Random Lifted Leads on QFP Components

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 QFPs. 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 three most 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 that component is not level, it's tilted, usually resulting from 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 two 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% 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.

These are all very solid points in the article as well as the comments. I would also like to add the surface finish of the PCB to the mix. Hot air leveled boards (HASL) have statistical variance on the crowning height and solder deposit due to surface tension. Other surface finishes such as ENIG do not suffer from this issue. Sometimes this can cause teeter-tottering or tilting and the pads that have the lower deposit amount during blow-off will be an open connection. Also look at your screen printing process for release consistency and also deposit consistency - this can cause an issue that looks similar but is a vastly different solution. Also if the part has a ground plane/heat-slug connection under the body, this can also add another tilting and deposit consistency variable to the mix. I hope this helps.

Both of the last comments are good. We fought this problem at my last job.

The root cause could be either non coplanar / bent leads before placement, or thermal stress relief on the board or parts during reflow. And it may be both.

First thing we did was to inspect the co planarity of the leads before placement. We found that we did have some non-conforming material and worked that with the supplier. But that was only part of our problem.

Even with a more consistent part supply, we had to do some work on the thermal profile and solder paste height to solve the issue.

First I would number then inspect some parts before and after going through a reflow cycle. This should let you know if you are fighting a part or heat problem (or both).

One of the major causes for lifted leads for QFP type components is handling. Examples are below:

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.

When I was with a prior company we experienced problems with lifted leads. We discovered something that was not mentioned in your material. The design of the board. The size and distribution of the components on the board provide different heating indexes in different places, this along with the material of the board itself. Let's say that a large QFP is properly aligned but the board flexes in the oven. You end up having what looks to be a lifted lead problem, but do you have a problem with the QFP?

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.