P

Solder Paste Design Considerations

By Dr. M. Warwick and P. Hedges

S older pastes based on RMA- type flux formulations may be used as no-clean prod- ucts for many, applications.

They offer a wide process window, and their residues are electrically in- ert. However, many users are con- cerned about residue levels and their effect on pin testing and the appear- ance of reflowed PCBs.

Typical RMA paste fluxes con- tain 60 to 70% non-volatile matter. This allows the products to show excellent printing, tack life and sol- der balling performance during re- flow in air. Early efforts to produce solder pastes with reduced residue levels resulted in materials with a non-volatile content of 40 to 50% in the flux, and working time in the printer was reduced from eight hours to between two and four hours. Tack performance was also reduced. Reflow was possible in air, but a nitrogen atmosphere was necessary to obtain the best results for fine-pitch applications.

Solder pastes with a non-volatile content of about 15% have been de- veloped for use where nitrogen re- flow atmospheres are commercially viable and very low residue levels are required. Typically, these have a very short open time and are not suitable for fine-pitch printing.

The electronics industry is faced with parallel changes in technology that are placing even greater de- mands on both process control and materials. The elimination of CFC cleaning has presented a range of options, including a greater use of no-clean soldering products. At the

FIGURE 1: The relative residue levels of three solder pastes.

36 CIRCUITS ASSEMBLY AUGUST 1994

same time, electronic components are shrinking, and lead pitch is de- creasing. Consequently, the user must adapt to new products with different characteristics while work- ing with the limited capabilities of the printing equipment. It goes without saying that all of this is oc- curring against a background of greater competition and a realiza- tion that “right the first time” is the only option available. One change that opens up the process window in these circumstances is nitrogen atmosphere reflow. This allows the solder paste manufactur- er to adopt a different approach to product design, which can lead to lower residue materials without sacrificing too much in terms of all-around performance.

No-Clean Fluxes Three principle factors have to be taken into consideration in defining a no-clean solder paste product. Some customers will be concerned by the cosmetics of the finished boards, while others will consider electrical reliability more impor- tant. Many users will be concerned about the testability of the board, including any debris that will pose a risk of both poor electrical con- tact and progressive fouling of the test pins. Aithough there is an ar- gument that having sufficient resin in the residues can effectively im- mobilize activators and metal salts, it is probable that all three factors bring about the conclusion that minimizing the quantity of residue is a prerequisite for no-clean solder paste formulations.

The major component of all solder pastes is, of course, the alloy powder, which for stencil printing products is typically 90% by weight of the mate-

rial. With solder having a composi- tion near the tin-lead eutectic and flux paste with a typical specific gravity of one, flux and powder are present in approximately equal pro- portions by volume. It is difficult to increase the proportion of powder significantly since the resulting for- mulation would not show acceptable rheology. Thus, a practical route for reducing residue levels is to elimi- nate non-volatile matter from the flux.

The major components of the flux are the solvents, resins, activators and rheology modifiers. The resin, usually rosin or modified rosin, con- tains some volatile fractions and does suffer some decomposition at reflow temperatures. But, in tradi- tional formulations, the residue lev- el is directly related to the quantity of resin in the flux. In these prod- ucts, comparatively small amounts of rheology modifiers are needed, although they do increase the resi- due burden. Activators in conven- tional products are usually carbox- ylic acids and amine hydrohalides, which are soluble in the resin, along with any tin and lead salts produced during reflow.

Obviously, the way to significantly reduce residue levels is to use less resin or to substitute part of the ro- sins with more volatile resins. This leads to a number of consequences including loss in tack, poorer solder balling performance and a reduced tolerance toward printing process variations. A reduction in the level of resin has to be partially compen- sated for by an increase in the quan- tity of rheology modifiers, which off- sets some of the residue reduction achieved. There is an incentive to use volatile activators since the rheology modifiers do not solubilize the residual activators effectively. There is also a need to reduce the ionic contamination on the reflowed board. This can have an adverse ef- fect on wetting and solder balling, particularly during extended reflow profiles.

One of the functions of the resin in the paste is to prevent reoxidation during reflow. But, reducing the res- in's level in the flux exaggerates the disadvantages of using volatile acti- vators. The flux paste designer has to address these conflicting factors. One way to address this is by replac- ing the normal air reflow atmo-

TABLE 1: The effect of flux non-volatile matter content on paste properties.

sphere with one essentially free of oxygen.

Fine-Pi tch Printing Conventional wisdom says that the finer the pitch of the components to be soldered, the smaller the solder particles should be. This is an obvi- ous rule that leaves room for manip- ulation. Solder particles should not be so large that they cause a restrict- ed flow into the stencil openings be- cause this might promote segrega- tion of the paste, which would cause inconsistent metal deposition.

