SMTAi Paper_Printing Small Aperture components Final Aug_2_2019.pdf - 第2页

Printed Circuit B oards (PCB) : As Process Engineers, most PCB d esigns are created long before we beco me involved. Design for manufacturing considerations are most often out weighted b y cost requirements. We are often…

STENCIL PRINTING 008004/0201 APERTURE COMPONENTS

Edward C. Nauss and Michael Butler

ITW EAE

Hopkinton, MA

enauss@itweae.com; mbutler@itweae.com

ABSTRACT:

This paper will focus on the application requirements of

solder printing small aperture designs, concentrating on

008004 (inch) / 0201 (metric) size components, and the

results of a design of experiment printing these challenging

apertures. As Moore’s law continues to be applied to

component miniaturization, the next installment of reduced

packaging has arrived in the form of the 008004/0201 for

resistors and capacitors. Component size roughly the size of

a grain of sand presents specific challenges to the solder

printing process. To address these challenges, each aspect of

the printing process will need be examined. This includes

essential machine requirements, including correct squeegee

blades, tooling support, and calibrations, to meet the

demanding specifications. The correct match and design of

materials will be addressed, focusing on the stencil and

substrate design along with solder paste and cleaning

solvent requirements. A design of experiment will be

reviewed that applies the machine and materials discussed,

including the printer and Solder Paste Inspection (SPI) setup

and the specific machine parameters used. The results of

these DOE’s will then be closely examined.

Key words: printing, miniaturization, component, stencil,

solder paste, 008004, 0201mm

BACKGROUND:

Moore’s Law states that the number of transistors a

microchip can contain is expected to double every two

years. The same trend can be applied to miniaturization of

components. Demands for high functionality from mobile

devices, smart watches, wearables, military, medical and

audio technology continues to drive the development of

component miniaturization. The SMT industry has seen the

introduction of the next generation of component

miniaturization every 4-7 years with the latest release of the

008004” (0201mm) component size. With a 6% reduction of

area and a 1.6% reduction in volume when compared to a

01005” (0402mm) package size, the equipment

manufacturers have quickly adapted to this next challenge.

However, when introducing micro devices to a

manufacturing process, the problematic hurdle that needs to

be addressed is the printing process. Focus on the materials,

machine and process that ties them together, needs to be

addressed in order to successfully introduce 008004”

(0201mm) components into a manufacturing process.

Developing a process that can produce a product containing

008004” (0201mm) with a Cp value that is twice a capable

can be achieved when each element of the process is

working together. This will require the engineer to

reevaluate their present process to adapt to the challenges

this component will require in order to successfully

implement a 008004” (0201mm) component. This may go

as far as implementing a clean room environment. The focus

of this paper will be on the individual elements of the

printing process and the results of testing performed at the

ITW EAE process lab in Hopkinton, Massachusetts.

Status of components today:

Referencing (Figure 1) most of the market has become

comfortable with products that contain components down to

0402” (1005mm). Recently 0201” (0603mm) components

are becoming more common in mass production with the

01005” (0402mm) just now entering the mainstream. With

each step, we have adapted stencil designs and materials to

implement these components into our designs with little or

no consideration to the other key elements. Printing

008004” (0201mm) requires us to examine each element of

the process prior to fabricating our first substrate to insure

each element is designed with this component challenge in

mind. The solution to micro component printing is in the

aggregate, where each element is using best practices for

optimum results. Any deviation or ignoring any element

will result in a less than repeatable process.

Figure 1: Component sizes

Printed Circuit Boards (PCB):

As Process Engineers, most PCB designs are created long

before we become involved. Design for manufacturing

considerations are most often out weighted by cost

requirements. We are often faced with having to design the

process to the board rather than the other way around where

adjustments and compromises are made to accommodate

shortfalls in the PCB design. This is where the Process

Engineer needs to take an active role to make sure that

success is designed into the PCB to insure repeatable

results. So far, applications have been focused on micro

components with little to no mixed technology on multi-up

panels. The problems with mixed technology, also referred

to as the broadband printing issue, is not the focus of this

paper though this would need to be addressed before

proceeding. Most applications to date have been using PCB

thicknesses of 0.030” (0.762mm) or less, so this should be

expected. Based on the PCB thickness and the amount of

routing, special carriers maybe needed to transfer the

product from machine to machine as well as stabilize it

during the reflow process. One of the key elements to

successful printing which the PCB plays a major role is the

capability to form a seal between the stencil and the PCB

commonly referred to as gasketing. Gasketing in turn plays

directly into the process to transfer the paste from the stencil

aperture to the PCB pad efficiently and repeatably.

