3D Electroform Stencils for Two Level PCB

3D Electroform Stencils for Two Level PCB

Rachel Miller-Short, VP Global Sales

Photo Stencil

Bill Coleman Photo Stencil

Dudi Amir Intel

Joe Perault Parmi

The requirements for two-level PCB with components on both levels have seen a recent increase. Stencil printing on both levels requires special stencil and squeegee blade designs. This paper will examine two such requirements. The first requirement is printing solder paste for .3um uBGA with pads on two levels of the PCB separated by 7 mils (175um). The second requirement is printing flux or solder paste into a recessed area on the PCB for an embedded flip chip. The cavity depth is 14 mils (350um). 3D Electroform stencils will be used for both of these print requirements. Rubber squeegee blades, metal squeegee blades, slit metal squeegee blades, and notched metal squeegee blades will be tested. Print results including paste volume and paste volume dispersion as well as pictures of the solder bricks and flux deposits will be presented.

Key Words

Stencil, Reservoir Printing, Squeegee Blade, Flip-Chip, Solder Paste, Flux, Embedded Components

Background

Step Stencils(1) are commonly used in SMT printing. The most common step stencil has different stencil thickness for printing different types of components. Normally the stencil is thinner for the fine pitch components to assure proper paste transfer and thicker for larger components to assure good solder filets. Two Print Stencil Systems(2) are also useful for a number of SMT printing applications. Applications include mixed technology for Through-Hole / SMT, SMT / Flip-Chip and Glue / solder paste. In these applications a 1st print stencil is used to print solder paste or flux. A second stencil which has a relief pocket anywhere the 1st print material was printed is used to print solder paste or glue. There has been previous work on reservoir solder paste printing(3) The circuit board to be printed is constructed with EMI shielding on the top surface with the component pads located in multiple cavities recessed 20 mils (500um) below the topside surface. The stencil was 5 mils (125um) thick in the cavity with a 20 mil high reservoir on top. Circular apertures were used and the smallest aperture for sufficient paste transfer was a 12 mil (300um) aperture. Flux reservoir

stencil printing(4) has been reported in an application where solder paste was printed for 01005 components with a 2 mil (50um) thick 1st print stencil and flux for flip chip with a 2nd print stencil. The 2nd print stencil is 8 mil (200um) thick with a 6 mil (150um) deep reservoir and a 6 mil (150um) relief pocket where the 01005 paste was previously printed.

Introduction

A flex circuit having a 7 mil (175um) thick stiffener on the back side and a .260mm pitch flip-chip on the front side is used as the test vehicle. Both the flex surface and the stiffener surface separated by the stiffener height are printed at the same time with a 3D Electroform stencil. The front side of the flex circuit is flat and has a .260mm pitch flip chip component as well as several uBGA’s. The plan was to have the Flip- chip embedded 14 mils (355um) deep, however it is very expensive to retool the flex into a two level board. Therefore a shim was attached to the flex providing the required cavity. A 3D electroform stencil was used to print down into the cavity containing the Flip-Chip. In an actual embedded circuit application a Two-Print stencil system would be employed. The 1st print would print flux or solder paste into the cavity on the Flip-Chip pad sites and the 2nd print stencil would print solder paste on the top surface pads.

Stencil, board, and squeegee blade set-up

Back side (Step Print) - The test vehicle for the present study is a flex circuit with a 7 mil (175um) stiffener attached to the back side of the circuit. Figure 1 shows the backside view. The task is to print on two surfaces of the backside of this flex; on the .4mm uBGA shown on the left side of the flex and on the surface of the stiffener. Figure 2 shows the location of the uBGA patterns printed on top of the stiffener, positioned 25 mil (.64mm), 50 mil (1.28mm), and 100 mil (2.54mm) from the step edge of the stencil. The 3D Electroform stencil is 4 mils (100um) thick in all locations across the stencil with a 7 mil (175um) step-up over the stiffener. Figure 3 shows the contact side of the stencil for both the small and large spacing from the step wall. Four different squeegee blades were used to print solder paste on the uneven surfaces: an 80 durometer rubber squeegee, a slit metal squeegee blade,

a notched metal squeegee blade and a straight metal squeegee blade. The slit and notched Electroform blades are shown in Figure 4.

Front side (Reservoir Print) – The front side was created by gluing a 10 mil (250um) shim to the flex circuit with a cavity for the Flip-Chip component pads. The shim is pin registered to the flex then glued in place as shown in Figure 5. The Flip-Chip pads are shown in the shim cavity in Figure 6. A 3D Electroform stencil 2 mils (50um) in thickness with a formed pocket 14 mils (355um) deep is shown in Figure 7. The formed pocket with apertures is shown in Figure 8. The squeegee blade of choice for Reservoir printing is either a contained head pump print system or a pump print rubber blade. A pump print rubber blade was used in this test, shown in Figure 9.

