Solder & PCB issues

Dr. Paul Goodman, Senior Materials Consultant at ERA Technology writes, exclusively for EDE World, reviewing the lead-free impact on components, behaviour of solders and PCB design issues.

Dr Paul Goodman

The RoHS directive is forcing manufacturers to use lead-free solders to manufacture a wide variety of electrical equipment. All lead-free solders are “different” to tin/lead solder but as yet there are no clear guidelines or standards for lead-free solder circuit design. Design engineers searching for publications recommending designs for pads, tracks and board layout specifically for lead-free will find very little information.

This lack of guidance does not, however, mean that PCB designers can ignore the change to lead-free solders.  Profound changes need to be considered when designing new products.

Many manufacturers who have already re-designed their products for lead-free processes have found that changes to circuit design do not need to change, but this is not always the case, especially with more complex PCBs. This short article is intended to indicate where design changes might be needed and how lead-free solders might influence circuit design.

1 Components

  • Material changes:  The main change has been to change tin/lead terminations to a lead-free substitute, which is often tin. For many components this is the only difference and will not affect the maximum reflow temperature of the component.

  • Maximum reflow temperature: The temperature above which components distort or are permanently damaged depends mainly on the choice of materials within the part. Plastics, for example, are susceptible to high temperature and either degrade or melt and both these can degrade or destroy the function of the component. As lead-free solders require a higher reflow temperature than tin/lead, a wide variety of components may be unsuitable for use unless component manufacturers have modified these parts to increase their maximum reflow temperature.  Most components are converted to comply with RoHS simply by changing tin/lead termination coatings to tin but changes to withstand the higher temperatures required are more difficult and often this is unchanged. This has a significant impact on circuit design if the reflow temperature that has to be used for a particular PCB is higher than the maximum reflow temperature of a component, as this component cannot be used. If a substitute with the same function is not available from another manufacturer, then re-design will be necessary. There are two possible options here;

    • Redesign circuit so that heat sensitive components can be attached using selective soldering or hand soldering after all other components have been attached, These soldering techniques do not heat components to as high a temperature as SMT but require space around the component.

    • As a last resort, the heat sensitive component will have to be changed to a different type, which often requires significant circuit design changes. SMT electrolytic capacitors are examples of particularly heat sensitive components.

  • Part numbers:  Some component manufacturers show this change by a different part number, a code prefix or suffix but some have not made changes to part numbers

  • Component obsolescence: Many older types of component, particularly ICs may not be available as lead-free versions. In some cases, the demand for components is low and RoHS has reduced this to a level where the manufacturer decides to make it obsolete. In either circumstance, this will necessitate circuit redesign to accommodate alternative components. Re-plating leadframes with tin is possible but expensive and so not usually an option.  Software rewrite may also be required for processor ICs. Early component obsolescence is increasingly an issue for electronics manufacturers. Life-time-buys are an option as a last resort but are usually unacceptable in the longer term. Some circuit designers of equipment  intended to be on the market for many years are planning for such eventualities by designing circuits in such a way that when a critical component becomes obsolete, it can easily be replaced by a substitute.
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2 Behaviour of lead-free solders

Lead-free solders are different to tin/lead in several ways, the main differences being:

- Higher melting point – typically 30 - 40°C higher
- Inferior wetting properties, mid-chip solder balls
- Higher surface tension – increased risk of tomb-stoning and bridging

  • Issues arising from higher temperature lead-free processes. All thermal effects increase with temperature:

    • Multiple heat cycles, pad solderability and component damage - Double sided PCBs may have two SMT reflows plus one wave soldering followed by selective soldering and hand soldering. Pads may be unsolderable after three lead-free reflow cycles as surface oxidation is increased at the higher lead-free temperature. Design should aim to minimise heat cycles. Components attached in the early heat cycles may be damage during later cycles if over-heated.

    • DT across pcb: This is an issue with surface mount reflow.
    • PCB warping
    • Cracks in PTH

  • Inferior wetting – smaller pads but as bridging is more likely, pads need to be elongated and spacing between pads increased.

  • Poor wetting of vias and through holes – alleviated

  • Tomb-stoning occurs with tin/lead solders for a variety of reasons. The risk of this occurring with lead-free solders is increased due to the higher surface tension of these alloys and so the techniques used avoid this defect with tin/lead solders are even more important with lead-free solders.
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2.1 Additional Suggestions:

  • Increase spacing between pads to minimise risk from bridging

  • Align multi-pin components parallel to solder transport direction

  • As wetting is inferior, use smaller pads to ensure that no pad areas remain without solder. This will be important for equipment used in corrosive environments. “Rounding” of corners of pads helps prevent thermal decomposition of flux at higher lead-free temperatures

SMR and SMT pads
Tin - lead SMR pads
Smaller lead-free SMT pads

  • Use "thief pads" to reduce risk of bridging

Plated through hole pads
Elongated PTH pads
Plated through hole pads
Elongated PTH pads to avoid bridging

Tin/lead pads
Lead-free pads

Elongated IC pads and larger end pads to act as thief pads to reduce risk of bridging

  • Use modified pad shape to reduce risk from mid-chip solder balls

Modified pad shape to reduce risk from mid-chip solder balls

Stencil aperture design recommended by Cookson Electronics to reduce risk of mid-chip solder balls.

3 PCB Design issues

There are no new formal standards for design of PCBs that use lead-free solders, but component manufacturers will include (in their technical datasheets) the dimensions of pads that they recommend for their components.  These should be used unless modifications as described above prove to be necessary or these problems are suspected from previous experience.

There are several other issues that should be considered for PCB design.

- PCB warping – The higher reflow temperature required for lead-free increases warping of laminate. At high temperature, the laminate becomes soft in reflow ovens with conveyers that support the sides of panels only, these are likely to sag in the middle. This can cause some components to have poor solder bonds and to produce warped boards. One approach suggested by NPL is to use a support wire along the length of the reflow oven. These prevent boards from sagging in the centre;

PCB panel

- Vias and plated through holes are stressed during reflow because the thermal coefficient of expansion of the laminate in the direction perpendicular to the board is greater than that of copper. This can cause cracks and open circuits, particularly if the hole drilling or plating are of poor quality. This risk increases with reflow temperature and is more likely with lead-free but can be reduced by using:

  • Larger hole diameters
  • Thinner laminate
  • Avoid paste in hole connectors (use wave or hand soldering)

- In most reflow ovens, the temperature at the sides is lower than at the centre. Therefore avoid locating large components at the edge and small heat sensitive components at the centre.

Dr. Paul Goodman Senior Materials Consultant Reliability & Failure Analysis

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