Tricks of Cost Effective Panelization

7/29/2019 Tricks of Cost Effective Panelization

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2/4www.pcdmag.com PRINTED CIRCUIT DESIGN MAY 2002 19

FEATURE >

T

he panelization of PCBs should

be of concern to anyone involved

in the design and engineering of

boards. Good panelizationtechniques will reduce

manufacturing cycle time and cost, while

bad panelization will cost time and money.

Determining the number of boards that can

fit onto a panel and whether the construction

uses common core material are the main

panelization issues that fabrication houses

face.

The number of boards up per panel

directly correlates to the price of the individ-

ual PCB. The availability of material directly

correlates to the ability to build the PCBs inthe shortest cycle time.The best way to design

for manufacture is to know your fab houses

panel sizes, cores and pre-preg inventories.

Size is everythingPanel sizes vary between fabrication houses

and they will depend on an order size. Keep

in mind that in a manufacturing environ-

ment, material waste and inventory are the

most important areas to control to attain

profitability. The most common raw lami-

nates produced are manufactured in 36 x48 and 36 x 72 sheets. Most fabrication

houses will decide on panel sizes that pro-

duce the best yields from these sizes. The

sizes that are most common are 12 x 12,

12x 18,16x 18,18x 24 and at some

of the larger shops, 24 x 36.

Most fabrication shops will limit them-

selves to two or three of these panel sizes.

This will reduce the inventory of materials

and equipment that is needed in manufactur-

ing, e.g., film sizes and tooling plates.

Limiting panel sizes also aids in the automa-tion of the individual departments. Smaller

board houses generally use the 12 x 12

through 18 x 24 panel sizes, while the

larger production houses generally use the

larger sizes of 18 x 24 and 24 x 36.

As mentioned earlier, the board houseuses the criteria of board size, quantity and

sometimes manufacturing difficulty to

decide on the panel size. A small board, lets

say 1.8 x 1.8, in a prototype run of 10

pieces would not be panelized onto an 18 x

24 panel because this would produce too

many boards; a 12 x 18 panel would prob-

ably be the best choice. And a design that is

pushing the boundaries of manufacturing

ability would probably be panelized onto a

smaller panel size even if its size and quantity

define a larger panel size.This way the man-ufacturing tolerances will be minimized on a

smaller panel size, thus improving yields.

These panel sizes are merely the rough

sizes used by the board houses for fabrica-

tion. The actual image areathe part of the

panel that can be used for the individual

PCBis smaller. Generally the board houses

need a one-inch border around the panel.

This area will incorporate all the tooling

holes required for lamination, the registra-

tion alignment holes, commercial coupons

and area needed by the manufacturingequipment to produce the panel.The image

area can be further reduced if impedance or

customer coupons are required.The follow-

ing is a list of panel sizes and image sizes:

Panel size Image area

12 x 12 10 x 10

12 x 18 10 x 16

16 x 18 14 x 16

18 x 24 16 x 22

24 x 36 22 x 34

The spacing between boards required by

the board house also has an impact on the

Tricks Of Cost-EffectivePanelizationYou pay for the whole panel, so why not use it

By Keith Schenk



image area.This spacing

is generally .100 to

.200 inches between

parts.This spacing helps

accommodate the rout-

ing out of the individ-

ual boards. If the boards

are to be palletized

that is, set into anarraythen the spacing

applies between pallets

as well as between the

boards inside the pallet.

The palletization of

the PCB has its own

unique criteria because

the assembly must be taken into account.

There are two schools of thought on this sub-

ject. I have seen customers/ designers send

data in as a one up PCB and ask us to pan-

elize it into an array for them, while othersprefer to send the data in as an array format.

The decision on which method to use to

send the design to the fabrication house

depends on whether you have direct knowl-

edge of how the PCBs will be assembled and

who will be doing the assembling.

If you know who is going to assemble the

boards, it is best to get the physical dimen-

sions of the array, number of boards per array

and any fiducial locations that will be

needed, and relay this information to the fab-

rication house.The fab house can then deter-mine for you any effect the physical

dimensions of the array will have on the pan-

elization of the PCBs.At this time you may be

able to alter the dimensions of the PCB to

better fit into the array and subsequently

onto the panel. If the number up per array of

the physical dimensions are not of great

importance, it is best to allow the fabrication

house to place the PCBs into an array that

best fits the panel.

In general, cost will be lower if the physi-

cal dimensions of a board can be manipu-lated to divide evenly into the image area of

a specific panel. A basic formula is used by

board houses to determine how many PCBs

will fit onto a given panel size (see Equation

1).

This formula can be used for different

panel sizes to reveal the optimum panel size

to be used for maximum panelization.

