Benchmark Design Engineering May 3, 2016 · 2016. 6. 8. · Benchmark Design Engineering May 3, 2016 Board Strain Analysis Process Page 3 of 9 Benchmark Electronics, Inc. Any procedure

Benchmark Design Engineering May 3, 2016

Board Strain Analysis Process Page 1 of 9

Benchmark Electronics, Inc.

Title:

Board Strain Analysis Process

Presenter:

Gary A. Manhart

Principal Test Development Engineer

Benchmark Electronics, Inc

4065 Theurer Blvd

PO Box 5025

Winona, MN 55987

[email protected]

Phone (507) 453-4560

Topic:

Benchmark Electronics uses a process (that is described in this paper) that evaluates strain that can be applied

to a PCBA during the manufacturing process This process can help determine if the strain level is safe for the

PCBA or not. This strain is referred to as board strain.

Abstract:

This paper will discuss the process that Benchmark Electronics uses to evaluate board strain. Board strain will

be explained and examples will be shared of where board strain can exist in the PCB assembly process. Next

the process Benchmark Electronics uses to evaluate board strain before and after a procedure or hardware is

released to manufacturing will be discussed. This process will include how Benchmark Electronics uses Finite

Element Analysis (FEA), Strain Gage Analysis (SGA), and Dye & Pry to determine if board strain is within

acceptable levels.




Purpose:

The purpose of this paper is to discuss and share Benchmark Electronics Board Strain Analysis Process. High

levels of board strain will cause damage to PCBAs. This is most evident when dealing with Ball Grid Array

(BGA) type components, but board strain can cause damage to other component types and even the printed

circuit board (PCB). This paper will discuss board strain, where it can be found, and what can be done to

predict, measure, and verify if the level of board strain is safe for the PCBA. After reading this paper the reader

will understand the risks of board strain and be aware of some of the tools that can be used to evaluate it.

What is Board Strain?

Strain is defined as a normalized measure of deformation

representing the displacement between particles in the body

relative to a reference length.

• Micro-strain is a dimensionless value labeled micro-

strain (µstrain), e.g. 1 µstrain = 0.000001 mm/mm

• Positive (+) and negative (-) strain values indicates the

direction of strain. Positive values reflect a tension

and negative values reflect a compress. Positive strain

values are not greater than negative strain values.

Board Strain is any strain that can be applied to the board

during the manufacturing process. Notice on the right; the

top picture of the BGA has no board strain. The bottom BGA

shows that the PCBA is being compressed causing the PCBA to

flex. Board Strain can exist in many different places.

Test Fixtures Press Operations Depanelization

Board Handling Hardware Attach/Detach




Any procedure or hardware that makes contact to a PCB should be evaluated for board strain. This evaluation

could be as simple as a quick review or include using tools such as Finite Element Analysis (FEA), Strain Gage

Analysis (SGA), and Dye & Pry which are discussed later in this paper. Be aware of high risk operations early

during manufacturing process development and plan to do some level of Board Strain Analysis before releasing

a procedure or hardware to manufacturing. Any procedure or hardware that has caused strain related damage

in the past should be tested!

BGA type components are probably the most common strain related defect. These parts usually have many

fine pitch balls (pins) that are soldered to pads on the PCB. These components can be small or cover an area of

a couple inches. Board Strain in these areas has to be kept at safe strain limit or fractures will happen.

Fractures usually happen close to the solder joint, but can be found in the bulk of the solder joint. See the

diagram below for areas where fractures can be found.

The picture above shows a solder fracture (top of picture with red arrows). The black line is a fracture. Notice

how the solder joint on the bottom is solid with no black line. Board strain can also cause damage to other

components and the PCB.

Multiple groups have created standards to deal with board strain. Benchmark Electronics has based their

board strain analysis off of the IPC/JEDEC-9704A Printed Circuit Assembly Strain Gage Test Guideline and IPC-

WP-011 Guidance for Strain Gage Limits for Printed Circuit Assemblies. The following graph from IPC/JEDEC-

9704A Printed Circuit Assembly Strain Gage Test Guideline displays where the Maximum Allowable Safe PWB

Strain is when factoring the PCB thickness and the Strain Rate.