Smaller solder particles produce a print that shows better definition, thus reducing the risk of bridging if there should be a tendency for paste to slump or if there is any deposit misregistration. In addition to these effects, stencil openings have a char- acteristic ratio between the perim- eter surface area in contact with the paste before it is printed and the sur- face area of the paste in contact with the board. This can dictate the re- quired rheological and tack proper- ties of a paste in order to give opti- mum printing performance. Solder powder particle size distribution in- fluences both rheology and tack, thus having a second-order effect on print quality.

It is generally f-elt that powder particles in the range of 38 to 53 micrometers can be used for print- ing lead pitches down to 25 mils. Good printers and, particularly, good stencils can improve process capabilities and even extend the range down to 20- or 16-mil-pitch components. However, for such fine- pitch applications, it is more com- mon to specify solder powder in the range of 25 to 38 micrometers. In- deed, solder pastes have been devel-

oped that use even finer powder for ultrafine-pitch printing. These prod- ucts tend to require more active flux systems because the very high sur- face area of fine solder powder in- creases the amount of tin and lead oxides to be dispersed during reflow. This is not always compatible with the needs of a no-clean process. The use of a nitrogen reflow atmosphere cannot completely solve the problem. One of the objectives in developing new no-clean solder pastes is to pro- duce materials that can yield good fine-pitch printing with coarser pow- der fractions than normal.

The Progression to Reduced Residues A typical solder paste for conven- tional PCB assembly would conform to the RMA classification and might contain one or more resins at up to 60 to 70% by weight of the flux. About 5% of the flux might be made up of rheology modifiers, and the to- tal activator content would be less than 1%.

Such a product would have a wide operating window showing good stability during printing un- der a range of conditions. Tack per- formance would be good, and the product would be insensitive to re- flow temperature profile variations. In most circumstances, the use of reflow atmosphere inerting is un- likely to bring any benefits.

This type of product is what users have come to expect, and it is the measure by which new materials are assessed. In many situations, this product is perfectly acceptable as a no-clean paste, and it easily passes the requirements of Bellcore, IPC and company test protocols.

The problems for some users with

38 CIRCUITS ASSEMBLY AUGUST 1994

FIGURE 2 Tack profiles for several solder creams.

a product, such as product “A”, are the quantity and appearance of the residues. The high level of modified rosin in the flux leaves a generous halo of residue over and around the solder joint.

The first obvious design step was to reduce the quantity of resin used in the formulation. Using this ap- proach, the non-volatile content of

the flux was reduced to two-thirds the original valbe. The appearance of the reflowed boards improved sig- nificantly. However, it was apparent that the open time for the product (product “B”) was reduced, and this affected the ability of the paste to hold components in place before re- flow if there was a significant delay before placement. The paste also

Custom/Special Feeder Delivery Systems Unique Component Applications On-line Systems Ski-Slope Tube Feeders Packaged Systems Tube Stacking Feeders And Lots More!

For a FREE Copy of our 32 page bound catalog including Current Pricing

Call 1-800-562-3648 Standard items in-stock, ready for immediate delivery

Can be integrated with no modifications to your work cell or feeder assemblies Cost effective- feeders and systems are versatile and totally interchangeable

Assembly Tech EXDO #I 31 17

3769 E. Hardy Drive Tuscon, AZ 8571 6 Phone: 602-326-3352 Fax: 602-326-3542

See us at SMI, Booth #440 CIRCLE 27

40 CIRCUITS ASSEMBLY AUGUST 1994

showed a tendency to dry out more rapidly on the printer than the con- ventional product.

Experience showed that the product demonstrated good solder balling behavior in air but was sometimes inadequate with fine powders when a nitrogen atmosphere was recom- mended. Noting these disadvan- tages, it is apparent why this paste could only be an interim solution to user’s demands. Although it did sat- isfy some customers’ requirements, significant improvements could be achieved by modifications to the res- in and solvent blend. Data in Figure 1 show the relative residue levels for three products subjected to a linear temperature ramp. And in another evaluation, it was seen that solvent mained in the paste longer for roduct “D”, possibly explaining rhy it showed superior solder balling erformance over “B”. Improvements produced in “D”

ieet the requirements of a large umber of users, but development as not ceased. The primary focus as been on the selection of different ilvent blends since it has been

JUMPER WIRES OR

ETCH CUTS?

NOT ANY MORE! Our patented, automated and UL recognize process has added a “fine line“ layer of cir- cuitry to this board prior to assembly!

“WIRELESS ECO’S OFFER” Improved assembly throughput L V d I auiings Reliability, repeatability and reduced handling Ability to add SMT components as well as new signals Customer appeal through better appearance

P,.*rrn.’

Call for free sample

SElm Awn@ Hudson NewHamDhue03051 phone (f83]88$+Wfax (6031889 1324

CIRCLE 28

shown that these can have an effect on both open time and solder balling. With these improvements, it has proved possible to reduce the residue level of the pastes even further.

Products for Reflow in Nitrogen Although improvements in solvent and resin types allow adequate print- ing performance, and open time and tack life can be retained in pastes with very low flux paste solids con- tents, the performance of these ma- terials in air reflow is not sufficient. An inert gas atmosphere is needed to provide reduced solder balling. The effect appears to be essentially pro- gressive as the oxygen content in the atmosphere is reduced and as the flux solids content falls (Table 1).