Print and Inspection Results

Back side (Step Print) – The printer set-up is summarized in Table 1. The slit blade was tried first. As seen from Figure 10 excessive paste was left on top of the stencil. The pressure was increased to 4kgm but the result was that the slit portion on top of the raised area bent back at a lower angle without improving the residue paste left on the surface. Figure 11-A shows much less paste residue at 2kgm pressure for the Notched and Figure 11-B shows similar results for the normal straight metal squeegee blade. There is paste residue at the edge or the step-up portion edges. This is caused by a slight lip at the edge of the step. The data will show that the set of apertures located closest to the step edge (25 mils 635um) produced solder bricks with the largest paste volume deviation. Parmi Sigma X Orange Solder Paste Inspection system was used to measure the solder paste volume and solder paste volume deviation for the 3 sites on the lower level of the board and the 3 sites on top of the stiffener. Figure 12 identifies the 6 sites. The Apertures are 10 mil (250um) circles and the Electroform stencil thickness is 4 mils (100um) thick. The measures volume is the measured volume divided by the aperture volume times 100% giving the % volume of paste transferred. The Parmi SPI is capable of measuring paste volumes on both levels even though they are separated by 7mil (175um). The printer set-up, as shown in Table 1, was the same for all 4 blades. Ten boards were run on the Speedline printer; the next 5 boards were run in the Parmi SPI for paste volume measurements. Pictures of the solder bricks for all 6 sites were captured for the 4th board.

Table 2 shows a summary of the paste volume and paste volume deviation results from the 4th board. As seen, the most consistent results are with the straight metal squeegee blade. Location # 6 provided the highest paste volume but the largest standard deviation for all 4 blade types. This location is the closest to the

step edge which is 25 mils (635um). Pictures of the solder bricks for location 1 and location 6 for the straight metal blade are shown in Figure 13 (Loc 1) and Figure 14 (Loc 6). The paste height, as seen in Figure 14 gets progressively higher for bricks closer to the step edge. This same effect is seen in Figure 15 for the notch blade in location 6 but is even more prominent. A more complete picture of the edge effect in location 6 is obtained by analyzing the solder paste volume data for all five runs and all four squeegee blades. Figure 16 shows the solder paste volume for location 6 broken down into the 8 columns of the 64 array pattern of the FC pad sites. Column 8 is closest to the step edge. The paste volume increases from column 1 to column 8 as well as the deviation. Column 8 has more paste volume due to paste being left on the top side of the stencil. The metal and rubber blade show the lowest paste variations compared to the notched and slit blades. These blades provide a cleaner wipe near the edge leaving less paste on top.

Front Side (Reservoir Print) – The printer set-up is summarized in Table 3. The only blade to transfer flux to the substrate for the flux reservoir print process was the pump rubber blade shown in Figure 9. The other blades: metal and 90 degree rubber could not apply enough pressure to force flux through the apertures in the reservoir. A self contained print head similar to Pro-Flow or Rheometric pump systems were available for this test but it is expected either would do an excellent job of forcing flux through the apertures in the reservoir.

The stencil had 3 separate flux reservoir cavities with different aperture sizes. Zone 1 had 4 mil (100um) apertures. Zone 2 had 4.5 mil (112um) apertures. Zone 3 had 5 mil (125um) apertures. The different Zones produced vastly different results. The flux was too transparent to the laser optics of the Parmi to measure flux volume. It was also difficult to distinguish the pad from the flux deposit because of the flux transparency. A raw board with a 14 mil (355um) cavity was rigged up providing good pictures of the flux deposits with the Parmi camera. Figure 17 shows the flux deposits on the flex circuit pads for the lowest viscosity flux (160 poise) for the forward and reverse squeegee stroke. The squeegee speed was 100mm/sec, the pressure 3kgms, and the stencil 2 mil (50um) thick for this set-up. There is flux bleed at the trailing edge of the squeegee stroke as seen in these pictures. On further review of the stencil cavity it was found that there is a slight ridge rising up at the edge of the pocket. The rubber squeegee blade pushes flux against the trailing cavity and forces flux under the stencil due to this ridge. The ridge is caused by the machined mandrel used to create the 3D Electroform stencil and can be corrected. The flux deposit definition improves at higher flux viscosities.

Figure 18 is a picture of the flux deposit on a raw FR4 board for the highest viscosity flux tested (250-350 poise). The squeegee speed is 100mm/sec and the pressure is 3kgms. The left side shows flux deposits for Zones 1, 2, 3 for a 2 mil (50um) thick stencil. The right side shows flux deposits for Zones 1, 2, 3 for a 3 mil thick stencil. The best result is for 2 mil thick stencil with apertures of 100um square (Zone 1). As the aperture size gets larger 112um (Zone 2) and 125um (Zone 3) the flux spreads and shorts between sites. The 3 mil (75um) thick stencil exhibits missing flux deposits for both Zone 1 and 2 as well as excess flux for Zone 3.

