New Molding Compound Enables Higher Assembly Throughput

Advanced post-cureless compounds show promise for the future.

By M. Shinahara, Ciba Specialty Chemicals, Los Angeles, California and T: Shiobara Shin-Etsu Chemical Co., Matsuida, Japan

Post-molding cure is a batch process, not suited to an inline, continuous device assembly process. Many efforts have been made by numerous molding compound suppliers to develop a molding compound that does not require post curing, but without much success.

However, many semiconductor companies have found that advanced molding compounds readily meet reliability requirements without a post cure because of their higher purity constituents and the combination of advanced epoxy resin, hardener and improved accelerator technology.

Researchers have also learned that even though satisfactory device reliability and performance can be achieved without post cure, the reliability of semiconductor devices, particularly at elevated temperatures, will improve if they are post cured for a time at the appropriate temperature.

This paper reviews a recent post-cureless molding compound, Ciba-Shin-Etsu X43-2442, which has been field-tested, but has not yet been released to production, and discusses the changes which take place during post cure.

Figure 1 -Barcol hot hardness at different cure time at 175°C.

Chemical Changes

During the 45-180 seconds in the transfer molding of semiconductor devices, with an epoxy molding compound at 170-185°C, an epoxy resin in a molding compound reacts with a hardener (normally a cross linker with phenol group) to induce cross linking. A standard cure molding compound of this type hardens (as often traced by Shore D or Barcol hot hardness), during the molding process as shown in Figure 1.

Figure 2 shows that some 90-95 percent of a cross-linking reaction takes place during a short transfer molding process for regular and post-cureless molding compounds. Once a cured molding compound composition builds up a sufficient green strength, along with proper hot hardness, the molding process is over and molded devices are removed.

Figure 2 -Progress of cure during molding at 175°C.

(Green strength implies hot mechanical strength achieved immediately after molding. If this has been inadequately developed, demolding is quite difficult, particularly with automated molding equipment.)

During post-mold cure, the cross-linking reaction between epoxides and phenol groups continues, and the cross-linked density slowly increases, as seen from the increase in glass transition temperature (Figure 3).

Glass transition temperature is the temperature at which a polymeric substance changes from a glassy mass to a rubbery state as the temperature is raised. At or near Tg, mechanical, electrical, dielectric, physical and chemical properties change to a great extent. In a glassy state, a polymeric material provides higher strength along with superior electric and dielectric properties as an insulator. The junction temperature of semiconductors is normally kept about 10°C lower than the Tg of an encapsulant; above Tg, the CTE becomes higher along with the diffusion rate of gasses.

The post cureless compound delivered "as molded" glass transition temperatures above 150 °C, while a regular grade molding compound needed about two hours for post-cure at 180°C to achieve the same glass transition temperature.

Ionic Conductivity

Extractable ionics (sodium, potassium and chloride ions) and water conductivity were monitored during post cure for up to four hours at 180°C and for up to two hours at 190 °C. The ionic conductivity of aqueous extracts (Table 1) steadily increased with post-cure time. Post cure at higher temperatures raised the conductivity even higher as shown. However, the levels of extracted tonics, such as sodium and chloride ions, remained virtually unchanged.

Figure 3 -Effect of post cure at 180°C on glass transition temperature.

Increased cross-linking generally enhances the solvent resistance of an epoxy molding compound and makes it more difficult to decapsulate a semiconductor device.

Post cure at an excessively high temperature (for example at 190°C or higher) often delivers a lower glass transition temperature than normal. This is particularly true when a high reliability accelerator is employed and is the result of a too rapid conversion into chemically and electrically inert substances.

The formation of extremely corrosive antimony oxyhalides from a flame retardant system (aromatic bromides and antimony oxide) becomes apparent, particularly above 200°C. For that reason, post cure at excessively high temperatures is not recommended.

Physical Properties

Molding compound compositions exhibit relatively low glass transition temperatures of 80-140°C in the "as molded" stage immediately after molding. When subjected to post cure, additional cross-linking reactions between an epoxy resin and a hardener take place, raising the cross-link density. The glass transition temperature increases to 110-135°C for biphenol-based molding compounds, to 145-160°C for o-cresol Novolac-based molding compounds and to 160-200°C for multifunctional epoxy resin-based compounds. The change of glass transition temperature during post cure at 180°C is shown for the same pair of molding compounds in Figure 3. An increasing glass transition temperature indicates a decreasing population of free dipoles in the cross-linked matrix of a cured molding compound and is related to improved electric and dielectric properties, particularly at elevated temperatures.

With post cure, flexural strength increases slightly (Figure 4), while strain at break shows a slight decrease. Flexural modulus also increases with post cure. The increases are, however, generally so small as to be insignificant.