Early very low-residue formula- tions were, in fact, developed for condensation reflow, which offers the dual advantages of a compara- tively short temperature profile and an inert atmosphere. Both of these advantages minimize the demands placed on the flux compared with IR reflow in air. Nevertheless, these early products suffered from other

TABLE 2: Printing performance of a very low-residue solder paste.

process deficiencies, including poor printer life and tack performance.

A typical formulation contained about 5% resin in the flux with near- ly the same proportion of rheology modifiers. Both of these constituents

Flexible to adjust to TLS-KIT-6OOOA any board size.

BENEFITS: Save money! Reuse

Rigid support to AUTOMATION common fixture elements

prevent sagging 101 Billerica Avenue, Bldg. 5 North Billerica, MA 01862 Phone: (800) 648-3338 0 Reduce set-up times

0 Reduce handling errors FAX: (508) 670-2300

See us at SMI, Baath #1245

Call 1-800-648-3338

CIRCLE 29

had to be increased to provide good reflow characteristics in a nitrogen atmosphere in an IR oven. Solder paste “F” was the result. It became obvious that the limitations of this product were unacceptable to some

Mcroscm’s new AT100 tapes 12,000

axial components per hour. That’s fast!

Want to see more? Like the photo detector that eliminates gap? Call Rick Martin at

206/221-8212 for an even faster response. t

microscan P.O. Box 1227 Freeland, WA 98249 (206)221-8212

CIRCLE 30 CIRCLE 31

AUGUST 1994 CIRCUITS ASSEMBLY 41

CAN HAVE IT ALL!

Want real-time data displayed as you are profiling? Use the SlimKlC as a wireless remote RF TRANSMITTER.

Want to download data after profiling? Use the SlimKlC as a DATA LOGGER.

YOU CAN with the SlimKIC.

0 Transmit data at up to 20 readingslsec.

0 Store data at 60,000+ data pointslu p to 60 sa m pleslsec .

0 Accurate Profile Prediction. 0 Accepts input from 12 Thermo-

couples. 0 Software in DOS or Microsoft@

Windows'" compatible versions 0 Handle directlv from heat soiirce

Only 3" wide, but what a bundle! Who says you can't have if all? Call KIC for more info, or a free brochure.

61 91673-6050 Fax: 61 9/673-0085

San Diego, CA. KIC Thermal Proliling IS a dlvislon oi Embedded Designs Inc

FIGURE 3: Relative residue levels for solder paste formulations.

customers, where extremely low resi- due levels were not actually re- quired. Consequently, the next suc- cessful developments were based on the idea that resin levels could be in- creased further. Data in Figure 2 show the significant effect this had on tack performance. Printing perfor- mance was also improved (Table 2).

Figure 3 shows the relative residue levels for a number of solder paste formulations designed for reflow in nitrogen. Data are for the flux only, but they may be directly related to paste performance since each prod- uct would contain the same amount 3f metal. As with the products for air reflow, the pastes named in Fig- ure 3 behave somewhat differently when subjected to a more normal paste reflow profile. These data again show that variations in resin and solvent types can have a marked effect on the rate at which flux de- grades to the final residues on the PCB. This probably controls factors such as resin melt volume, fluidness and spread, and has an effect on sol- der spread and solder balling.

Experience shows that such prod- ucts are approaching the conventional, higher non-volatile matter solder -)astes, but further improvements are ;till being sought. In particular, it is xobable that the current generation If low-residue products are more sus- :eptible to shear thinning during ieavy working at high squeegee ;peeds and high repeat print rates. rhis is partly related to the tendency If rheology modifiers (on which these products depend heavily) to form a jolvent gel structure that can be da- maged easily. As a result, there is an

effort to find rheology modifiers that produce a powerful gelling effect at low concentrations in order to mini- mize residue levels but with an en- hanced resistance to breakdown. It is also important that these materials also leave electrically safe residues after exposure to reflow temperatures.

Conclusion Solder paste manufacturers have developed products designed to de- liver what the customer wants. Conventional RMA products pro- vide robust, no-clean pastes that leave electrically safe residues, which is entirely acceptable in many situa- tions. If a customer wishes to reduce residue levels, then pastes with a lower non-volatile matter content can be used in standard production processes with only a modest reduc- tion in ease of use. Currently, there is no such thing as a no-residue paste. It is questionable whether such a product could be developed. However, this has not stopped paste manufacturers from developing their products so that they yield very low residue levels, provided they are re- flowed in nitrogen. !I3

4 cknowledgements The authors would like to acknowl- d g e the assistance of many col- leagues in collecting the data used in this paper and Multicore Solders Ltd. for permission to publish this paper.

Dr. M. Wavwick is group technical director and I? Hedges is a senior re- search chemist with Multicore Sol- ders, Richardson, TX.

CIRCLE 32

42 CIRCUITS ASSEMBLY AUGUST 1994