Question 1

TDK's Guide to Electrically Conductive Adhesives (1): What is an electrically conductive adhesive?

Accepted Answer

A. It is a material that consists of an epoxy resin mixed with a conductive material, and is used to form conductive joints that connect electronic components to PCBs (Printed Circuit Boards). Due to its physical characteristics, electrically conductive adhesives are used as alternatives to the conventional SMT (Surface Mount Technology) in many applications. It is a material that frequently consists of an epoxy resin mixed with a conductive filler. Higher conductivity can be achieved by using more filler, however, this reduces the strength of the bond due to less adhesive material.
The conductive material contained in the epoxy is often silver (Ag), or, less frequently, a conductive form of carbon (e.g. graphite).

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Question 2

TDK's Guide to Electrically Conductive Adhesives (2): Why should I use conductive epoxy?

Accepted Answer

A. Conductive epoxies are mainly used in applications where the risks of mechanical and thermal cracking are very high, as well as when there are concerns about damaging heat sensitive components on the PCB during soldering.

Although tin/lead solders have been widely used for many years, leaded solders are becoming much less frequently used because of concerns over lead's toxicity. Unfortunately, unleaded solders have higher melting points, and because of this, sensitive electrical components near the solder joint can be damaged during the soldering process.

Another issue with lead-free solder is that it is less elastic than leaded solder. This means that lead-free solder joints will be more brittle and more susceptible to developing cracks than with lead-based solder, especially during temperature cycling.

Conductive epoxies are an alternative to leaded and unleaded solders. They have low curing temperatures, reducing the risk that heat sensitive components will be damaged during manufacturing, and they are more resilient to thermal and mechanical stress.

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Question 3

TDK's Guide to Electrically Conductive Adhesives (3): How does epoxy differ from solder during manufacturing?

Accepted Answer

A. The general process for mounting components with conductive epoxy is similar to solder mounting. The following table displays the differences.

Process	Epoxy	Solder
1	Disperse glue	Disperse solder
2	Place component	Place component
3	Cure	Heat/Reflow
4	Cool (if heated cure)	Cool

Table 1 : Epoxy and solder mounting processes.

The curing durations for epoxies are significantly longer than the heating and cooling times for solders, and care must be taken to make sure they are maintained at the proper temperature during curing, or the epoxy may not fully cure.

Conductive epoxies can be made in one or two part systems. One part systems have the advantage of not needing to mix the resins to form the final epoxy. They often have high curing temperatures (150°C), and often need to be refrigerated until used.

Two part epoxies have the advantage of being able to cure at lower temperatures (often room temperature) than single compound epoxies, plus they do not need to be refrigerated. These compounds need to be used immediately after mixing, and can be ruined by an improper mixing ratio of the curing agent.

Special care must be taken when placing the conductive epoxy. In the case of solder, excessive solder can be moved back to the landing point when heated because of the surface tension of the solder. However, epoxies cure without moving, and this can create shorts and silver migration issues (which could eventually result in a short).

Figure 1: Excessive solder and glue placement.

Figure 2: Short risk from excessive glue.

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Question 4

TDK's Guide to Electrically Conductive Adhesives (4): Does TDK offer MLCCs for use with conductive epoxy?

Accepted Answer

A. TDK offers many of our MLCC’s with a termination specifically designed for use with conductive adhesives. The termination for use with conductive epoxy uses an outer layer of an alloy consisting of silver (Ag), palladium (Pd), and copper (Cu) on top of a Cu layer. The standard termination of TDK’s MLCC’s consists of electroplated layers of nickel (Ni), and tin (Sn) on top of a copper layer.

Figure 1: MLCC termination for use with epoxy.

Figure 2: Standard MLCC termination.

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Question 5

TDK's Guide to Electrically Conductive Adhesives (5): Can I use an MLCC (Multilayer Ceramic Capacitor)  with a standard termination with conductive epoxy?

Accepted Answer

A. No. The termination used for epoxy mounting serves several purposes.

The addition of copper reduces the silver migration rate of the termination, which increases the amount of time it takes for a short to develop across the capacitor.

The conductive glue can absorb water, which would cause the Sn plating on a standard termination to oxidize. This increases the contact resistance between the MLCC and the conductive epoxy significantly (approximately 100 times greater after 100 hours), which leads to energy losses and heat generation. The AgPdCu alloy does not have the same oxidation issues as Cu/Ni/Sn and maintains a stable contact resistance.

Finally, during operation, temperature cycling can result in micro-cracks developing between the standard Cu/Ni/Sn termination and the epoxy, resulting in drastic reduction in shear force required to remove the chip from the board. The AgPdCu termination reduces the rate at which these micro-cracks develop because the silver contained in the termination and the conductive adhesive have a strong affinity for each other.

Figure 1: Contact resistance over time.

Figure 2: Shear force after temperature cycles.

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Question 6

TDK's Guide to Electrically Conductive Adhesives (6): What is the Difference Between “Soft Termination” and “Conductive Epoxy Termination”?

Accepted Answer

A. TDK's Soft Termination series is an MLCC with a layer of conductive resin built in to the standard termination.

The Conductive Epoxy series uses an AgPdCu termination for use with conductive epoxy as a mounting adhesive (solder replacement).

Both series allow for greater resistance to cracking due to mechanical stress, and significantly more resistance to cracking due to thermal shock. Additionally, the low curing temperatures of the conductive epoxy series preserve heat sensitive components during manufacturing.