Adhesion characteristics degrade with an increasing crosslink density during post cure, since a molding compound matrix becomes increasingly hard and brittle. A molding compound with marginal adhesion to a leadframe (or substrate) and a die surface often exhibits more delamination after post cure.

Electric and Dielectric Properties

Post cure greatly influences the electric and dielectric properties of a cured molding compound. Volume resistivity increases and the electrical insulating capability of a cured molding compound improves during post cure. The volume resistivity increase at 150°C is particularly remarkable versus post cure time at 180°C (Figure 5). The increase from the volume resistivity at 150°C often increases by a factor of 20X-100X during post cure at 180°C.

Figure 4-Effect of post cure 180°C on flexural strength at RT.

Even after the glass transition temperature reaches a plateau value during post cure, volume resistivity often shows a small but steady increase (Figure 6). This increase in resistivity is considered attributable to decreasing free dipoles by enhanced cross linking, as well as to the deactivation of an accelerator.

A necessary post-cure time has markedly decreased with an advanced accelerator which delivers enhanced reliability performance to semiconductor devices under the stresses of humidity and temperature.

Figure 5 -Volume resistivity change during post cure at 180°C.

Post Cure Influence

The dielectric constant at elevated temperatures (150-200°C) is greatly influenced by post cure, since post cure effectively reduces the population of free dipoles by enhancing the formation of a complete cross-linked matrix. Indeed the dielectric relation spectrometer has often been used in analysis to determine the extent of cure.

A post-cureless epoxy molding compound must deliver a relatively complete cross-linked matrix during a short transfer molding cycle of 1-2 minutes at 170-185°C. An accelerator is converted to chemically and electrically inert substances.

Target performance was set for the latest post-cureless molding compound as follows: 1. A glass transition temperature of 140°C or higher (10-15°C above junction temperature of an encapsulated semiconductor device), 2. Volume resistivity of lx10¹² ohm-cm or higher at 150°C and 3. Satisfactory reliability performance under moisture and temperature stresses.

Properties and performance of the latest post-cureless molding compound showing the contrast between a regular grade of molding compound are listed in Table 2.

Post-Cureless Compounds

The latest post-cureless molding compound delivered a glass transition temperature of above 150°C and targeted volume resistivity at 150°C as well as satisfactory reliability. However, post cure, even for a few hours at 175-180°C, further improved electrical and dielectric properties at elevated temperatures.

These results suggest that it is an extremely difficult task to create an accelerator for an epoxy molding compound that can deliver the rapid formation of a sufficiently cured matrix and a satisfactory glass transition temperature during a short molding cycle. Additionally, after the molding cycle, the accelerator must be converted to a chemically and electrically inert substance.

Failures during high temperature parasitic gate leakage tests by many automotive companies seem to be related to electric/dielectric fatigue and are considered to be related to the population of free dipoles and charge carriers.

Summary

Post cure of an epoxy molding compound greatly improves its glass transition temperature, high temperature volume resistivity and high temperature dielectric performance. These improvements during post cure are due to the formation of an increasingly complex matrix, reduction of free dipoles and the conversion of an accelerator to chemically and electrically inert substances that cross link.

Table 2 -Typical properties of regular and PC-less molding compounds.

	Unit	Regular compd	PC-less compd
Spiral Flow@175°C/ 1,000 psi	inches	30	30
Gel Time@175°C	seconds	20	17
Melt Viscosity @175°C	poises	350	350
Hot Hardness@ 60 secs/175°C		57	83
Specific Gravity		1.82	1.89
Tg(as molded)	°C	135	160
CTE-1	ppm°C	17	13
CTE-2	ppm°C	70	55
Flexural Strength	kg/sq-mm	13	13
Flexural Modulus	kg/sq-mm	1,200	1,350
150°C Volume Resistivity (as molded)	ohm-cm	0.4E(11)	1.0E(12)

See the interview, "Mak Shinohara on Encapsulation" in this issue. Dr. Shinohara may be contacted at 818.265.7103,fax 818.265.7454 or by email at mak.shinohara@cibacs.com.

Mr. Shiobara is a Deputy General Manager of the 6th Department of Silicone Electronic Materials Research Center at Shin-Etsu Chemical Co. Ltd. He has led the development of advanced packaging materials at the company and holds a number of patents. He earned B. Sc. and M. Sc. degrees in applied chemistry from Nigata University, Japan, and Joined Shin-Etsu in 1972. Contact the coauthor at 81.273.84.5360 or by fax at 81.273.84.5369.

(This paper, originally presented at APCON 1997, has been edited for Chip Scale Review and is used by permission.)