Printed Circuit Board Assembly & PCB Design Forum

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# PCB Warpage

Dhanish

PCB Warpage | 7 April, 2005

Need help from the experts on 1)simple way to measure the PCB warpage 2)what is the maximum warpage can the SMT machine allow the to accept and place the component without problem. 3)I have seen some people are using 7mils/inch as a Warpage spec. and some are using the Bow & Twist percentage formula to calculate the warpage.When do we use the first method and the second method.

PCB Warpage | 7 April, 2005

Both methods are fine, but you should create or use a spec that fits your process needs. You should also consider if this warp will directly impact the performance of your final product once it's completely built. Then make sure your customer won't reject it for this warpage as well.

This problem has reoccured world wide again due to a lot of shops opening up in China. We first saw this happen with the wave of boards crossing there. Since then they have learned a lot. Now it's Chinas turn.

PCB Warpage | 7 April, 2005

Q1) Simple way to measure the PCB warpage A1) IPC-TM-650; Method 2.4.22 Bow & Twist

Q2) What is the maximum warpage can the SMT machine allow the to accept and place the component without problem. A2) That depends. IPC-A-610 Acceptability of Electronic Assemblies, allows 1.5% for PTH only and 0.75% for SMT

Q3) I have seen some people are using 7mils/inch as a Warpage spec. and some are using the Bow & Twist percentage formula to calculate the warpage.When do we use the first method and the second method? A3) Using the methods of IPC-TM-650; Method 2.4.22 Bow & Twist provides consistancy

vinitverma

PCB Warpage | 8 April, 2005

7mils/inch is 0.7% and the IPC-A-610 calls for 0.75% (for SMT), so both are approx. the same. So there are no two methods in what you described. The 7mil/inch is also a percentage.

Regards Vinit

PCB Warpage | 8 April, 2005

What is Your customers requirements? Is the PCBA mounted with front/back connectors? Will the PCBA be mounded in a rack and connected to opposite connectors on the other side?

In this case, if the warpage is more than approx 1-1.5 % You will then need to bow/twist the PCBA manually to be able to fit the PCBA in the slot and the connectors into each other. THIS is NOT good!! You will even hear the cracks from the solder joints and the components.

Press Fit connectors? In this case the PCBA will be somewhat "flatten out" during the press-fit process. THIS is NOT good.

Micro vias in the PCB? Warpage is then critical. Regards,

Dhanish

PCB Warpage | 8 April, 2005

What would be the problem during SMT process if the board warps?What is normal warpage allowed in SMT equipment?

KEN

PCB Warpage | 8 April, 2005

Warpage factors can cause:

1. Boards can not transfer down conveyor belts or wedge at transitions. 2. May not enter into machinery due to interferences (like board clamps) 3. May fall off conveyor chain in furnace. Especially true of thin (PCMCIA and v-score panels). 4. May cause print issues with stencil printer.

THe bottom line is IPC is a starting point. You need to determine what works with your machinery / process.

Dhanish

PCB Warpage | 8 April, 2005

The problem is IPC spec. calls for 7 mils/inch but my equipment spec. calls for Warpage as 40 mils.Our Equipment folks saying if a board warps more than 40 mils ,there will be placement issue.

WarpSpeed

PCB Warpage | 10 April, 2005

1.) First of all: What is Your customers requirements???

2.) If the customer or You don't have a specification; I will strongly recommend that you follow the "IPC-IPC-TM-650 Method 2.4.22 Bow & Twist" that [DaveF] suggested.

3.) If You have "critical final pcba-assembly" (like previous posting mentioned) to do; return the bad pcbs's to the manufactories. You should also have a seriuos conversation with them.

4.) About the board warpage: is the pcb's already twisted from start, or did it happend after first/second reflow?

5.) The main questions remain...

Dhanish

PCB Warpage | 11 April, 2005

The problem that I am facing is the Equipment Engieer refuse to run this lot as the boards warp as high as 60 mils but the machine spec.call max 40 mils warpage.The boards length is 16 inches and the thickness is 93 mils.The supplier feedback this is within IPC spec.I am in dilemma.

PCB Warpage | 11 April, 2005

Why not simply try one?