Figure 1: MLCC with soft termination.

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Question 7

TDK's Guide to Electrically Conductive Adhesives (7): What type of epoxy should I use?

Accepted Answer

A. Because epoxy technology is so mature, many companies can engineer a variety of compounds for an even greater variety of specifications. Conductive epoxies can be modified and manufactured to meet many different performance characteristics and specifications including cohesive strength, hardness, durability, flexibility, temperature resistance, viscosity, thermal conductivity, electrical conductivity, moisture resistance, chemical resistance, resistance to mechanical shock and vibration, as well as the ability to withstand ultra-high or cryogenic temperatures.

As with all engineering, there are trade-offs for every type of epoxy. For example, epoxies with higher conductivity have a weaker bond strength because of the greater filler to resin ratio. Thorough research of the advantages and disadvantages of each type of epoxy should be conducted in order to ensure the epoxy will meet the required specifications.

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Question 8

Terminal Adhesion (1): What can cause MLCC (Multilayer Ceramic Chip Capacitor) to come off the printed circuit board (PCB) after soldering?

Accepted Answer

A. Any number of things can cause this problem, however the most common are solder joint quality and harsh handing. Solder joint quality here means the solder fillet volume and its coverage on the component’s termination.

Figure 1: Side view of soldered component

The adhesion between the component and the PCB is directly proportional to the amount of solder used in most cases.

Solder fillets typically become smaller as the component becomes smaller, and the lack of solder mass will reduce the adhesion strength also.

Figure 2: Terminal adhesion increases with solder amount

Figure 3: Solder amount increases with case size increase

Abusive handling obviously can cause the capacitors to come off after soldering, but even normal handling can dislodge a component from the PCB with small solder fillets. Touching the populated PCB with the hand or instrument should be a point of training and minimized as much as possible. Also, aqueous washes offer use higher water pressure than necessary when cleaning flux from the board. Brushes, conveyors, and robotics are all areas of handling risk.

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Question 9

Terminal Adhesion (2): What is “terminal adhesion” of MLCCs (Multilayer Ceramic Chip Capacitors)?

Accepted Answer

A. Terminal adhesion is the amount of force required to remove a component's termination from its body. It is most often evaluated by an axial lead pull or side push test. It is typically expressed in kilograms and is dependent upon component case size.

Figure 1: Traditional terminal adhesion tests

The axial lead pull is the best measure of a component’s true terminal strength, but the side push method takes into consideration the SMT process as a whole.

Note: The side push method is heavily dependent upon solder amount and the push point on the component.

The magnitude of terminal adhesion should always be discussed together with the failure mode. The three basic modes of failure are illustrated below.

Figure 2: Failure modes, lead pull

Figure 3: Failure modes, side push

The failure mode can change with varied solder fillet heights. A ceramic-to-ceramic or metal-to-metal failure mode typically yields the highest strengths and is therefore generally preferred.

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Question 10

Terminal Adhesion (3): What is the proper amount of solder when using an MLCC (Multilayer Ceramic Capacitor)?

Accepted Answer

A. The solder fillet is most practically expressed in terms of coverage of the component’s height or thickness.

TDK recommends the solder fillet to be 50% of the MLCC thickness. Additionally a thin layer of solder typically extends beyond the fillet’s bulbous volume, indicating the termination was properly “wet” by the solder and therefore exhibits good solderability. EIA*1 recommends >25% of the component thickness.

Note: Often manufactures will try to minimize the amount of solder used for cost control or tombstone prevention, however great care must be used to avoid sacrificing adequate solder fillets and proper terminal adhesion.

Figure 1: Solder fillet*2

*1 ANSI/J-STD-001B, Oct 1996
*2 TDK Multilayer Ceramic Chip Capacitors. Application Manual, Sept 1988 (publication BBE-017A).

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Question 11

Terminal Adhesion (4): What MLCC (Multilayer Ceramic Capacitor) variables effect terminal adhesion?

Accepted Answer

A. Solderability and plating thickness4 play an important role in terminal adhesion, however it is the MLCCs material system that is most significant.

Figure 1: MLCC inner structure

The material system used in a ceramic capacitor is rather complex, especially when considering the bond between the ceramic dielectric and the metal electrodes. The type and amount of glass frit used in the termination paste as well as the ceramic dielectric formulation are key variables for terminal adhesion. The co-firing temperature profile and atmosphere provide the catalyst for the termination-ceramic bond to occur.

Figure 2: Glass frit penetrates the ceramic dielectric

* C.F.Smith, Jr., “End Termination Compositions for Multilayer Chip Capacitor: Platable Bases and Plating”, 37th Electronics Components Conference, 1986.

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Question 12

Terminal Adhesion (5): What should I do to ensure good terminal adhesion?

Accepted Answer

A. By far the most common problem associated with poor terminal adhesion is too small of a solder fillet. Properly sized stencil apertures and a printing process free of "blinding" are crucial. Avoiding too fast of a solder profile will help ensure good solder flow and provide proper solder coverage.

Contaminated solder lands could effect the adhesion between the solder fillet and the PCB. Only clean PCBs (free of oils, moisture-free, etc.) should be used to ensure good terminal adhesion.

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Question 13

Commentary video of flex crack countermeasures for MLCCs (Multilayer Ceramic Capacitors)

Accepted Answer

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