Parmi happened to have a type 6 Low Viscosity dispensing solder paste on hand that was also tested for paste reservoir printing. The viscosity of this solder paste was about the same as the high viscosity flux (~300 poise). Results were very encouraging. Figure 19 shows the solder brick deposits when printing at 25mm/sec at 2kgm pressure for a 3 mil (75um) thick stencil. The top portion shows solder deposits along the leading edge and the bottom portion shows solder deposits along the trailing edge. The trailing edge effect is still evident.

Conclusion

Two Level Printing - Printing with a 3D Electroform single thickness on two levels of a PCB is possible. The solder paste volume is slightly higher on the top surface

for all four blade types tested. Straight metal blade provided the lowest paste volume standard deviation of all the blades tested. Edge effect due to raised edge near the step edge caused higher paste deviation near the step edge. Parmi optics is capable of inspecting paste volume on two separate levels.

Reservoir Flux / Paste printing – Higher flux viscosities gave better flux deposit results. Rubber pump squeegee blade is necessary for flux to transfer out of the reservoir onto the substrate FC pads. Solder paste reservoir printing of small apertures (100um) is possible with Type 6 Low Viscosity Dispersion solder paste.

Future Work

(1) Correct stencil step lip / reservoir cavity lip problem. (2) Improve Parmi inspection capability to include very small volume paste deposits and very small flux deposits that are transparent to present optics.

References

1- “Step Stencils” W.E. Coleman, APEX Proceedings’ 2006

2- “Two Print Stencil Systems” W.E. Coleman, APEX Poster paper 2014

3- “Reservoir solder Paste Printing “ Michael Burgess et all SMTAI 2007

4- “Flux Reservoir Stencil Printing” W.E Coleman and Matt Read SMTAI 2012

Figure 1 Back Side of Flex with Stiffener Figure 2 Apertures created on top of Stiffener

Figure 3a Apertures 100 mil from edge Figure 3b Apertures 25 mil from edge

Figure 5 PCB Cavity created by glued shim Figure 6 FC pads in the 14 mil cavity

Figure 7 3D Electroform Stencil (2 mil thick with 14 mil deep cavity) and Laser-cut Apertures

Table 1 Printer Set-Up for Backside Printing

Figure 10 Residue Paste Slit Fig 11a Residue Paste Notch Figure 11b Residue Paste Metal Squeegee Blade Squeegee Blade Straight squeegee Blade

Printer Set-Up Back Side Paste Printing

Printer

MPM Momentum

SPI

Parmi Sigma

X Orange

Paste

Alpha 338 LF Type 4

Indium 9HFA LF Type 4.5

Pressure

2kgm for 12" blade

Speed

25mm/sec

Wipe

Dry wipe after each print

Blade

Slit Electroform Blade

Notched Electroform Blade

Rubber 80 durometer

Figure 12 Locations 1-6 of print sites on Flex and Stiffener

Table 2 Paste Volume and Paste Volume STD for 4 Blade Types

Results % Solder Paste and % Solder Paste Standard Standard Deviation for 4 blade types

Blade Type % Paste Vol % Paste Vol STD

Slit Loc 1 54 11 Loc 2 70 13 Loc 3 85 15 Loc 4 95 6 Loc 5 100 11 Loc 6 104 20 Notch Loc 1 61 10 Loc 2 74 6 Loc 3 72 14 Loc 4 70 15 Loc 5 101 14 Loc 6 115 33 Rubber Loc 1 55 11 Loc 2 59 4 Loc 3 58 12 Loc 4 54 6 Loc 5 73 9 Loc 6 75 22 Straight Metal Loc 1 73 7 Loc 2 71 6 Loc 3 72 6 Loc 4 77 6 Loc 5 82 9 Loc 6 98 15

Figure 13 Solder Bricks Location 1 Straight Metal Blade

Figure 14 Solder Bricks Location 6 Straight Metal Blade

Figure 16 Solder Paste Volume Shown by Row 1-8 of the FC Array for Location 6 (8 is closest to step-edge)

Table 3 Printer Set-Up for Reservoir Flux printing

Printer Set-Up Front Side Flux Printing

Printer

MPM Momentum

SPI

P

armi Sigma X Orange

Flux

Type

Viscosity Poise

Indium 3600

160

Indium 676

200

Alpha 353 Black

<150

Alpha 390

140-250

Alpha 338

250-350

Paste

Alpha Type 6 LV Dispense

Pressure

1.5 - 6kgn for 12" blade

Speed

.12mm/sec - 150mm/sec

Wipe

Dry wipe after each print

Blade

90 degree rubber

Pump Rubber

Metal

Figure 17a Flux Deposits on Flex FC Pads using 160 poise viscosity Flux (Front-Rear Squeegee Direction)

2 mil (50um) Thick Stencil 3 mil (75um) Thick Stencil

Zone 1 100um Aperture Zone 1 100um Aperture

Zone 2 112um Aperture Zone 2 112um Aperture

ZZ

Zone 3 125um Aperture Zone 3 125um Aperture

Figure 19 Type 6 Low Viscosity Dispensing Solder Paste Solder Bricks on Top and Bottom of FC Pad sites Bottom picture is Trailing Edge of Squeegee Direction