.2g-s’2d- P 8 F

. Accelerated Life Testing F?B3-81

of Semiaqueous-Cleaned Surface Mount Devices A side-by-side comparison (with CFCs) reveals how processes affect component reliability. by Satn Anderson arid Williatri C. Rornnn, Moturolri Sernicondiictur I’rudiicts, Phoenix, Arizona

In surface mount technology, the surface mount device (SMD) must rely on the quality of the solder joint for both electri- cal and mechanical connections to the substrate. A good solder joint meets three basic criteria: good wetting of sur- faces, a complete fillet, and the right amount of solder. Other factors influenc- ing quality and reliability include: the sol- dering process itself, condition of compo- nents and substrates being soldered, the choice and application of the flux, and the post solder cleaning method.

Flux and Post Solder Cleaning The choice, application, and method of flux-residue removal are important factors for SMT reliability. However, a highly acti- vated flux should not be used to compen- sate for i l l -prepared sur faces. F lux residues are corrosive and a mildly acti- vated flux should be used whenever pos- sible. The major flux types together with cleaning processes suitable for removal of each type are shown in table 1.

The main characteristics of flux are its ability to promote wetting of surfaces (flux act iv i ty ) and the corros iv i ty of f lux residues after soldering. As a general rule, the more active the flux, the more corrosive the residues. In electronic assemblies exposed to elevated tempera- tures and humidity, flux residues can cor- rode the metallic traces and component leads. Post-solder cleaning removes any contamination, such as surface deposits, inclusions, occlusions, or absorbed mat- ter that may degrade the chemical, physi- cal, or electrical properties of the elec- tronic assemblies.

Such contaminants fa l l in to two groups, polar and nonpolar. Polar con- taminants are compounds that dissociate into free ions, which are very reactive (corrosive) with metals (e.g., rosin acti- vated fluxes (RMA) and soldering salts). They must be removed from electrical

assemblies to prevent reliability failures. Nonpolar contaminants do not dissociate into free ions and are noncorrosive (e.g., pure rosin flux and silicones). They do not present a reliabil i ty problem and removal is generally not required.

Polar contaminants containing ionic residues are the most damaging to elec- tronic assembly performance. They can be both corrosive and electrically conduc- tive. The activating agents that are added to the rosin flux to produce RMA and RA fluxes are typically formulated with organi- cally bound halogen compounds. Activa- tors such as amine hydrohalides and alka- no1 amines hydrohalides are added to the rosin flux to place the flux in the RA cate- gory as specified by MIL-F-14256. When heated, these activators can release the hydrohalide (HCL or HBr) through dissoci- ation as shown by the equation:

R2+ NHCI’ + RzNH + HCI

This strong mineral acid, HCI, easily reacts with the native oxide layer of the

metal, facilitating its removal and expos- ing pure metal to the solder.

It is essential that the activated flux residues are removed from the assembly soon after the soldering process. These residues can reduce s t rong mineral acids, such as HCI, that not only react with the metal oxides but also readily attack the cleaned metal leads and sol- der itself.

Semi-Aqueous Cleaning or Terpenes for Defluxing Flux residues feature physical and chemi- cal bonding mechanisms between the contaminant and the substrate. These can cause contamination to adhere to the substrate. To remove the contaminant, it is necessary to break down the bonds via energy applied universally in the form of a solvent. The process of application is accomplished by dissolution of the con- taminant. This technique simply involves dissolving the flux residues in the solvent and repeating this process until all con- taminants are dissolved and driven away.

SURFACE MOUNTTECHNOLOGY OCTOBER 1991 51

KCD-9438 Makeup

I Supply Water

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.., ... :-

Solvent. . . Emulsion -1

Water Rinse Stage

9488Stage I -~

.. ... Water + Soap

To Burn or Reconstitute

To Waste Water Treatment or Recycle

Figure 1, a semiaqueous solvent-emulsion cleaning system. Reparation of contami- nants in the decanter opens the possibility for recycling of solvents and wastewater.

However, the solvent also must remove residues under very small clearances such as those typically formed under SMDs, component leads, and specific component types. In order to determine the best solvent for a particular applica- tion an understanding of the physical, chemical, and mechanical characteristics of solvents that affect the assembly is necessary. It is also necessary to deter- mine whether the solvent or cleaning agent has an adverse ef fect on the SMD's reliability.

Though CFCs have long been the sol- vents of choice, by now their environmen- tally deleterious effects are well under- stood and the use of substitute agents is recommended. One commercially avail- able hydrocarbon cleaning agent* used in

semiaqueous cleaning processes has been developed as an environmentally acceptable alternative to CFC-I 13. A ter- pene solvent** also has been developed. It features zero ozone depletion potential but it is a flammable substance with a flash point of 11 7" F, and requires cold cleaning. In this evaluation the hydrocar- bon cleaning agent is used.

In a semiaqueous process the con- taminants are dissolved from the assem- blies and the solvent/residue mixture is rinsed from the surface in a water stage. A solvent-emulsion cleaning system is shown in figure 1. The first step in the process is a solvent stage. This is fol- lowed by an emulsion stage dragout from the solvent stage and a small amount of water i s established in the first rinse.

I .

Overflow from the emulsion stage ;an be allowed to settle in the decanter vessel, permitting the contaminants generated in the soldering process to be separated from the aqueous effluent. The contami- nants collect in the hydrocarbon layer of the decanter. The organics in the water primarily consist of extracted surfactants from the cleaning agent formula. This is different from aqueous cleaning where all dirt and saponifiers are flushed down the drain. Such filtration makes the elimina- t ion of water discharge possible in a semiaqueous system.

