Phil
Welcome to board talk. This is Jim Hall and Phil Zarrow, the assembly brothers, sometimes known as pick and place, but also known as those fun loving consultants from ITM consulting.
We're here to talk about process problems, equipment problems, methodologies, materials, and whatever else is bothering you in your SMT process. And Jim, what is today's question?
Jim
Today's question comes from EL. We have a number of QFP, quad flat pack, components on our circuit boards. After placement and reflow, we have a random lifted lead problem on some QFP's. The fallout rate is small at less than 1 percent of components. Is this fall our rate normal? What might be the cause? Where should we look?
Phil
I guess the first thing that comes to mind is based on our experiences in trouble shooting and I would have to say there are three popular areas that come to mind. The first of the more blatant ones is the popcorn effect due to improper MSD recognition and handling.
Jim
Exposing plastic parts for to long out of their moisture barrier bags before you do final reflow. Causing them to absorb moisture, to expand, crack the component. But I would think you would know the solution because a popcorned part is permanently damaged.
Phil
A more possible thing is coplanarity.
Jim
If you have a single or few leads at random locations, particularly in the middle of a row along one side of your QFP, I would say it's a coplanarity problem and that strictly a result of handling. Something somewhere in the handling of that package caused the lead to get bent and is not coplanar with the rest of the leads so it sticks up. You may not see it, but it's high enough that it's lifted.
The other possibility would be tilted, where the component is not level it's tilted, usually resulting in an improper placement operation, but there you would typically see it as the corners one side of the package all lifted because the entire package is tilted. Those are the typical factors and the solutions. The handling for coplanarity or improper placements through a worn nozzle or an off centered mechanism.
Jim
As far as the defect rate, 1 percent random coplanarity reject even coming out of matrix trays, there is always some fall out. Some of your better placement machines have coplanarity checks on them before they'll place the part and they'll reject it if they are out of spec. Only some machines have that capability, usually requires relatively costly addition to hardware in the vision system.
Phil
Okay, well, thank you for the question. This is Phil Zarrow and Jim Hall, the pick and place assembly brothers, saying, whatever you do,—
Jim
Don't solder like my brother.
Phil
And please don't solder like my brother and keep those kids away from the flux pot.
Use the form below to submit a comment.
1. QFP tray holds 20 components
2. Purchasing orders 110
3. Vendor/manufacturer has to re-package 10 components in another tray
3a. They might not use the right tray which can cause bent leads
3b. Just moving these parts to a good tray can cause bent leads
4. You buy components from your customer stock and their handling can cause bent leads, plus 3a and 3b
5. Internal kitting might be moving parts from 1 tray to another causing bent leads
6. Most machines can be stepped through a process, with QFP attached to nozzle, to determine if the machine is bending any leads
6a. Check program to see where failed QFP's are rejected ... dump box or belt.
A good inspection/training program would be able to reduce or eliminate the possibility of bent leads.
Dean Edwards, Quality Engineer, APT Electronics, Inc.
What if you don't have a center guide to keep the board from flexing? What if the board is already bent from the first reflow?
The problem we saw was because the board was large and the QFP was placed near one corner. The flexing of the QFP and the flexing of the board had an effect on this defect. The solution was to change the board, change our design spec and add a temp stress simulation to the design cycle.
Guillerma Velazquez, RainBird