Maintaining a consistent board thickness, by elimination of

variance in mask thicknesses from PCB to PCB, is key as

this determines the position of the PCB to the bottom stencil

surface. Mask encroachment on to the pad surface should be

eliminated as this will greatly affect the capability to seal

the aperture. Nomenclature and silk screens should be

avoided in the PCB design. Issues arise when silkscreen is

equal to or greater than the stencil thickness being applied

along with being located adjacent to the component. Logos

and identification information should be presented on the

back of the PCB or located in consideration to cause the

least effect on the critical components. The use of barcode

labels should be avoided as this is the source of many

printing issues with less challenging components. Barcode

labels should be applied after the print process is completed

if possible. One of the critical mistakes when designing

PCBs for micro component printing is to make sure the pad

is not significantly below the mask height. When designing

a board with a bare copper pad, the pad should be just below

or equal to the mask height. If the pad is below the mask,

this creates a gap that the paste must now overcome when it

is printed into the aperture. Because the paste must flow

between the aperture opening on the bottom of the stencil to

the pad surface, the paste will be unable to secure a proper

adhesion to pad surface. The result is erratic print results

that is represented in opens and insufficient volume or pad

coverage. In some cases, the PCBs with this issue become

non-manufacturable due to erratic results from the printer.

To insure the pad is positioned correctly to the mask height

and has a flat planar surface, an electroless nickel

immersion gold (ENIG) plating should be considered. ENIG

plating, consisting of an electroless nickel plating covered

with a thin layer of immersion gold, protects the nickel from

oxidation has shown to have the best results. In the board

design, the decision to use mask defined pads often present

issues. Accuracy of the pad locations as well as pads that are

sized larger than the specification have been issues when

using mask defined pads. Location of the fiducials,

especially when implementing a multi-up panel, is critical

for the machine vision systems to properly perform

alignment. The fiducials should be part of the artwork and

be present on the PCB image for best results. Avoid locating

the fiducials on the breakout panel as this tends to add to

any alignment error. Stretch and step and repeat errors

should be avoided as we are dealing with a recommended

pad size of 0.005” X 0.006” (0.127 x 0.1524mm) where as

much as a 0.001” (0.0254mm) error can have significant

consequences. Investment up front in the PCB design and

manufacturing will insure success where some of the

previously described issues are often difficult if not

impossible to overcome.

Squeegee Blades/Enclosed Heads:

A best practice is to separate a set of squeegee blades

specifically used for micro-component printing. This insures

that the blades being used are undamaged and not worn. The

squeegee requirements for micro-component printing is

simple, spring steel blades with a squared edge is all that is

required. A blade angle of 55 degrees is also recommended

where standard blade angles are set to 60 degrees. This

change in angle allows more surface area of the blade over

the aperture to promote an improved aperture fill. It also

improves sheering off the paste at this angle when the blade

passes over the aperture to prevent paste drag out and erratic

aperture fill. Blade length should match the PCB as closely

as possible with a maximum size range within 2 inches of

the PCB size in X direction. This will center the squeegee

pressure on the PCB as well as prevent long term damage to

the stencil. Inspect the blades every time prior to use for

cleanliness and for damage. It should be noted that enclosed

heads have been used for fine featured printing applications

with great success in the market today. The extrusion flow

from the pressurized chambers are compatible for repeatable

aperture fill for micro-component printing. Some Type 6

pastes have a limited stencil life where enclosed chambers

address this issue and minimizes paste waste. For this test

we focused on squeegee blades as this represents most of

the process applications on the market.

Stencils:

For this experiment we used a 29” x 29” (736 x 736mm)

fine grain, laser -cut, Nano-coated, 0.002” (0.0508mm)

thick stencil. The aperture size is a square, 0.005” x 0.006”

(0.127 x 0.1524mm) that is one to one with no reductions or

variations in shape. Based on previous experiments we

determined that the 0.002” thick stencil had the best transfer

efficiency. When specifying a stencil thickness more often

we take in consideration the two ends of the spectrum for

paste requirements and find a compromise in-between. Most

applications so far using micro-component printing, have

had compatible component mixes where there was not a

significant difference in requirements. The frame size we

used was 29” x 29” (736 x 736mm), however, 23” x 23”

