards can be exposed to have to be observed.  Only the simultaneous adherence to all these demands allows rework of Lead Free assemblies to be successful.

Preheat Phase:
This phase lies between the start temperature and the melting point of the solder.  It should be as short as possible, as at higher temperatures flux is used by continuing oxidation of the metal surfaces.  If the preheat phase is too long the flux may have lost its effectiveness before the melting temperature is reached.  Experience shows that preheating should not exceed 240 s, beyond which wetting deficiencies are to be expected.  Solder paste contains substances whose boiling points lie between 125 und 250°C.  These have to evaporate before the peak temperature is reached as otherwise deficiencies, such as solder balls and voids are likely.  This means that the preheat time must not be too short.  Bell (2005) suggests preheat times of between 65 and 190 s for SnAgCu solder, the ideal being between 95 and 120 s.

Time above Melting Point:
The time above the melting point is best selected so that all solder joints melt as closely as possible at the same time and sufficient time remains for wetting to take place.  There should also be enough time for the molten solder to fill the gap between component and PCB and to form a fillet.  On the other hand this time should not be to long to keep the growth of the intermetallic phase and alloying effects to a minimum. Experience shows that up to one minute ideal.  Bell (2005) specifies a time span of between 20 and 90 s for SnAgCu solder.

Peak Temperature:
Reflow soldering, on the one hand, requires a high peak temperature.  This accelerates the wetting reaction and flowing is improved by lower surface tension.   Complete melting of the solder has to be ensured.  On the other hand, the maximum permitted temperatures for components (IPC / JEDEC) and printed circuit boards represent an absolute upper limit.  In practice a temperature 5% higher than that of the solder melting point has proved successful (Bell, 2005).  For solder with a melting point of 217°C the actual temperature of 230°C should be maintained for 10 – 30 s.  Tolerances in  the measurement and regulation of temperature by the equipment used needs to be taken into account.

Cooling Phase:
The cooling phase is the time between solidification of the solder and the end temperature.  While observing the maximum permitted cooling gradient (6ºC/s) it should be kept as short as possible.  The purpose of a short cooling time, amongst others, is the minimisation of alloying and intermetallic growth effects.  At the same time a short cooling phase also reduces the time the assembly is exposed to higher temperatures with commensurate reduction in stress of all components.  A too rapid cooling rate can lead to micro cracks in component bodies and broken solder joints.

The optimum rework profile needs to take into account the different guidelines from the literature.

The assumption is made that by adhering to the rules for SnAgCu solder the majority of applications will result in adequate soldering quality during rework.

Taking the requirements of a Lead Free soldering profile on board a FINEPLACER® Pico was set up for a practical application test.

As examples two BGA components were selected from a FINETECH Lead Free Evaluation Kit: BGA 13x13 und BGA 27x27, together with a multilayer test board.  Open type soldering head were used.  Hot gas reflow module COMISS IV and a full area underheater provided the necessary heat energy. The new COMISS IVPLUS reflow module will be presented on the SMT show. Providing even more temperature homogeneity and process repeatability.

A special feature of the COMISS module is the internal mixing of hot and cold gases.  There is a software-supervised choice to use air or nitrogen as processing gas.  In contrast to rework stations that provide their energy input from radiated heat the inert gas option achieves heating and protection at the same time, reduces the risk of voids being formed and that of the oxidisation of the solder joints.

To quantify the heat energy already present in the board a temperature sensor was placed near the component to be reworked.  Lead Free rework benefits from this feature, as it ensures a precisely reproducible process.

The actual temperature/time graph was generated from three thermocouples type K (Ni/CrNi) to DIN IEC 584 with a measuring accuracy of ± 1.5ºC: one on the surface of the component body and two directly at the solder joints.

The illustrations show clean contact arrangements for both components.  The top views confirm that there are no shorts and the perspective views show good connections between boards and components.
 
There is no doubt that the introduction of Lead Free solders brings with it increased demands for rework equipment and users.  These results clearly show that the FINEPLACER® is more than ready to face the challenge of Lead Free rework.  FINETECH offers equipment that takes the fear out of Lead Free. Please visit our booth 9/525 at the SMT show in Nuremberg. More information about our SMT highlights: www.finetech.de/enid/SMT

Bell, Hans (2005): Reflowlöten, Leuze Verlag
Rahn, Armin (2005): Bleifrei löten. Ein Leitfaden für die Praxis, Leuze Verlag

FINEPLACER^® Pico RS (modular rework system for smallest components and mobile devices)
FINEPLACER^® Micro RS (modular rework system for advanced and standard rework)
FINEPLACER^® Micro HVR (automatic rework station for advanced and standard rework)
FINEPLACER^® CRS (compact rework system for advanced and standard rework)
FINEPLACER^® Jumbo (modular rework system for very large board sizes)
FINEPLACER^® Jumbo HVR (automatic rework station for very large board sizes and standard rework applications)

^{*the configuration shown on the picture may contain optional modules}