A Defluxing Matrix An accelerated life testing side-by-side comparison of surface mount assemblies was developed to compare post solder cleaning with CFC-113 and the hydrocar- bon-based semiaqueous cleaning agent. The defluxing matrix consisted of surface mount diodes on printed circuit boards (PCBs) using two solder f luxes, two cleaning methods, and an infrared reflow process. In addition, a group of no-clean boards were assembled that featured a low-solids flux. The clean boards were all assembled with RMA flux and split into two groups for the cleaning evaluation. Fifty percent of the boards were cleaned with CFC-113 and 50 percent with the semiaqueous cleaning agent.

The infrared reflow soldering process is optimized for the maximum limits of temperature and t ime exposure to achieve reflow of 63/37 (Sn/Pb) solder while minimizing thermal exposure to the board and components. The solder is screen printed using 63/37 solder eutec- tic RMA solder paste. The SMDs' internal connections use 88/10/2 solder with a liq- uidus of 260" C. After assembly, the boards were submitted for accelerated- stress l i fe test ing. The layout of the matrix is shown in table 2.

i

Lifetest Definitions and Results The SMDs assembled in the evaluation were 1 .O A Schottky rectifiers (figure 2). The recti f iers' rel iabi l i ty test p lan is designed to evaluate their performance under the influence of temperature, rela- t ive humidi ty, and bias. A technique ca l led accelerated test ing seeks to assess reliability since the failure rate is inherently low.

High humidity reverse bias is an envi- ronmental test to measure moisture resis- tance of plastic encapsulated devices.

High temperature reverse bias test is performed to align mobile ions by means of temperature and bias stress.

Temperature cycle exposes excessive

Four different tests were run:

52 OCTOBER 1991 SURFACE MOUNTTECHNOLOGY

C’

/+0 .08s”(

c

Figure 2, Schottky rectifier package dimensions and footprint. Four tests of accelerated life program were performed after solder assembly and cleaning.

thermal mismatch in the temperature coefficients of expansions between the device materials.

Autoclave measures the device resis- tance to moisture penetration and the resultant effects of galavanic corrosion. The conditions of these tests are shown in table 3. Life test t ime zero device parameters and the first readout observa- tions are shown in table 4.

Results The major observation from the evalua- tion is that boards 9, IO, 1 1, and 12 left a flux film on the SMDs’ leads, preventing good electrical contact during parametric measurements. Once the mechanical probes removed this residue, good con- tact t o the mounted dev ices was achieved and the parameters recorded.

From the results of 168 hours of test- ing no significant shift in device parame- ters are observed in any group, although the no-clean boards continued to be diffi- cult to test. None of the groups tested showed any indication of an “infant mor- tality” problem and it appeared that the semiaqueous cleaning agent demon- strates equivalent performance to CFC- 113. However, testing to 2000 hours or 25 percent failure will be a more accurate determination. Furthermore, based on the electrical contact problems at parametric measurements, the no-clean with low solids fluxes approach is not an accept- able alternative for any process that requires automatic parametric testing.

Reliability Analysis Approach Reliability is the probability that a semi- conductor device will perform its specified function in a given environment for a specified period. Reliability comes from the design of the product and the degree of effort the designer has directed against known failure mechanisms, such as ionic contamination, corrosion, cracked dice, package seal, electromigration, and in this case, process change compatibility with existing components. For most fail- ure mechanisms, life tests (such as those shown in table 3 and reported in table 4), can drastically accelerate the onset of failure. The reaction rate of chemical pro- cesses is related to temperature accord- ing to the “Arrhenius” equation:

R(T) = C(exp)-Ea/kT

where: R = reaction rate C = constant Ea = activation energy k = Boltzmann’s constant T = temperature K

Thermodynamics states that reactant molecules must exceed a minimum acti- vat ion energy before react ion takes place. The fraction of reactants having energies that exceed Ea is given by the exponential term. Accelerated life testing to failure allows the actual failure mecha- nism to be determined.

In calculating failure rates, the com- prehensive method is to use the activa- tion energy for each failure mechanism applicable to the technology and circuit application under consideration. Howev- er, information about circuit application is not always available to the component manufacturer. The alternative approach is to use the single lowest activation energy for the “expected” failure mechanisms and to predict a worst case failure rate.

-

Future Work The life tests of this experiment will be continued to 2000 hours. If failures occur before that t ime their modes wil l be recorded and their failure mechanism determined. The Arrhenius model will be

54 OCTOBER 1991 SURFACE MOUNTTECHNOLOGY

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used to predict failure rate. If all compo- nents reach the end points of the l ife tests with zero failures or significant shifts in parameters then the test will be termi- nated because the devices would have demonstrated acceptable reliability per- formance.

A comparison of terpenes, HCFCs, and semiaqueous cleaning solvents on SMDs and high-density surface mount assem- blies containing both passive and active components should also be investigated.

Acknowledgements The authors wish to acknowledge the assistance of Dr. Randy Fields of DuPont Electronics for assembling the semicon- ductor devices and providing the cleaning process described in this report. Thanks also are extended to Mr. Fritz Balkau of the United Nations Environment Pro- gramme for providing environmental data.

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*Axare1 38, DuPont Company. **Bioact EC-7, Petroferm, Inc.

Contact the authors at Motorola Inc., Semicon- ductor Products Sector , 5005 E. McDowel l Rd., Phoenix, Arizona 85008; telephone: 602 244.6900.

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