Bow & Twist (Part I)
Bow & Twist (Part I) – Understand the NonConformance & its Calculation Methods
Before we learn how to calculate the Bow and Twist as per IPCTM650 2.4.22 test method, let us understand the basics of the Bow and Twist.
What is Bow & Twist?
The Bow and Twist is the unintended change in the geometry of a PCB that determines the level of flatness in a circuit board. Board warpage is a common term used to describe altered shape of PCB, which can either be a Bow or a Twist.
A Bow is a condition in a PCB where all 4 corners more or less stay in the same plane but the board material displays cylindrical or spherical curvature.
A Twist is the board deformation parallel to the diagonal of the board such that one or both diagonal corners are not in the same plane compared to the remaining corners that normally stay flat.
How to Calculate Bow & Twist?
The Bow and Twist in a Printed Circuit Board (PCB) rank among the highest levels of falsely identified nonconformance because it is perhaps the least understood concept in terms of allowable IPC limits.
Most QC inspectors (for incoming materials in PCB assembly) understand that a perfectly flat PCB is the minimum standard that every circuit board manufacture must maintain. Though few accept that a little bit of Bow and Twist can be allowed in a PCB, how much to allow has remained a conundrum since they lack a clear understanding of a test method provided in IPCA600.
Let us understand the method of measurement for Bow and Twist and their maximum allowable limits with respect to the IPCTM650 2.4.22 test method.
Table for Acceptable Criteria of Bow & Twist (according to IPCTM650 2.4.22 test method)
Bow  Twist  
% Limit  Calculation Formula  % Limit  Calculation Formula  
Circuit Board (without SMD parts)  1.5% 
Longer Side: RL = L(B)/100 Shorter Side: RW = W(B)/100 L = Board Length W = Board Width B = % Bow Limit RL & RW = Go/NoGo feeler/pin gauge size for PCB length & width respectively 
1.5% 
Production Test Method R = 2 (D)(T)/100 Referee Test Method T= (R1R2)/D x 100 D = Diagonal Measurement T = % Twist Limit R = Go/NoGo feeler/pin gauge size R1 & R2 = Refer Step 3 & Step 4 
Circuit Board (with SMD parts)  0.75  0.75% 
Example of a Bow Calculation:
Assume the PCB size is 200mmx300mm. Thus, the width will be 200mm and the length will be 300mm.

Board without SMD Part >
Bow (300mm side of the board) RL = L(B)/100 = 300 (1.5) /100 = 4.5mm
Bow (200mm side of the board) RL = W(B)/100 = 200 (1.5) /100 = 3.0mm 
Board with SMD Part >
Bow (300mm side of the board) RW = L(B)/100 = 300 (0.75) /100 = 2.25mm
Bow (200mm side of the board) RW = W(B)/100 = 200 (0.75) /100 = 1.5mm
Example of a Twist Calculation:
Assume the PCB size is 200mmx300mm. Thus, the width will be 200mm, the length will be 300mm and the Diagonal measurement will be 360.56mm as per the Pythagorean Theorem.
Now, the Twist can be measured by the following two different methods:
Production Test Method
Step 1
Take the diagonal measurement of the PCB. There are two ways of taking the measurement (1) directly from the gerber data (2) place the PCB on the surface plate and while applying sufficient pressure to flatten the PCB, take the measurement. Record this measurement as D.
Step 2
Calculate the size of the feeler/pin gauge (Go/NoGo) to be used for maximum allowable Twist by using the following formula:

Board without SMD Part >
Maximum Twist R = 2 (D)(T)/100 = 2 (360.56) (1.5)/100 = 10.82mm 
Board with SMD Part >
Maximum Twist R = 2 (D)(T)/100 = 2 (360.56) (0.75)/100 = 5.41mm
(Refer Picture in Step 3)
Step 3
Place the PCB on the surface plate with any three corners of the PCB touching the surface. Apply sufficient pressure (if necessary) to only one corner of the PCB to ensure three of the four corners are in contact with the surface plate. It may be necessary to turn the PCB over to accomplish this.
Caution > If it is not possible to get three corners of the PCB to touch the surface plate by restraining only one corner, the "Production Testing Method" cannot be used to measure the Twist. In this case, “Referee Test Method” should be used for an accurate measurement of a Twist.
Step 4
If the feeler/pin gauge can slide underneath the PCB corner without touching it then the Twist is considered to be more than the allowable IPC limit.
Referee Test Method
Step 1
Place the PCB to be measured on a datum surface with the two lower opposite corners touching the datum surface or on a raised parallel surface of equal height from the datum surface.
Step 2
Support the other two corners with leveling jacks or some other appropriate devices, ensuring the two raised corners are of equal height from the datum surface. This may be checked by using a dial indicator.
Step 3
Using a dial indicator, measure the highest raised portion on the board and record the reading as R1
Step 4
Without disturbing the PCB, take a reading with the dial indicator on one of the corners contacting the surface (R2) and record the reading.
(Refer Picture in Step 3)
Step 5
Find out the Twist percentage using the following formula:
T = (R1R2)/D x 100
If T is less than 1.5 (for a Board without an SMD Part) or 0.75 (for a Board with an SMD Part), the Twist is considered to be within the IPC limit.
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