(584 x 584mm) stencil may be better suited based on

common average board size for 008004” (0201mm)

applications and stencil tensioning requirements outlined

below. It is recommended using a fine grain stainless steel

stencil that is laser cut. Electroform stencils have fallen out

of favor with reported issues such as variation on aperture

size and foil thickness and stretch being introduced to the

image. Recommended for this application is to use high

tensioned foils. Stencils have a range for tensioning

normally 28 – 40N/cm² (Newton/centimeter). Most stencil

tension falls into the lower 30-Newton range. Increasing the

tension into the upper 30-Newton range prevents stencil

drag. Stencil drag is when you are using a thin stencil foil

with a significant amount of aperture openings. The surface

tension of the paste that has now adhered to the PCB, pulls

at the stencil foil during release, resulting in lower paste

transfer efficiency. The higher tension results in a cleaner

more balanced release with no transfer issues. Nano-coating

is recommended as studies have proven it improves transfer

efficiency. Stencil manufactures have improved the

application methods for applying Nano-coating to stencils

that has improved its manufacturing life. However,

aggressive fluxes and repeated aggressive wipes will

eventually wear the coating off. Careful handling of thin

stencils should be emphasized as they are easily damage.

Special care should be used when storing and transferring to

and from the machine. Take care when handling blades over

the stencil in the machine as a dropped blade could ruin a

stencil quickly. Cleaning the stencil using ultrasonic

methods after printing is essential for continued stencil life.

Type 6 paste is difficult to clean and can become difficult if

not impossible to remove if not removed promptly after use.

Solder Paste:

The recommended solder paste for this aperture size is a

Type 6 powder size. The specification for Type 6 is a mesh

size of +635 mesh size with the ball size range of less than

20µ with an average of 10µ. Type 4 paste is the prevailing

powder size presently being used in the SMT market.

Significant improvements in powder size yields have eased

pricing for Type 4 and Type 5. However, Type 6 pricing has

remained constant where comparative pricing can be three

times the cost of the Type 4, they are presently using.

Squeegee speeds and release parameters are dictated by the

paste formulation and flux type. From the printing

prospective, Type 6 prints like any other paste, however

considerations of the requirements down steam need also be

considered. Matching the paste to the Pick and Place as well

as the requirement for using nitrogen during reflow must be

also considered when using Type 6 paste. As best practice

for a 0.005” x 0.006” (0.127 x 0.1524mm) aperture is a

Type 6 paste – experiments going forward need to be

performed to see if a hybrid Type 5.5 powder size or a Type

5 can be substituted for a Type 6.

PCB Support:

The consensus in printing is that tooling support is essential

to successful, repeatable print results. The aluminum tooling

plate is still the touchstone that all other forms of support

are tested against. Since most applications for micro-

component printing use PCBs 0.030” (0.762mm) or less, the

tooling in combination with vacuum assist to insure the

PCB is flat, level and supported will give the best results.

The plate should be designed so the PCB fits in a recessed

pocket with the PCB surface positioned above the tooling

surface. Support wings are also recommended to support the

squeegee outside of the print area to prevent long term

damage to the stencil. Recommended is a Venturi vacuum

system as the Hg (inches of mercury) produced by standard

vacuum systems may not be enough to flatten the PCB.

When implementing vacuum openings on the plate, take in

consideration the thickness of the PCB relative to the hole

size to prevent deflection or “dimpling” of the PCB surface.

Special attention needs to be focused on the leveling of the

bottom of the tooling plate fixture. This will be reflected on

how well the PCB gaskets to the stencil. Addressing how to

hold the PCB in place during the print process, vacuum is

the preferred method to insure a flat surface over top or side

clamping for PCBs thickness below 0.030” (0.508mm).

Wiping:

Wiping is the first defense against defects and can have both

a positive and negative impact on the process. Determining

the correct frequency, wipe sequence, compatible

chemistries and materials will impact repeatability and

eliminate potential defects. Micro sized apertures require

more frequent wiping where a simple experiment can

determine the starting point, however, over-wiping with

solvent can have the same negative effect as under-wiping.

The test involves printing a board and then drive the vision

camera under the stencil to inspect the apertures for any

paste squeeze out or clogged apertures. Note, that the

apertures will contain some paste that was not released

based on transfer efficiency and stencil quality. In most

cases this paste will be pushed out on the next print

sequence and will not require a wipe, please judge

accordingly. Continue this process of inspection until

defects are starting to form. Subtract 1 board from the total

and this can be your starting frequency. If a Solder Paste

Inspection machine is available, then based on results, this

can be used to determine the correct frequency of wipes.