The table below is an example of strain measurements that were made. Measurements below the red line are

acceptable while ones above the line are failing measurements.




Dealing with Board Strain and what can be done about it.

Now that we have an understanding of how board strain can be an issue, how do we handle it? There are

tools that are available. Benchmark Electronics relies on Finite Element Analysis (FEA), Strain Gage Analysis

(SGA), and Dye & Pry Analysis to determine if Board Strain is acceptable. Many other tools exist, but they vary

in cost and effectiveness. These tools allow objective analysis of the strain and strain rate levels to which

components may be subjected during assembly, test, and handling operations.

When designing hardware Benchmark Electronics recommends using Finite Element Analysis (FEA). FEA is a

theoretical computer aided analysis of strain that is applied to the PCB. FEAs can be done for almost anything

that is being mechanically designed. We will focus on how FEA is used for an ICT Test fixture. Most FEAs will

factor in board thickness, components, push fingers, board stops, and probe forces. FEA is very helpful in

pointing out areas of concern before the fixture is built. This information is used to make adjustments to the

fixture design that will lower the strain that is applied to the PCBA during testing. The images below show a

graphical representation of strain. Higher levels of strain are indicated by yellow and red areas. The fixture

vendor can make changes to the fixture design to lower the strain in these areas. Changes may include adding

or removing push fingers or board stops. Probe spring forces may even need to be lowered to obtain safe

levels. In some case probes may need to be removed. The nice part about using FEA is that all the changes at

this point are done with software so changes can be made easily. Once acceptable strain levels are reached

the fixture can then be fabricated. If passing FEA results are not possible the fixture vendor will notify their

customer and decide if fixture fabrication should be continued or halted.

After the fixture is fabricated Strain Gage Analysis (SGA) can be used. SGA is a measurement of actual board

strain recorded on the PCBA during some action. This data is then calculated and analyzed to see if the strain

is at safe levels. Since SGA is an actual measurement hardware and instrumentation are needed to perform

the measurements. A fully populated current revision PCBA that matches what will be built in production

should be used. This PCBA will need to have sensors mounted to it. These sensors are calleded rosettes and

are normally mounted in areas around parts that are easily damaged by

board strain. Normally for ICT each corner of a BGA would get a rosette. A

rosette contains three low value resistors that overlap at different angles as

shown on the left. The resistance value of these resistors will change as the

rosette is flexed. The rosette can measure very slight changes in board strain.

The rosette needs to be glued to the PCB. This is considered a destructive

test and the PCB will not be usable after rosettes are mounted to the board.

The picture on the next page shows 4 rosettes mounted on the corners of a

BGA. (Please refer to the IPC/JEDEC-9704A and IPC-WP-011 for more

information on rosette placement ).The BGA is placed on the other side of the board, but you can see the

1/16

e1 e2

e3




outline of it by looking at the

VIAs that are in a square shape

pattern. This board was used

to evaluate a board assembly

process where the board was

placed into hardware. Notice

that each screw is labeled with

a number ranging from 1-6.

SGA was used to measure the

board strain that would be

generated when the screws

were secured in the hardware.

The graph below shows how the board strain reacted when the screws were put in. Notice that there was a

spike to the board strain around the BGA when screw 3 was secured. In order to lower this strain the speed

and torque setting had to be changed.

In order to record board strain values a PC and Vishay 7000 scanner, shown in the lower right, or similar

equipment is required. During an SGA measurement thousands of measurements are taken. The Vishay 7000

comes with Strain Smart software that allows you to display the board strain in a graph as above or you can

export the data in multiple formats. The graph is usually used when performing the SGA to indicate to the

operator that the measured board strain looks acceptable or not.

It is very important to export the data and analyze it giving additional

scrutiny. The graph does not always show outlying measurements.

