So You Want to Print to and Below .6 AAR?

Jim PriceWestern Regional Sales Manager

What is the Goal?

Print to .6 and lower area aperture ratios (AAR) without the need to use exotic stencils or restricted PCA designs.

Consider: The volume of solder paste on a pad to support 0.3mm CSP’s is about 1.8 nanolitres.

To do this in a production worthy manner requires strict attention to the printer configuration and all contributing materials and methods.

Properly executed you can “Move the Boundaries of Physics” that stop traditional SMT printing methods

today historical

0%10%

20%30%

40%50%60%

70%80%

90%100%

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Area Ratio (Opening Area / Wall Area)

Tran

sfer Efficiency

01005’s0.3CSP

FutureFuture

0201’s0.4CSP

SMT TodaySMT TodayLeading Edge

Heterogeneous Area Aperture Ratios (AAR), Cont’d

What do You Need to Do?

The following things must happen when printing those world class applications:

Make certain the bottom of the stencil is as clean as is practically possible in a production setting.

The board to the rail to the stencil must be as flat as is physically possible.

The board is flat to the table and supremely supported by the tooling.

Make sure the apertures are as fully loaded with solder paste as possible.

Make certain the PCB and the stencil separate at the same time over the entire printed surface.

Confirm the squeegee pressure and support tooling are stable

No board clamps!

What Features are Needed? Con’t

With standard PCB clamps, up to 50% of stencil release control is lost and purely dependant on stencil rebound speed.

Things that affect rebound speed:• Mesh or no mesh on the stencil• Speed of squeegees travel

What Features are Needed? Con’t

Vacuum tooling capabilityVacuum capable hard toolingWhy?

Assume the sum of all the aperture areas is 2”^2.At sea level there is 14 PSI of air pressureThat means the total force required to break the gasket seal is 28 pounds!

This area requires more force to separate than-

This area!

Closed Loop Squeegee Pressure and Tooling Deviation Monitor

Super co-planed PCB rails to the printing table to the stencil+/- 10 micron flatness

Ultrasonic squeegees or the functional equivalent.

What Features are Needed? Con’t

Activation of the squeegee blade

- locally modifies the rheology of the solder paste increasing fluidity and…

- increases the downward interaction at the blade tip

This combination improves the packing density of particles into apertures and enhances the cohesive bond between solder paste particles.

What Ultrasonic Squeegees do

What About Other Needed Process Items?

High tension stainless steel stencilNo mesh mount to distort over timeStays rigid for uniform separation over the entire PCB

4 mil thickness is typical for .6 AAR or lower AAR’s when 0201 / .4mm CSP’s or smaller are being printed

Square apertures will increase the available volume of past by 23%

Nano coating stencil treatment

Resists paste leakage when gasketing is compromised

Type 4 solder paste is typically used as type 3 solder particles are too large

Test Stencil Design

.5 AAR

.44 AAR

.38 AAR

.31 AAR

.25 AAR

Printing Results

Before Required Changes After Required Changes

The “Before Required Changes” example has everything done to the printer except ultrasonic squeegees

BLISTRE board Area Ratio quadrant test print data.   A broad range of area ratios is covered.Average volume paste transfer trends are showing ultra sonic squeegees capable of maintaininghigher average transfer efficiency through lower Area Ratios relative to Standard Squeegees.

Ultra Sonic

Transfer Efficiency

0.3mm pitch CSP data containing a much larger print deposit sample size (compared to BLISTREdata in previous slide) in order to compute standard deviations. Although this data covers a narrower range of area ratios, it does show standard deviation differences favoring ultra sonicSqueegees over Standard.

Ultra Sonic

Transfer Standard Deviation