Delaminating of LCD polarizers
LCD displays, especially older, mature display technologies such as segment and character LCDs have a very low fallout rate; so when we receive a request to return a large number of failures, we are concerned and want to solve the problem right away. A few weeks ago, one of our customers, a manufacture of test and measurement equipment, contacted us to report a 1% LCD Display failure rate. One percent failure is cause for alarm; after some investigation we found that the delaminating of LCD polarizers (or polarizers peeling away from the glass) was a result of excessive heat being applied to the display.
The photo below shows an example of a segment LCD with a delaminating of LCD polarizer due to excessive heat.
Maximum recommended temperature for a segment LCD Display?
LCD displays, like microprocessors, are very sensitive to excessive heat and must be protected from exposure to high temperatures for long periods of time. In order to avoid the delaminating of LCD polarizers, the only time to expose the display to high heat is when it is being attached to the PCB (Printed Circuit Board).
A new challenge to this is the move to ROHS (Restriction of Hazardous Substances) which requires a higher melting temperature than older solder technology.
When soldering a LCD, try to use hand soldering. If you do wave-solder, we recommend the display not be exposed to heat in excess of 300C (572 F) and then only for a few seconds.
We do not recommend running the LCD through the solder process more than once.
Heat damage caused during LCD manufacturing
There are two times when a display will face this extreme environment: manufacturing of the LCD and when being soldered to a PCB. Great care must be taken to avoid damage.
A segment display is manufactured by applying polarizers to the top and bottom plate of glass, pins (connections) are attached to each side of the LCD and held in place with an adhesive and then a TN (Twisted-Nematic) type of fluid is inserted between the two layers of glass.
Seven segment LCD Glass with pins and polarizer
The entire LCD assembly is then “baked” to cure the adhesive with the temperature of the oven and the amount of time in the oven being critical. If the temperature is excessive, or the LCDs are left in too long, the polarizer will peel away from the glass.
All completed LCDs should be 100% visually inspected to identify the delaminating of LCD polarizer issue. If there is a problem, it should be pulled from the lot as defective. It should not reach the customers manufacturing location.
Heat damage caused due to soldering of LCD
LCD Displays, like most electronic components are soldered to a PCB, either by wave or hand solder. When the temperature of the solder is excessive, or the heat source is kept in contact for an extended period of time, the polarizer will begin to peel away.
If you find that during your manufacturing process the LCD will be exposed to excessive heat, it is recommended to incorporate a heat shunt and divert the excessive heat away from the adhesive and polarizer.
LCD Display Manufacturing: RoHS and Lead based solder
LCD’s are manufactured with ROHS (Restriction of Certain Hazardous Substances) or non-RoHS (Lead based) solder with each type of solder having its own unique melting point.
ROHS solder is the default solder that is used in all our manufactured LCD Displays and has a melting temperature of 227C (441F).
Lead-based solder is mainly used in aviation, military and some medical industries with a melting temperature of 180C (356F). If requested, we are able to manufacture the LCD with non-RoHS solder.
Do you have any questions regarding delaminating of LCD polarizers? Contact engineering support at 480-503-4295 or fill out the Contact Us form.