Benchmark Electronics uses a script that will record the highest strain

values for each rosette as well as the strain rate and outputs them to a

text file. Following is an excerpt from the text file: Worst Case Strains Per Rosette

Rosette Strain Strain Rate Max Strain Percent of Max Time

------- ---------- ----------- ---------- ------------- ------

R1 513.240 5415.751 1195.114 42.94% 5.730

R2 436.294 15858.905 980.603 44.49% 1.678

R3 243.631 30188.867 852.079 28.59% 5.605

R4 -569.383 -5631.652 1187.309 47.96% 9.710

Warning: Strain imbalance on U1000_U1200, R2 was more than double other rosettes

Time = 1.679

R2 strain = 439.744

R9 strain = -211.554

Warning: Strain imbalance on U1000_U1200, R4 was more than double other rosettes

Time = 9.766

R4 strain = -586.304

R2 strain = 281.008 R9 strain = -209.747




This text file allows us to pin point which rosette may be failing. The script will list out any strain that is above

the IPC specification as an error. It will also analyze corners of each BGA and point out areas that should be

examined as warnings if corner to corner limits exceed what is recommended by the IPC specification.

SGA does a very good job of measuring actual board strain. But even if board strain levels are within

acceptable values according to the IPC specifications, component damage can still occur. Benchmark

Electronics uses Dye & Pry to verify that the SGA values measured are safe and will not cause damage to BGAs.

Dye & Pry isn’t high tech and is low cost analysis (except you have to sacrifice a fully populated PCBA). To

perform Dye & Pry execute the action multiple times to the PCBA to be used for Dye & Pry. For example, for

ICT fixtures we actuate the fixture 50 times. Then BGA sites are flooded with Dykem red dye penetrant and

exposed to 20inHg vacuum for a minimum of 30 seconds to highlight any evidence of separation. The samples

are allowed to dry to set the dye/penetrant, and the components are pried or sheared from the PCBA.

A stereomicroscope is used to inspect the BGA sites for any

evidence of dye penetrant at critical surfaces. Where

junction cracks may have formed, dye penetrates and

reveals the damage. The image below, shows a fracture

that was found on a ball grid. The entire area of the ball

grid should be a shiny solder color. You can see where the

fracture is in the lower left of the ball grid. If damage is

found during Dye & Pry, the procedure or hardware that

was being evaluated should not be used until the reason for

excessive board strain is found and reduced.




Dye & Pry should be done again to verify damage will not happen during manufacturing. When Dye & Pry fails,

start looking at the area where that damage occurred. Look for any hardware that may be contacting that

area of the PCBA. It may be worthwhile to go back to the SGA data and see what the board strain value was on

the closest rosette. Keep in mind that sometimes the damage may not have been caused by the equipment

being evaluated, but could have happened when the board was being built or while the board was being

handled. We’ve had two cases where PCBAs from a previous proto build that had already been shipped to the

customer for evaluation were used for Dye & Pry and failed. After extensive investigation it was found that

one board had been dropped at the customer facility and another board had been damaged when the board

was assembled in the final housing.

Now that FEA, SGA, and Dye & Pry have all been used and the hardware is determined to be safe it can be

released to production. SGA should still be used during the life of the procedure/hardware. Previous SGA

results can be used for a base line and SGA should be done periodically. This is especially important when

ECOs to the fixture are completed. It should also be done when a probe, push finger, or board stop is repaired

or replaced. By keeping at least two previously passing SGA results you can determine if an SGA failure is new

or if the failure has been getting worse over time. If you have a failure, you can determine a date range based

off of the other SGA reports of product that may be at risk of having damage.

Summary

To summarize, high levels of Board Strain can cause damage to PCBA. Several tools exist to help indicate

where this may occur. Benchmark Electronics has standardized on using FEA, SGA, and Dye & Pry to minimize

the risk of damage during the PCBAs assembly process. Board strain should be examined on any

procedure/hardware that could cause the board to flex or bend and cause damage. Benchmark Electronics

recommends that some type of Board Strain Analysis Process be in place to help prevent damage during

manufacturing.

Documents

Benchmark Design Engineering May 3, 2016 · 2016. 6. 8. · Benchmark Design Engineering May 3, 2016 Board Strain Analysis Process Page 3 of 9 Benchmark Electronics, Inc. Any procedure