Figure 1 shows a typical panelized PCB on a

12 x 18 panel.

Another key area in which money and

time can be saved is the dielectric spacing

between layers in the construction of the

PCB. As with minimizing the number of

panel sizes, a board house also will try toreduce the number of different core thick-

nesses and pre-preg types it keeps in inven-

tory. The core material is the raw laminate

that has been laminated with copper on each

side by a supplier and shipped to the board

house; this is commonly referred to as C-

stage material.The pre-preg is the pre-woven

glass that has been impregnated with the

epoxy resin and is used between the core

material to laminate a multilayer board; this

is commonly referred to as B-stage material.

A board house can also save money by uti-lizing foil and single-ply construction. Foil

construction allows the board house to use

one less piece of core material in the con-

struction of a PCB. Single-ply construction

allows the board house to use only one sheet

of pre-preg between layers. A board house

will first look at any design and try to apply

these principles for simpler manufacturing

and cost benefits. Below is a comparison of

an older conventional construction to a foil

single-ply construction:

A board house can order an abundance ofdifferent core thicknesses from their supplier

but most choose to limit their inventory and

only keep the most common thicknesses on

hand at any given time. A board design will

dictate how many pieces of core material will

be needed for a job.And the dielectric spacing

specified by a customer will detail the core

and pre-preg thicknesses that must be used.

Figure 1 -A panelized PCB on a 12 x18 panel.

Equation 1

Imagecourtesyof

theCirexxCorp.



To specify or not?It is best for a designer not to specify the

dielectric thicknesses of a board unless it is

needed for the functionality of the boards.

Not specifying dielectric thicknesses will

allow the board house to utilize the common

cores and pre-pregs that it has in house.

There is a large potential for lost time at

the fab house when there are both dielectricand impedance call-outs for the PCB. When

an order is booked and dielectrics and

impedance are specified, the fabrication

house is now held responsible for the imped-

ance outcome of the PCB.There are hundreds

of impedance modeling software tools and

the chances are that the fabricator is not

using the same one you are. Since the

responsibility rests upon the fabricator to

achieve the targeted impedance, he will use

his modeling software based on the dielectric

call-outs that have been supplied.

It is very common that the impedance

numbers will not match, and at this point the

fabrication house has no choice but to put

the job on hold and contact the designer for

approval to change the dielectrics. If the his-

tory of given dielectrics for a designfrom

an earlier revision or similar boardhave

generated good performance, then dielectric

call-outs are probably better than impedance

call-outs because there will be no variation in

the measurements between layers. A cross-

section can be supplied by the fabricationhouse to verify dielectric call-outs for a given

design.

If dielectrics must be controlled for pur-

poses of impedance or buried capacitance,

then it is best to contact the board manufac-

turer for a construction that will work best

for the design and its manufacture.

Keep the following in mind when com-

municating with your fabrication house:

Ask for a list of the panel sizes, image areas

and spacing between parts that the fabrica-

tion house uses. Ask what panel size the fabricator would

use to panelize a PCB at a given size, quan-

tity and level of difficulty. The smaller the

panel size the greater the cost savings.

If the PCB is to be palletized, get the phys-

ical dimensions and number up (if applic-

able) to the fabrication house, and ask how

the shop would panelize it.

Ask for a list of common core materials

and pre-pregs that they use.

If impedance or controlled dielectrics areto specified, get a construction model

from the fabrication house.

Do not specify both controlled dielectrics

and impedance unless it is also specified

that the dielectrics are for reference only.

Decide which of the two is more important.

In conclusion, cost reductions can be

achieved through minimal effort by paying

attention to panelization and construction

concerns. Contact the fabrication house and

ask what panel sizes are used and what the

given image area is for each size.The formulathat was given ear-

lier can be used to

determine the best

panel size to be

used. Generally, the

smaller the panel

size that can be used

with minimal pan-

els, the lower the

cost of the order.

Contact the fab-

rication house andwork with their in-house engineers for a

dielectric stack-up that works best for the

inventory of raw materials on hand that will

also work best for the functionality of the

board.

Fabrication houses are generally eager to

assist designers in these areas because it

increases the chances that the shop will

receive the order. Work out these issues

before the design is done and before the

order is placed with the board house to help

speed up the quoting and engineeringprocess.

Keith Schenk is an engineering manager atCirexx Corp. in Santa Clara, CA. He has23 years of experience in PCB manufactur-ing. Schenk holds an undergraduatedegree in business management and amasters degree in business administrationand technical management.

Figure 2 -Conventional construction vs. foil singly-plyconstruction

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Tricks of Cost Effective Panelization