Adhesive Selection For Semiconductor Packaging, Flip-Chip Attachment, And Wafer Bonding - Q&A

Semiconductor packaging adhesives are used to attach dies or chips to the substrates on which semiconductor devices are packaged or fabricated. Examples of these substrate materials include glass, ceramics, metals, and plastics. In addition to die attachment and chip bonding, semiconductor packaging adhesives are used to attach heat sinks and stiffeners, components that are used for testing semiconductor devices. For design engineers who are looking for the right solution, this Q&A from Gluespec® provides answers to frequently asked questions.

Q: What are the basics of product selection?

Semiconductor packaging adhesives can be categorized by chemistry, form, processing method, and curing mechanism. The type of bond, temporary or permanent, is also important to consider, as are application-specific considerations such as exposure to high temperatures, moisture, chemicals, shock, and vibration. Design engineers can also choose products that are either electrically conductive or electrically insulating. Semiconductor packaging adhesives are stress-relieving and help to prevent warpage, but their specific properties differ by chemistry.   

Q: What some common chemistries?

There are three common chemistries for semiconductor packaging adhesives.

  • Epoxy adhesives are strong, hard, and rigid. Because they form tough, durable bonds, they are a good choice for ruggedized products; however, epoxies are also susceptible to cracking and may not be suitable for flexible displays.  

  • Silicone adhesives aren’t as hard and strong as epoxies but combine elasticity with high-temperature resistance and thermal stability. They are a good choice when there are coefficient of thermal expansion (CTE) mismatches between components made of different materials.  

  • Polyimide adhesives are also resistant to heat, but have relatively low impact strength, a potential issue for devices (such as smartphone and laptops) that may be dropped.

Today, hybrid silicone-epoxy formulations are also available. Applications for these advanced semiconductor packaging adhesives include flip-chip packaging, which can experience delamination during reliability testing.

Q: How are semiconductor packaging adhesives supplied?

Most products are supplied as either pastes or films.

  • Pastes tend to flow readily and will “wet out” the surface of the substrate so that there is a larger contact area for adhesive bonding. Typically, these products come in syringes, tubes, pails or drums.

  • Films are pre-formed, semi-solid products that come in rolls, sheets, and die cut shapes. They support a smaller adhesive footprint and are increasingly used for die-attachment.

Liquid products are also available but are often used for temporary bonding or encapsulation.

Q: How are pastes applied?

Paste adhesives are dispensed sequentially as multiple bond locations on a substrate are indexed under a dispensing head. They can also be applied through conventional printing methods for increased throughputs. Today, newer semiconductor packaging adhesives have rheologies that are optimized for screen printing and stencil printing. Compared to syringe-style adhesive dispensers, screen and stencil printing equipment can support high-volume manufacturing more cost-effectively.

Q: How are films applied?

Film adhesives are usually applied through molding and eliminate the need to apply paste to individual chips or dies. For large-area chip bonding or encapsulation, vacuum lamination can be used instead. Film cartridges support compression molding, which provides better workability, but film tablets are processed with transfer molding. Some film-type adhesives support continuous-roll processing or the use of precut shapes for higher throughputs and faster processing.     

Q: Are these the only application methods?

No. Liquid adhesives for die attachment may support stamping or dipping. 

Q: What are some issues with pastes?

There are several issues with semiconductor adhesive pastes. Sagging and slumping can cause the adhesive to flow beyond the application site on the semiconductor substrate. Resin bleed-out, a problem in which a portion of the resin separates from the adhesive matrix, can also occur. Edge roll-up or overflow, another processing challenge, can cause pastes to flow up and over the top of the substrate. If this occurs, surrounding components may become contaminated. In addition, controlling bond-line thickness with pastes can be challenging. If the thickness of the bond line varies across the plane of the substrate, the resulting inclination can impede die bonding in a phenomenon known as die tilt.

There are solutions to these issues, however. To avoid sagging, slumping, and edge roll-up or overflow, design engineers can choose thixotropic pastes that flow easily due to the shear forces developed during dispensing but then stay in place once they are dispensed. Resin bleed-out, an issue that tends to occur on high-energy surfaces, can be addressed by using pastes with anti-bleeding agents. Some of these additives form a thin layer with a surface energy that is lower than the substrate. Others enhance the cohesiveness of the paste itself. For bond lines with a more uniform thickness, there are adhesives with self-leveling properties.

Q: What are some issues with films?

Die attach films and other types of film adhesives may have difficulty “wetting out” substrates with non-planar features. In other words, the adhesive won’t flow readily across the substrates. Because films are supplied in a semi-solid state, these adhesives tend to flow less readily than pastes and are less conformable to substrate topography. If voids occur during bonding, electrically-conductive die attach materials may experience increased volume resistivity and decreased electrical isolation. Films also cost more than pastes and are not as cost-effective to apply in low volumes.

Although semiconductor adhesive films cost more than pastes, films may promote greater assembly efficiency at higher production volumes. Importantly, the other issues with films also have solutions. Although semiconductor topography is a factor in how well an adhesive flows across a substrate, wet-out is a mainly a function of the substrate’s surface energy. Surface modification is already an important part of semiconductor manufacturing, and coatings with surface energy modifiers are used commonly. Chemical mechanical planarization (CMP) is designed to reduce surface defects such as pits and scratches on the wafer, so the presence of excessive voids suggests upstream issues, rather than adhesive-related ones, during the semiconductor manufacturing process.

Q: How are semiconductor packaging adhesives cured?

Pastes and films require curing, a process that results in crosslinking and hardening and imparts the semiconductor packaging adhesive’s end-use properties. There are two main curing methods for these adhesives: heat and ultraviolet (UV) light. Dual-cure adhesives that use both heat and UV light are also available. When considering curing methods for high-volume production, energy usage and speed of throughput are both important considerations.

Heat curing, or thermal curing, uses thermal ovens or infrared lamps to either initiative or accelerate curing. Oven curing is faster, but it also ends energy costs. UV / light curing requires a UV light source and, typically, a secondary curing mechanism for “shadowed” areas that aren’t reachable by the UV lamp.   

Q: Are semiconductor packaging adhesives the only way to attach dies?

No. Other methods of attachment include eutectic bonding, solder attachment, and flip-chip technology.

Eutectic bonding, also called eutectic soldering, is often used in wafer bonding. The term “eutectic” refers to how a metal or solder melts at a single specific temperature – in this case, a temperature that is lower than the substrate melting point. By using a metal alloy as an intermediate layer, eutectic bonding attaches a die to package.

Solder attachment attaches the die with solder paste, a combination of metal particles and sticky flux that has the consistency of putty. Flip-chip is a die-attach method that inverts the die face-down on the package or substrate. 

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